Periodic Properties and Electronegativity

This is the notes for periodic properties. Don't forget to scroll down the notes.

 Nuclear Charge: 

The strength of attractive force of a nucleus to attract electrons is the nuclear charge. As the no. of protons in a nucleus increases, the nuclear charge increases.

Shielding/Screening effect: 

In a poly-electronic atom or ion, a valence electron does not experience the total nuclear charge due to the presence of inner electrons which is known as shielding effect.

Effective nuclear charge: 

Due to the shielding effect, the attraction of nucleus on the valence electron decreases. The resultant magnitude of the nuclear charge which is experienced by the valence electron is called effective nuclear charge.

Effective nuclear charge (Zeff ) = Total nuclear charge (Z) – Shielding effect (σ)

Atomic Radius (Atomic Size) 

Atomic radius is defined as the distance between the center of the nucleus and the valence shell of the atom. It is difficult to determine the exact radius of the atom because there is not any specific distance from the nucleus to the valence electron and the distribution of electron in an atom is affected by neighboring atoms.

To overcome these difficulties, atomic radius can be defined only in the bonding situation and is expressed as; 

 a) covalent radius 

 b) ionic radius 

a) Covalent radius is defined as the distance between the center of the nucleus of an atom and the mean position of the shared pair of electrons between the bonded atoms.

b) Ionic radius is defined as the effective distance from the nucleus of the ion to which it has an influence in the ionic bond.

• The size (ionic radius) of the cation is smaller than that of the parent atom. Why? 

The cation is formed by the loss of electron from the valence shell but the no. of protons remain same. Due to this, nuclear charge per electron increases and the outermost electrons of the cation are more strongly attracted towards the nucleus. This results in decrease in size. Hence, The size (ionic radius) of the cation is smaller than that of the parent atom. For e.g. 

 Na+ has smaller size than Na atom. 

 Mg++ has smaller size than Mg atom.

• The size (ionic radius) of the anion is greater than that of the parent atom. Why? 

The anion is formed by the gain of electron in the valence shell but the no. of protons remain same. Due to this, nuclear charge per electron decreases and the outermost electrons of the anion are less effectively attracted towards the nucleus. This results in increase in size. Hence, the size (ionic radius) of the anion is greater than that of the parent atom. For e.g. 

 Cl- anion has greater size than Cl atom. 

Variation of atomic radius (size) across a period 

The atomic size goes on decreasing on moving left to right in a period. This is due to increase in nuclear charge across a period and as a result the electrons are more tightly attracted towards the nucleus.

Variation of atomic radius (size) along the group 

On moving top to bottom down the particular group, the atomic size goes on increasing. This is due to the addition of extra shell and increase in shielding effect which reduces the attractive force between the nucleus and the valence electrons resulting in the increase in size. 

Electronegativity (E.N.): 

It is defined as the relative tendency of an atom in a molecule to attract a shared pair of electrons towards itself. For e.g. In H-Cl molecule, the shared pair of electrons are attracted more towards chlorine atom. Therefore, chlorine is said to be more EN than hydrogen.

The EN values are scaled in Pauling scale and Mulliken scale. According to Pauling’s scale, the EN of Fluorine is assigned (4) which is the highest of all other elements.

Factors affecting Electronegativity:

I. Atomic Size: The EN increases with the decrease in the size of the atom because smaller the size of the atom larger will be the effective nuclear charge. As a result, small sized atom has more tendency to attract the shared pair of electrons.

 

II. Nature of atoms bonded to it: The EN of an atom depends upon nature of atoms bonded to it. For e.g. the EN of P in PCl5 is higher than that in PF5 because F is more EN than Cl. 

III. Type of hybridization: The EN increases as the s-character increases in the carbon atom.

Hybridization:     sp             sp2             sp3  

% of s-character: 25          33.33             50

Molecule:      Ethyne        Ethene         Ethane 

IV. Charge on the ion: 

 Since a cation is smaller in size than the parent atom, it attracts electrons more strongly and therefore has higher EN than the parent atom. E.g. Na+ has greater EN than Na atom .

 Since an anion is greater in size than the parent atom, it attracts electrons less effectively and therefore has lower EN than the parent atom. E.g. F- has less EN than F atom.

V. Shielding effect: Increase in shielding effect increases the size which in turn decreases the EN of the atom/ion.

Variation of EN across a period: 

The EN of the element increases from left to right in a period. This is due to the increase in nuclear charge and decrease in atomic size which causes to attract the shared pair of electrons more strongly.

 

Variation of EN along the group: 

The EN of the elements decreases on moving down the group. This is because of increase in atomic size due to addition of extra shell and increase in shielding effect which decreases the tendency to attract the shared pair of electrons.

Hope this will help you a lot.

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