Minggu, 30 Mei 2010
Study Tips for Chemistry Students
Getting good grades in Chemistry is really based on having good performances on exams and assignments. Everything that you learned about performing well in athletics or music applies to having a good performance in chemistry. As in music or athletics, you must practice regularly and develop a lifestyle that does not get in the way of your performance. Here are a few examples of how to obtain better performance skills in your courses. Remember that each person learns a little differently. What works for others may not necessarily work for you. You need to find out how to maximize your performance. The tips below should help most students.
Write everything out.
It would be foolish for a musician to watch someone else play a piece or to glance at a piece of music and say "that doesn’t look too hard, I can do that". The concert would be a very bad place to realize that they couldn’t play it. You don’t really know how hard it is until you try it yourself. In chemistry, you must work it out for yourself in writing. Solve the problem on paper or write out your explanation before you are being tested. What you think you know and what you can successfully write down may not be the same. The test is a terrible place to find this out.
Practice daily.
Wouldn’t it seem ridiculous for an athlete to put off practicing until the night before the competition and then stay up all night "cramming" for the event? Not only is there insufficient preparation but the problem is compounded by not getting enough sleep. Several shorter practices spread out over a period of time will do much more good than a marathon session where your progress is impaired by fatigue. When studying, don’t be afraid to take a short break and then return to your work. Don’t forget that the quality of your study time is as important as the quantity of your studies. If athletes put on their gear and spend two hours standing around drinking Gatorade, they should not claim to have practiced for two hours. Likewise, a student sitting in the library with the book open but socializing should not kid themselves into thinking that they are studying. Find a place where you can work with out being interrupted. Being a full time college student is a full time job with lots of overtime involved. (And that does not even include the extracurricular activities). It takes a lot of effort, but the rewards are enormous.
Do your best work.
Have you ever heard the expression "how you practice is how you will play the game"? Just as sloppy play will often lose the game, and sloppy playing will ruin the best piece of music, sloppy work habits will ruin a good academic performance. The only way to avoid a sloppy performance is to practice not being sloppy. When working a problem, neatly and clearly write out your answer. Be sure your drawings and figures are clear and labeled. Write out explanations in clear and complete sentences. Check to make sure you chose the best words and that they say what you really intended them to say. Being close to the right answer may not get credit just as being close to the basket will not score the points. Indeed, many points have been sacrificed for inexact or unclear answers.
Think about the material all the time.
Loving what you do and being good at it often go hand in hand. People who love what they do think about it all the time and relate it to their everyday lives. Good athletes seem to talk about their sport all the time and always seem to be looking for a way to do it better. Even when you are not formally studying, think about the concepts in the course. While going for a walk, showering, or before you fall asleep, think about the concepts and how you might explain it to someone else. Relate the concept to what you see in life. This can be done formally by thinking about phenomena in your daily life such the fizzing of a glass of soda and thinking about what gas pressure and solubility properties give rise to it. This can also be done less formally by relating some abstract concept to a silly analogy such as relating the concept of limiting reagent to making sandwiches. Don’t forget that underneath the details is a topic that you used to find interesting.
Learn the material in small chunks.
There may seem to be an overwhelming amount of material and students have a tendency to go over all the material many times. With so much information, very little is really learned even after several repetitions. When learning a complicated piece of music, it is fruitless to struggle all the way through a song day after day. Instead, break the material into little pieces that you can concentrate on until they are mastered. You may feel like you are spending a lot of time to learn a small amount, but if the material is really learned you will know it the next time you see it, and then, more can be added to it. You may also find that once you really know a few concepts well, the rest is easier to learn because it is related to what you already know well. Athletes do not learn every play by running through all of them quickly day after day. The plays are best learned one at a time, step by step, until they become second nature. Don’t be afraid to invest the time to learn it right. Take it one day at a time.
Concentrate on your work and let the grade take care of itself.
The best performances in music or athletics require total concentration. Paying attention to the score of the game or what the audience is thinking takes away from you doing your best job. When studying or taking a test, give it your complete attention. There will be plenty of time later to think about the grade. The students who seem to do the best in class give their full attention to learning the material and, in the end, are often surprised by how good a grade they get.
Prepare for class.
