Module 3 Revision notes - Atomic Structure & The Periodic Table

These notes are not intended as a complete set of revision materials; instead they are here to give you a quick reminder of the most important points in this module.

Atomic Structure

• The way that the atoms of different elements react together to form compounds can only be explained properly by understanding the structure of atoms. The PERIODIC TABLE is organised in the way it is because of the structure of atoms.
  
• Atoms are made up of three types of subatomic particles called protons, neutrons and electrons.

• The protons and neutrons are in the nucleus (centre) of the atom and the electrons orbit round the outside in shells (energy levels or layers). So you will often see pictures of atoms that look a little like this:

How many protons, neutrons and electrons does an atom have?

You can work this out using the periodic table. Every element in the periodic table has two numbers with it: the atomic number and the mass number. For example for lithium, the numbers are:

The atomic number is the number of protons that the atom has. It is also the number of electrons that the atom has. So lithium has 3 protons and 3 electrons.

The mass number is the number of protons and neutrons added together. So, for lithium there are 7 protons and neutrons combined, and we know that 3 of them are protons so there must be 4 neutrons.

RULE: The number is neutrons is (mass number - atomic number)

How are the electrons arranged?

The electrons are arranged in energy levels (or electron shells as they are sometimes known). The number of electrons that you can fit into each energy level is as follows:

• In the first energy level you can fit 2 electrons
• In the second energy level you can fit 8 electrons
• In the third energy level you can fit 8 electrons

You need to be able to work out the electron configuration of the first 20 elements in the periodic table. We write electron configurations like this:

2.8.1

which means that there are 2 electrons in the first energy level, 8 in the second energy level and 1 in the third energy level. Here is a complete list of the electron configurations of the first 20 elements:

ˆBack to top of page

The Periodic Table

• The rows (horizontal) of the periodic table are called PERIODS
• The columns (vertical) are called GROUPS
• Every group has a NUMBER
• Elements in the same group have SIMILAR CHEMICAL PROPERTIES
• This is because elements in the same group have the SAME NUMBER of electrons in their outer shell.
• For example, all of the elements in Group 1 (The alkali metals) - lithium, sodium, potassium etc. - have 1 ELECTRON IN THEIR OUTER SHELL.

As you move through the elements in a group the REACTIVITY (hoe reactive they are) changes. The way that it changes depends on the group you are looking at:

• For GROUPS ON THE LEFT HAND SIDE of the periodic table (Group 1, Group 2, Group 3) as you move DOWN THE GROUP the elements become MORE REACTIVE. For example in Group 1, the order of reactivity is:

POTASSIUM > SODIUM > LITHIUM

• For GROUPS ON THE RIGHT HAND SIDE of the periodic table (Group 6, Group 7) as you move DOWN THE GROUP the elements become LESS REACTIVE. For example in Group 7, the order of reactivity is:

FLUORINE > CHLORINE > BROMINE > IODINE

ˆBack to top of page