Trends in the Modern Periodic Table

If you observe the long form of the periodic table or the modern periodic table, then you will find that there are certain trends that are followed. These trends help us in determining various properties of the elements. For example, elements such as sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon are present in the third period. Do you know why they are placed like this?

It is because all these elements have one thing in common. You must have observed that these elements contain the same number of shells and the last electron enters shell M as shown in the given table:

Elements of the third period
Element	Electronic configuration K L M
Sodium	2, 8, 1
Magnesium	2, 8, 2
Aluminium	2, 8, 3
Silicon	2, 8, 4
Phosphorus	2, 8, 5
Sulphur	2, 8, 6
Chlorine	2, 8, 7
Argon	2, 8, 8

Do you know the valency of the elements present in the third period?

Valency is defined as the number of electrons an atom requires to lose, gain, or share in order to complete its valence shell to attain the stable noble gas configuration. Valencies of the elements can also be determined by the number of electrons present in the outermost shell known as the valence shell.

Therefore, on moving across a period, from left to right, the valency first increases from 1 to 4 and then decreases from 4 to 0.

Element	Valence electrons	Valency
Sodium, Na	1	1
Magnesium, Mg	2	2
Aluminium, Al	3	3
Silicon, Si	4	4
Phosphorus, P	5	3, 5
Sulphur, S	6	2
Chlorine, Cl	7	1
Argon, Ar	8	0

Periodicity of Properties of Elements in the Periodic Table

Elements show periodicity because of their valence shell configuration. All elements showing periodicity in properties have the same number of electrons in the last or valence shell. The properties that will be discussed here are:

• Atomic radius
• Ionisation potential
• Electron affinity
• Electronegativity
• Metallic character

Atomic Radius: The atomic radius is usually considered as the distance from the centre of the nucleus to the outermost shell i.e., to a point where the electron density is effectively zero.

Across the period i.e., from left to right: Atomic radius decreases

Down the group i.e., from top to bottom: Atomic radius increases

Reason: Across the period, the effective nuclear charge increases. This is due to the fact that the number of electrons increase (in the same subshell), increasing the number of protons in the nucleus. This pulls the valence shell of electrons in an atom towards itself, thus decreasing the atomic radius. But as we move down the group, the number of orbits keeps on increasing along with the number of protons. The space required to accommodate the extra orbits takes prevalence and therefore the atomic size increases.

The given animation explains the change in atomic size as we move down the group.

Ionisation Potential: It is the energy required to remove one mole of electrons from the valence shell of one mole of isolated gaseous atoms.

Across the period i.e., from left to right: Ionisation potential increases

Down the group i.e., from top to bottom: Ionisation potential decreases

Reason: Across the period, the effective nuclear charge increases. This causes the atomic radius to decrease, thus getting the valence shell closer to the nucleus. This makes it difficult to remove electrons. But as we move down the group, the number of orbits keeps on increasing along with the number of electrons. The distance from the nucleus coupled with the interference of the electron between the nucleus and the valence shell renders the valence electrons weakly bound to the nucleus.

Electronegativity: The tendency of an atom to attract a bonding pair of electron towards itself when combined in a compound is called electronegativity.

Across the period i.e., from left to right: Electronegativity increases

Down the group i.e., from top to bottom: Electronegativity decreases

Reason: Across the period, the effective nuclear charge increases, thus decreasing the atomic radius. This favours the increase in electronegativity of elements across the period. But as we move down the group, the number of orbits keeps on increasing and therefore the atomic size increases and the electronegativity decreases.

Metallic character: It is defined as the tendency of an atom to lose electrons.

Across the period i.e., from left to right: Metallic character decreases

Down the group i.e., from top to bottom: Metallic character increases

Reason:Across the period, the effective nuclear charge increases, thus increasing its atomic radius. This favours the increase of electronegativity and therefore the tendency to lose electrons is low. This accounts for the decrease in the metallic character along a period. But as we move down the group, the number of orbits keeps on increasing and therefore the atomic size increases. This means that the electronegativity decreases. This enhances the loss of electrons and therefore the metallic character increases down a group.