10.1    Periodicity in Physical Properties

What is periodicity?

1) Periodicity is the recurrence of similar properties at regular intervals when the elements are arranged in increasing atomic  number.

Variation in size of atoms

• Covalent radius is half the internuclear distance between two like atoms bonded by a single covalent bond
• Van der Waal’s radius is half the average distance between two adjacent non-bonded atoms
• For example, the covalent radius of Cl2 is 0.099 nm while the van der Waal’s radius of Cl2 is 0.180 nm

Note: van der Waal’s radius is always larger than covalent radius.

• Metallic radius is half the distance between two like metal atoms bonded by metallic bond
• All these measurable quantities can be given a general name called ‘atomic radii’.
• Across Period 3, the atomic radius decreases This is because the nuclear charge increases while the shielding effect remains constant. The outer electrons are more attracted towards the nucleus, making the atoms smaller.
• For comparison, metallic radii are used for Na, Mg and Al, covalent radii are used for Si, P, S and Cl. For argon, van der Waal’s radius is used(argon do not form any bonds)Note: This trend excludes argon because comparing van der Waal’s radius with covalent and metallic radius is not fair.

Variation in ionic radius

• Cations are formed when an atom loses electron(s). In Period 3, Na, Mg, Al and Si form cations by losing electron(s) to achieve stable octet electronic configuration. The ions formed are Na⁺, Mg²⁺, Al³⁺ and Si⁴⁺ respectively
• Cations are smaller than their respective atoms because a whole layer of electrons are lost. The remaining electrons are attracted more strongly towards the centre by the same nuclear charge
• Anions are formed when an atom gains electron(s). In Period 3, P, S and Cl form anions by gaining electron(s) to achieve stable octet electronic configurat The ions formed are P³⁻, S²⁻ and Cl⁻ respectively.
• Anions are bigger than their respective atoms because they have more electrons than The electrons are held less strongly by the nucleus. Besides, a repulsion is created between the electrons when a new electron is introduced and this causes the ion to expand.
• Anions are bigger than cations because anions have one more shell of electrons compared to cations
• In the isoelectronic series(from Na⁺ to Si⁴⁺ and P³⁻ to Cl⁻), the ionic radius decreases This is because the same number of electrons are attracted more strongly by the increasing nuclear charge.

Variation in melting and boiling points

• Across a Period,
  1. melting point increases from Na to Al because the strength of the metallic bond increases
  2. melting point of Si is highest because Si has a giant covalent structure, a lot of energy is required to overcome the strong covalent bonds
  3. melting points of P, S, Cl and Ar are lower because these have simple molecular structures, only weak van der Waal’s forces of attraction exist between them
• Melting point of S > P > Cl > Ar because these elements exist as S8, P4, Cl2 and Ar S8 contains the most number of electrons, followed by P4, Cl2 and Ar. Van der Waal’s forces get stronger with increasing number of electrons.

Variation in electrical conductivity

• Across the Period, the elements change from metals(Na to Al) to semi-metal(Si) and then to non-metals(P to Ar).
• Electrical conductivity is highest is metals, lower in semi-metals and lowest in non-metals(Most non-metals do not conduct electricity at all).
• The electrical conductivity of Period 3 elements:
  1. increases from Na to Al because the number of electrons contributed by per atom to the sea of delocalised electrons increases from one in Na, two in Mg and three in Al. There are more electrons to conduct electricity
  2. decreases from Al onwards. Si is a semi-metal therefore it is a semi- The remaining elements do not conduct electricity because there are no mobile electrons.

Variation is first ionisation energy (Refer Chapter 3)