National 5 Chemistry Revision

Atomic Structure and Bonding related to properties

Isotopes

Atoms of the same element MUST have the same number of protons (or they would be different elements) but the number of neutrons may vary. This means the atomic number will stay the same but the mass number can change.

ISOTOPES are atoms of the same element with different mass numbers.

Isotopes have identical properties except mass (because the numbers of protons and electrons are identical). Most elements are a mixture of isotopes. Because the relative proportions of each isotope is always the same, the mass of an average atom can be calculated- this is called the RELATIVE ATOMIC MASS (RAM).

Relative Proportions

The RAM is the average of the masses of the isotopes in an element.

Electron Shells

We know that electrons move around the nucleus in definite layers. The simplest representation is to think of "target diagrams", sometimes called electron shell diagrams.

These are drawn as simple circles around the central nucleus (often with the symbol for the element in the middle). The maximum number of electrons that can fill a layer or "shell" is fixed. the first shell can take 2 electrons, the second and third each and so on. You will only need to worry about the first 20 or so elements. 

In actual fact, the only electrons chemists are really interested in are the outer ones, as they are the only ones involved in bonding, hence chemical reactions! So, we often miss out the inner shell electrons.

When filling up each shell, start at the position "N" on a compass, them "S". Fill the first shell. For the next shells, start at the position "N" again, then "S", then "E" then "W" (or 12, 6, 3, 9 on a clock!) Once you've put 4 electrons in, go back to "N" and PAIR up the next electrons(N, S, E, W as before). It actually doesn't matter what order you do the "compass points" - it's important to realize that in reality you are dealing with 3 dimensions (i.e not flat circles!!). The electron cloud models (later) give you more detail than the shell diagrams.

Filling Shells

Only the outer level of an atom is important in determining the group of the Periodic Table the atom will be in. e.g. Cl- 2,8,7 - will be in group 7.