Before coming to class, it is important to adequately prepare. You should read the material several times if necessary. It may be helpful to quickly scan the chapter to get an overview and to get a feel for how the material will be presented and then go back and read more carefully. Don’t forget to read the assigned questions as well. It is always helpful to see what kind of skills you will be expected to have so you can pay attention to the most important information. The reading may be difficult and you may feel that you don’t get much out of it. Remember that a chemistry book is not a novel that can be read briskly but must be read slowly, several times, and digested as you go. One of the most important skills that you will get from your college education is to learn how to teach yourself. That is what you will take with you when you forget most of the course material. By reading technically difficult material and struggling through it, you improve your reading skills and your ability to learn on your own.
Take an active part in class.
Don’t forget the value of each class. With current tuition rates, this course is costing you (or someone else) approximately $50/hr, so pay attention! Coming to class overly tired or with a hangover can be quite costly, especially if you remember that this is probably the only time you will have to devote yourself completely to academic pursuits. If you are able to convince yourself that this is important to you, being involved will be easier. You should be involved enough that you have an answer for each question posed during a lecture, even if it is a wrong answer it is better than no answer at all. You should be relating the lecture to the material that you read in the book and thinking about whether it is consistent. You could also be asking yourself the questions "does this make sense with what I know from everyday life?" If you are really tuned in to a lecture, you will often anticipate the next step of what is being presented.
Re-read the material.
Now that you have gone to class and have some familiarity with the material, it is important to re-read the chapter. This gives your brain another chance to go over the material and it develops your ability to read technically difficult material. Remember that your reading skills are one of the most important things you will take with you when you leave college. A musicians ability to read music is enhanced by reading through a piece which is known well so that the brain can make connections between the symbols and the ideas behind them. This helps you to think in terms of those symbols. The material must be read again when it will make sense. You are learning the language, you need to practice reading it.
Write out everything you know.
Reading and working problems are an important part of learning chemistry. It is also important to take a blank piece of paper and write out what you know about the topic as if you had to teach it to someone else. This will force you to sift thought the mountain of material and pull out the most important parts. Write out what you think are the most important parts of the material and give examples, draw pictures, make up a problem or think of an analogy to some other topic. This is a great learning exercise as well as a confidence builder. You need to practice facing a blank page so that you are familiar with doing it before you get to the exam.
Work the problems without looking back at the chapter.
Many students have a tendency to read a problem, find the relevant section in the book, take the approach the author used and apply it to their problem, quickly write down an answer and think that they are done. Working problems in this manner gets students good at finding answers in the book and perhaps recalling key words or recognizing correct answers when they see them. The problem is that exams do not usually ask you to find a section in the book or relate a few key words. You need to be able to generate the answers on your own. Again, "how you practice is how you will play the game". Being able to play the chemistry game well, means a student can generate correct answers without assistance. This skill is required on an exam, so you will need to practice it. When you read a problem and you do not immediately know the answer, resist the temptation to look back in the book. Close the book, take a blank piece of paper and write out anything you know about the problem. Try any way you can think of to solve the problem. Many ways may not work, but try something. Some people who are perfectionists have a very difficult time with this. They do not want to write down wrong answers so they don’t write down anything at all. By not writing anything down they can not solve the problem so they get stuck. When you get stuck, start writing. When you first try this you may feel like a rat crawling through a maze and you will make a lot of wrong mental turns and bump into a lot of walls. But after going through this maze several times you will be able to travel it rapidly and get back on track even after making a wrong turn. This maze that I am referring to is your thought process and it is different for everyone. Only you can figure out how to get through yours. The sooner you do this the better you will perform.
Study offensively rather than defensively.
In sports, it is often said that the best defense is a good offense. If you are only concerned about defending yourself on an exam, you will make very little forward progress in your education. Many students look at the material and say "I better go over this in case it is on the test". With this attitude the student has already determined that the only value this material has to them is that it may be on the exam. The chances that the material will truly be learned, much less retained, is small. Instead, try studying offensively. Say "I am going to master this topic because it is important (and maybe even interesting)to me. If I see it on the exam I will know it". See the exam as a challenge and an opportunity to score some points, not as a defensive play in which you may lose the game. It is important to be balanced here because being too confident may also be detrimental; don’t celebrate until after you are in the end zone because you may still drop the ball. In short, attack the material and avoid overconfidence.
Check your answers.
