Magnesium (Mg) has an electronic configuration of 2.8.8, showing that its electrons enter in 3-shells. It is therefore found in period 3.

Potassium (K) has an electronic configuration of 2.8.8.1, showing that its electrons enter in ‘-shell and so is found in period 4.

Therefore, the number of shells to which the electrons of an element enters tells us the period in which the element is found on the periodic table.

Recall: on the periodic table, the metallic elements are found at the left corner, and metallic characters increases down the group. The mot reactive metal amongst the first 20 elements is potassium. This reactivity is referred to as electro-positivity.

Therefore, electro-positivity increases across the period to the left and down the group. This is because, the larger the atom the easier it loses its electrons0

Non-metallic characters (electro-negativity) increases across the period to the right and down the group. The most electronegative element among the first 20 elements is fluorine.

The first 2 vertical columns are called group 1 and 2 (alkali and alkali earth) metals. In between group 2 and 3 are the transition metals with characteristic properties such as:

• More than one valency
• Form colored compounds
• Act as catalyst

The group 1 elements are salt formers (halogens). The group 7 elements are not reactive because they have completely filled outermost (valence) shell.

Recall: Atoms comes from the Greek word “Atomos” meaning indivisible.

John Dalton in1808 worked on atoms with the notion that atoms cannot be divided and came up with the following suggestions that are called Dalton Atomic Theory. It can be summarized as:

• Elements are made up of very small indivisible particles called atoms.
• Atoms cannot be created nor destroyed in a chemical reaction.
• Atoms of the same element are alike in all aspects but different from atoms of other elements.
• When atoms combine to form compounds, they do so in small whole number ratio.

Modern discoveries have shown that Dalton’s ideas were not all correct. It is now known that:

• Atoms are made up of three important particles named protons, electrons and neutrons.
• During radioactivity, atoms are destroyed and completely new ones are formed. An example is;

92235U→90231U+24He

Also, we say that Radium can break down to smaller particles like helium   and Radom.

• The discovery of isotopes has also proven that atoms of some elements are not identical in all aspects.
• That atoms combine into small whole number ratio can still be accepted for organic chemistry, but in inorganic chemistry, it is seen that carbon atoms usually combine in large whole number ratios.