TSS-Connected: Chemistry

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Models for the atom

Before any experimental evidence, the existence of atoms was posited by ancient philosophers as a consequence of the idea that there must be some limit to the divisibility of matter into smaller and smaller portions. Democritus (466-370 BCE) is usually credited with developing the concept during a period in which classical Greek civilization gave rise to an interest in natural philosophy. Democritus' atoms (the term was derived from the Greek atomos, meaning "uncuttable") were solid, indivisible, and indestructable. Furthermore, the atoms could stick together upon collision, and such agglomeration of atoms gave rise to the variety of substances observed in the world^{1, 2}. Remarkably, these speculative propositions bore a striking resemblance to Dalton's atomic theory, put forward 1200 years later. As chemistry developed in the 18th century, indirect evidence began to accumulate for the existence of atoms and for the proposition that the variety of pure substances is a result of the tendency of atoms to form different stable combinations in definite proportions. In other words, atoms combine to form molecules, which are thus the smallest unit of the pure substances known as compounds. By the first decade of the 19th century, the English scientist John Dalton had proposed the first recognizably modern version of atomic theory. Let's outline the development of atomic theory, and in so doing we will arrive at a model for the atom that will prove to be of great utility in understanding much of chemistry.

Atomic theory

One of the major themes of general chemistry - which also illustrates how chemistry relates to everyday life - is the correlation of the properties of substances that we observe - often with our unaided senses - on a macroscopic scale with processes that occur on the ultra-microscopic scale of atoms and molecules. In fact, the events occurring at the atomic and molecular level are fundamental in explaining the observations of chemistry in the macroscopic realm. This is what a good scientific theory does of course: it serves as a powerful explanatory principle. We need to start our study of chemistry with a basic understanding of the major features of atomic theory, one of the greatest achievements of 20th-century science.

กฎออกเตต - The octet rule

กฎออกเตต หรือกฏครบ 8 กล่าวคือ อะตอมพยายามที่จะทำให้เวเลนซ์อิเล็กตรอนของตัวมันเองให้ครบแปดโดยมีหลักการ ดังนี้

อะตอมต่าง ๆ จะเข้ารวมตัวกัน (สร้างพันธะระหว่างกัน) โดยจัดอิเล็กตรอนให้มีเสถียรภาพสูงขึ้น และเสถียรภาพสูงสุดจะเกิดขึ้นเมื่ออะตอมมีการจัดอิเล็กตรอนเหมือนแก๊สมีตระกูล (ครบ 8)

แต่โมเลกุลบางชนิดไม่เป็นไปตามกฏออกเตต เช่น BeCl2, BF3, PCl5, SF6, XeF4