Many people think that they can sing quite well, until they listen to a recording of themselves. It is only then that they hear what they sound like to others. You can’t sing well unless you learn to listen to yourself. The same is true in academics. Many times, students think that they have a good performance on an exam and put down answers that make sense to them at the time. After getting the exam back they realize that their answers didn’t make sense. Each student needs to learn to be critical of their own work. Again, this takes practice. When working the problems, before you check your answers with the book, take a minute and ask yourself "does this make sense?" "Is this as complete as I could make it?" Double check your answers and make sure they are perfect. It is important for you to be more critical of your own work than the grader will be. Also go back and make sure that you answer the question which was asked. Get in the habit of correcting arithmetic, punctuation, spelling, grammar, and clarity since these elements are essential for good communication (i.e. full credit).
Relax on the test.
The superstar athletes are the ones who perform the best under pressure. Those who become anxious, go down in flames. Anxiety destroys your concentration and detracts from you showing what you can really do. And when you think about it, what good does worrying do anyway? Remember that in a few million years the sun will blow up and the Earth will be destroyed and then it won’t really matter how you did on one little exam. Seriously though, instead of worrying, take the attitude that you have worked your hardest to prepare and that you will just concentrate on doing your best and that you will accept what you get on the exam. Just do your best and stop worrying that you are not good enough. You wouldn’t be here if you weren’t.
Here are a few suggestions to help you relax on exams:
- Realize that being a little bit nervous is a good thing. It means that you care enough to want to do well.
-Get a good night’s sleep. A sharp mind does a lot more good that an overused memory. More than one student has stayed up late studying and then slept through the exam.
-Don’t consume more caffeine than usual. Caffeine is not known for it’s ability to make you relax or to clear your head. -Arrive early and relax. You don’t need the added stress of getting to the exam and fumbling though your stuff while everybody else has already started. Besides this is not fair to them. (How many good athletes arrive just as the game is starting and then scramble to get their equipment ready?)
-Stop worrying about how everybody else is doing. You only need to concentrate on doing your best. If seeing the other students makes you worry about how they are doing, sit up front so you won’t have to look at them.
-Work the easiest problems first. This will get you warmed up for the more difficult problems and give you a little confidence.
-Don’t get stuck on one problem. If you are not making progress, move on. There is no sense in missing five easy questions while struggling with one difficult one.
-Learn a few relaxation techniques. A slow, deep breath or a quick muscle stretch can go a long way in helping you relax. -Stop and think before you write. This will allow you to give a more clear answer. Think of key words and make a rough outline of the points you will talk about in your essay answer. Make a rough sketch of the picture you will draw.
-Ask if you don’t understand a question. You have a right to know what is being asked.
-Don’t forget to check your answers to make sure that you have answered the question which was asked. It is difficult for a grader to give full credit for a correct answer to a different question that what was asked for.
-Block off enough time in your schedule for the exam. Don’t miss points because you need to hurry to catch a plane or move out of your dorm room. This sounds ridiculous but it happens.
Learn from your mistakes.
Many students have a tendency to get back an exam, look at their score, get upset and use that energy to make them more nervous on the next exam. This is missing a very important opportunity to do some serious learning. When you get an exam back and after you have gotten over the shock of the grade, it is time to look at your mistakes and ask yourself what went wrong. Were you concentrating? Were you nervous? Were you clueless about the material? Did you know the answer but have trouble writing what you thought you knew? Did you think that all your answers were correct until you got the exam back? Would you give the same answers if you took the exam again? Most of these situations can be remedied with the advice above. A bad exam score is trying to tell you something, stop and listen to what it is. If you can truly say that you did your best, then you need to change the way you study. This is a golden opportunity to improve your performance next time.
Get help when you need it.
The responsibility for learning the material is yours and no one else’s. Talk to the instructor, or find a tutor. If you are having trouble understanding the material from the reading, find another source such as a different textbook that may present the material in a different style. Find a student who has had the course in the past. There is a tremendous amount of assistance out there, but it won’t help you unless you ask for it. This is your education, you should care enough to take the initiative.
Decide if you are willing to "pay the price".
