Classification of Elements and Periodicity — Practice

Question 1

Arrange Na, Mg, Al, Si in increasing order of first ionisation enthalpy and justify any anomaly.

Accepted Answer

Na < Al < Mg < Si. IE generally increases across a period, but Mg has a stable filled 3s2 configuration, so its IE is higher than Al (3s2 3p1), whose loosely-held 3p electron is easier to remove.

Question 2

Why is the ionic radius of Cl- larger than the atomic radius of Cl?

Accepted Answer

Cl- has gained an electron, increasing electron-electron repulsion in the same shell, which expands the electron cloud and increases the radius.

Question 3

What is the lanthanoid contraction and one of its consequences?

Accepted Answer

It is the steady decrease in atomic and ionic radius across the lanthanoid series due to the poor shielding of the 4f electrons. A consequence is that Zr and Hf have nearly identical radii and very similar chemical properties.

Question 4

Which element has the highest electronegativity and why?

Accepted Answer

Fluorine (4.0 on the Pauling scale) -- it has a small size, high nuclear charge, and needs only one electron to complete its octet, giving it a strong pull on shared electrons.

Question 5

Explain why boron has a lower first ionisation enthalpy than beryllium.

Accepted Answer

Be has a stable filled 2s2 configuration. B is [He]2s2 2p1, and its 2p electron is at higher energy and better shielded, so it is easier to remove than a 2s electron of Be -- giving B a lower IE.

Practice — Classification of Elements and Periodicity

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