Getting a superior grade is like winning an athletic or musical competition, it takes some talent and a lot of hard work. Very often, this comes at a price and those who do the best are typically the ones who consistently work the hardest. Each student needs to determine the importance of their academic performance and make the necessary adjustments in their life. I point this out so that you at least become aware of the importance of your studies in your life. If you are honest with yourself about the importance of your academic career and make the necessary lifestyle adjustments, your life will be much less stressful. Think about your long term goals and decide what value your academic performance, extracurricular activities, and social life will have in the long run. Make the necessary adjustments in your schedule. Balance does not necessarily mean equal time for all. Remember, there is no room on your transcript for excuses.
Remember that you are here for more than a grade.
One of the most important things that you can learn in college is to learn how to teach yourself. Most of the material from the course will eventually be forgotten unless it is regularly reviewed. What you will take with you will be your education; the ability to independently gather and analyze information, make informed decisions and communicate them clearly. The course material is the medium by which we exercise these abilities. The course material is important and interesting on its own, but it is also a vital part of a liberal education. Learning the material is important but not as important as the process of learning how to learn.
Finally, don’t forget that you are training for the real world. It is important to practice things which are valued in the real world, such as showing up on time, doing consistent work, getting along with others, taking initiative, working independently, being motivated, mature, and responsible. These skills are so important to future employers, as well as graduate and medical schools, that they are the main concern in letters of recommendation. You are being evaluated on far more qualities than just your academic grades. You are taking far more than a diploma, a transcript, and a sizable debt with you when you leave.
Bentuk - Bentuk Molekul dan Ion
Teori tolakan pasangan elektron
Bentuk molekul dan ion ditentukan oleh penataan pasangan elektron disekeliling atom pusat. Semua yang kamu butuhkan untuk menyusunnya adalah seberapa banyak pasanganelektron yang berada pada tingkat ikatan, dan kemudian tertatanya untuk menghasilkan jumlah tolakan minimum antara pasangan elektron. Kamu juga perlu memasukkan pasangan elektron ikatan dan pasangan elektron mandiri.
Bagaimana cara menyusun jumlah pasangan elektron
Kamu dapat melakukannya dengan menggambar titik-silang, atau dengan menyusun atom-atom dengan menggunakan elektron dalam kotak dan mengkhawatirkan tentang promosi, hibridisasi dan yang lainnya. Akan tetapi hal ini sangat membosankan! Kamu dapat memperoleh informasi yang sama dengan tepat dengan cara yang lebih mudah dan cepat untuk contoh-contoh yang akan kamu temukan.
Hal pertama yang perlu kamu susun adalah seberapa banyak elektron yang terdapat pada sekeliling atom pusat:
*Tuliskan jumlah elektron pada tingkat terluar dari atom pusat. Hal ini akan sama dengan nomor grup pada tabel periodik, kecuali pada kasus gas mulia yang membentuk senyawa, ketika jumlah elektron terluar menjadi delapan.
*Tambahkan satu elektron untuk tiap ikatan yang terbentuk. (Hal ini diperbolehkan untuk elektron yang berasal dari atom yang lain).
*Berikan muatan untuk tiap ion. Sebagai contoh, jika ion memiliki muatan 1-, tambahkan satu kelebihan elektron. Untuk muatan 1+, hilangkan satu elektron
Sekarang susun seberapa banyak pasangan elektron ikatan dan pasangan elektron mandiri yang ada:
*Dengan membagi dua untuk menemukan jumlah total pasangan elektron disekeliling atom pusat.
*Susun seberapa banyak pasangan ikatan, dan seberapa banyak pasangan elektron mandiri. Kamu tahu seberapa banyak pasangan elektron ikatan yang ada karena kamu mengetahui seberapa banyak atom yang lain yang bergabung dengan atom pusat (dengan asumsi bahwa hanya terbentuk ikatan tunggal).
Sebagai contoh, jika kamu mempunyai 4 pasangan elektron tetapi hanya terdapat 3 ikatan, hal itu harus ada 1 pasangan elektron mandiri selain tiga pasangan elektron ikatan
Akhirnya, kamu dapat menggunakan informasi ini untuk menyusun bentuk molekul atau ion:
*Susunlah semua pasangan elektron pada jarak yang mengalami tolakan minimum. Bagaimana caranya melakukan hal ini akan menjadi jelas pada contoh-contoh berikut.
Dua pasangan elektron disekeliling atom pusat
Kasus yang paling sederhana adalah berilium klorida, BeCl2. Perbedaan elektronegatifitas antara berilium dan klor tidak cukup untuk menghasilkan pembentukan ion.
Berilium memiliki dua elektron terluar karena terletak pada golongan dua. Berilium membentuk ikatan kepada dua klor, tiap atom klor menambhkan elektron yang lain ke tingkat terluar dari berilium. Tidak terdapat muatan ionik yang perlu ditakutkan, karena itu terdapat 4 elektron yang bersama-sama – 2 pasang.
Hal ini membentuk 2 ikatan dan karena itu tidak terdapat pasangan elektron mandiri. Dua pasangan ikatan tertata dengan sendirinya pada sudut 180o satu sama lain, karena hal ini sebagai yang paling jauh yang dapat mereka capai. Molekul digambarkan dengan linear.
Tiga pasangan elektron disekeliling atom pusat
Kasus yang paling sederhana adalah BF3 atau BCl3.
Boron terletak pada golongan 3, karena itu dimulai dengan 3 elektron. Tidak terdapat muatan, karena itu totalnya 6 elektron – 3 pasang.
Karena boron membentuk 3 ikatan maka tidak terdapat pasangan elektron mandiri. Tiga pasang ikatan tertata dengan sendirinya sejauh mungkin. Semuanya terletak dalam suatu bidang yang memiliki sudut 120° satu sama lain. Susunan seperti ini disebut trigonal planar.
Pada diagram, elektron yang lain pada fluor dapat dihilangkan karena tidak relevan dengan ikatan
Empat pasangan elektron disekeliling atom pusat
Terdapat banyak contoh untuk ini. Yang paling sederhana adalah metana, CH4.
Karbon terletak pada golongan 4, dan karena itu memiliki 4 elektron terluar. Karbon membentuk 4 ikatan dengan hidrogen, penambahan 4 elektron yang lain – seluruhnya 8, dalam 4 pasang. Karena membentuk 4 ikatan, semuanya harus menjadi pasangan ikatan.
Empat pasangan elektron tertata dengan sendirinya pada jarak yang disebut susunan tetrahedral. Tetrahedron adalah piramida dengan dasar segitiga. Atom karbon terletak di tengah-tengah dan hidrogen pada empat sudutnya. Semua sudut ikatan adalah 109.5°.
Contoh lain dengan empat pasang elektron disekeliling atom pusat
Amonia, NH3
Nitrogen terletak pada golongan 5 dan karena itu memiliki 5 elektron terluar. Tiap-tiap atom hidrogen yang tiga menambahkan elektron yang lain ke elektron nitrogen pada tingkat terluar, menjadikannya total 8 elektron dalam 4 pasang. Karena nitrogen hanya membentuk tiga ikatan, satu pasang harus menjadi pasangan elektron mandiri. Pasangan elektron tertata dengan sendirinya pada bentuk tetrahedral seperti metana.
Pada kasus ini, Faktor tambahan masuk. Pasangan elektron mandiri terletak pada orbital yang lebih pendek dan lebih bulat dibandingkan orbital yang ditempati pasangan elektron ikatan. Karena hal ini, terjadi tolakan yang lebih besar antara pasangan elektron mandiri dengan pasangan elektron ikatan dibandingkan antara dua pasangan elektron ikatan
Gaya pasangan elektron ikatan tersebut sedikt rapuh ? terjadi reduksi sudut ikatan dari 109.5o menjadi 107o. Ini tidak terlelu banyak, tetapi penguji akan mengharapkan kamu mengetahuinya
Ingat ini:
Tolakan paling besar pasangan mandiri – pasangan mandiri
pasangan mandiri – pasangan ikatan
Tolakan paling kecil pasangan ikatan – pasangan ikatan
Hati-hati ketika kamu menggambarkan bentuk amonia. Meskipun pasangan elektron tersusun tetrahedral, ketika kamu menggambarkan bentuknya, kamu hanya memperhatikan atom-atomnya. Amonia adalah piramidal – seperti piramida dengan tiga hidrogen pada bagian dasar dan nitrogen pada bagian puncak.
Air, H2O
Mengikuti logika yang sama dengan sebelumnya, kamu akan menemukan bahwa oksigen memiliki empat pasang elektron, dua diantaranya adalah pasangan mandiri. Air juga akan mengambil susunan tetrahedral. Saat ini sudut ikatan lebih sempit dari 104°, karena tolakan dua pasangan mandiri.
Bentuknya tidak dapat digambarkan dengan tetrahedral, karena kita hanya “melihat� oksigen dan hidrogen ? bukan pasangan mandiri. Air digambarkan dengan bengkok atau bentuk V.
Ion amonium, NH4+
Nitrogen memiliki 5 elektron terluar, ditambah 4 elektron dari empat hidrogen ? sehinga totalnya jadi 9.
Tetapi hati-hati! Ion amonium adalah ion positif. Ion ini memiliki muatan +1 karena kehilangan satu elektron. Sehingga tinggal 8 elektron pada tingkat terluar nitrogen. Karena itu menjadi 4 pasangan, yang semuanya berikatan karena adanya empat hidrogen
Ion amonium memiliki bentuk yang sama dengan metana, karena ion amonium memiliki susunan elektronik yang sama. NH4+ adalah tetrahedral
Metana dan ion amonium dikatakan isoelektronik. Dua spesi (atom, molekul atau ion) dikatakan isoelektronik jika keduanya memiliki bilangan dan susunan elektron yang sama (termasuk perbedaan antara pasangan ikatan dan pasangan mandiri).
Ion hidroksonium, H3O+
Oksigen terletak pada golongan 6 – karena itu memiliki 6 elektron terluar. Tambahan tiap 1 atom hidrogen, memberikan 9. Ambil satu untuk ion +1, tinggal 8. Hal ini memberikan 4 pasang, 3 diantaranya adalah pasangan ikatan. Ion hidroksonium adalah isoelektronik dengan amonia, dan memiliki bentuk yang identik – piramidal.
Lima pasangan elektron disekeliling atom pusat
Contoh yang sederhana: fosfor(V) fluorida PF5
(Argumen untuk fosfor(V) klorida, PCl5, akan identik)
Fosfor (terletak pada golongan 5) memberikan kontribusi 5 elektron, dan lima fluor memberikan 5 lagi, memberikan 10 elektron dengan 5 pasang disekeliling atom pusat. Karena fosfor membentuk lima ikatan, tidak dapat membentuk pasangan mandiri.
Lima pasang elektron disusun dengan menggambarkan bentuk trigonal bipyramid -tiga fluor terletak pada bidang 120o satu sama lain; dua yang lainnya terletak pada sudut sebelah kanan bidang. Trigonal bipiramid karena itu memiliki dua sudut yang berbeda – 120odan 90o.
Contoh yang rumit, ClF3
Klor terletak pada golongan 7 dan karena itu memiliki 7 elektron terluar. Tiga fluor masing-masing memberikan kontribusi 1 elektron, menghasilkan total 10 – dalam 5 pasang. Klor membentuk tiga ikatan ? meninggalkan 3 elektron ikatan dan 2 pasangan mandiri, yang akan tersusun dengan sendirinya ke dalam bentuk trigonal bipiramida.
Akan tetapi jangan meloncat ke kesimpulan. Terdapat tiga cara yang dapat kamu lakukan untuk menyususun 3 pasangan ikatan dan 2 pasangan mandiri menjadi bentuk trigonal bipiramida. Susunan yang baik akan menjadi menghasilkan satu susunan dengan jumlah minimum tolakan – dan kamu tidak akan dapat menganbil keputusan tanpa menggambarkannya terlebih dahulu semua kemungkinannya.
Hanya terdapat satu susunan memungkinkan. Sesuatu yang lain mungkin kamu pikirkan sebagai satu yang sederhana pada perputaran dalam jarak tertentu.
Kita perlu menyusun susunan yang memiliki tolakan minimum diantara berbagai pasangan elektron.
Aturan yang baru diterapkan pada kasus seperti ini:
Jika kamu mempunyai pasangan elektron lebih dari empat yang disusun disekeliling atom pusat, kamu dapat mengabaikan tolakan pada sudut yang lebih besar dari 90o.
Salah satu struktur yang memiliki jumlah tolakan besar yang jelas.
Pada diagram ini, dua pasangan mendiri terletak pada sudut 90o satu sama lain, dimana pada kasus yang lain keduanya terletak pada sudut lebih besar dari 90o, dan karena itu tolakan dapat diabaikan. ClF3 memang tidak dapat disusun melalui bentuk ini karena tolakan yang sangat kuat antara pasangan mandiri dengan pasangan mandiri.
Untuk memilih salah satu diantara dua, kamu perlu menghitung tolakan yang paling kecil.
Pada gambar berikutnya, tiap pasangan mandiri terletak pada sudut 90o terhadap 3 pasangan mandiri, dan karena itu tiap pasangan mandiri bertanggung jawab terhadap tolakan 3 pasangan mandiri dengan pasangan ikatan.
Karena terdapat dua pasangan mandiri karena itu terdapat 6 tolakan pasangan mandiri-pasangan ikatan. Dan itu semuanya. Pasangan ikatan terletak pada sudut 120o satu sama lain, dan tolakannya dapat diabaikan.
Sekarang mempertimbangkan struktur akhir.
Tiap pasangan mandiri terletak pada sudut 90o terhadap 2 pasangan mandiri – satu diatas bidang dan yang lainnya dibawah bidang. Hal ini membuat total 4 tolakan pasangan mandiri-pasangan ikatan ? dibandingkan dengan 6, hal tersebut memiliki tolakan relatif kuat pada gambar yang terakhir. Fluor yang lain (satu pada bidang) terletak pada sudut 120o, dan merasakan tolakan yang tidak berarti dari pasangan mandiri.
Ikatan ke arah fluor pada bidang adalah 90o ke arah ikatan diatas dan dibawah bidang, karena itu terdapat total 2 tolakan pasangan ikatan dengan pasangan ikatan.
Struktur dengan jumlah minimum tolakan adalah yang terakhir, karena tolakan pasangan ikatan dengan pasangan ikatan lebih kecil dibandingkan tolakan pasangan mandiri dengan pasangan ikatan. ClF3 digambarkan dengan bentuk T.
Enam pasangan elektron disekeliling atom pusat
Sebuah contoh yang sederhana: SF6
6 elektron pada tingkat terluar belerang, ditambah 1 dari masing-masing fluor, menghasilkan total 12 – dalam 6 pasangan. Karena belerang membentuk 6 ikatan, semuanya adalah pasangan ikatan. Semuanya tertata dengan sendirinya pada sudut 90o, pada bentuk yang digambarkan dengan oktahedral.
Dua contoh yang sedikit lebih sulit
XeF4
Xenon dapat membentuk jajaran senyawa, terutama dengan fluor atau oksigen, dan semuanya khas. Xenon memiliki 8 elektron terluar, ditambah 1 dari masing-masing fluor – menghasilkan 12, dalam 6 pasang. Semuanya akan membentuk empat pasang ikatan (karena empat fluor) dan 2 pasangan mandiri.
Terdapat dua struktur yang memungkinkan, akan tetapi pada salah satunya terdapat pasangan mandiri pada 90o. Malahan, beroposisi satu sama lain. XeF4 digambarkan dengan bentuk square planar.
ClF4-
Klor terletak pada golongan 7 dan karena itu memiliki 7 elektron terluar. Ditambah 4 dari 4 fluor. Ditambah satu karena memiliki muatan +1. hal ini memberikan total 12 elektron dalam 6 pasang – 4 pasangan ikatan dan 2 pasangan mandiri. Bentuknya akan identik dengan XeF4.
Firework in a Glass
Fireworks are a beautiful and fun part of many celebrations, but not something you want kids to make themselves. However, even very young explorers can experiment with these safe underwater 'fireworks'.
What You Need
- water
- oil
- food coloring
- tall clear glass
- another cup or glass
- fork
Create Fireworks in a Glass
- Fill the tall glass almost to the top with room-temperature water. Warm water is ok, too.
- Pour a little oil into the other glass. (1-2 tablespoons)
- Add a couple of drops of food coloring. I used one drop of blue and one drop of red, but you can use any colors.
- Briefly stir the oil and food coloring mixture with a fork. You want to break up the food coloring drops into smaller drops, but not thoroughly mix the liquid.
- Pour the oil and coloring mixture into the tall glass.
- Now watch! The food coloring will slowly sink in the glass, with each droplet expanding outward as it falls, resembling fireworks falling into water.
How It Works
Food coloring dissolves in water, but not in oil. When you stir the food coloring in the oil, you are breaking up the coloring droplets (though drops that come into contact with each other will merge... blue + red = purple). Oil is less dense than water, so the oil will float at the top of the glass. As the colored drops sink to the bottom of the oil, they mix with the water. The color diffuses outward as the heavier colored drop falls to the bottom.
Good Luck!
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