Electronic Configurations & Periodicity

What is the basis of the periodicity of properties?

Electrons are held in shells.

• The first shell holds only 2 electrons in what is called the 1s orbital. Thus helium has its first shell filled, so it can't react by picking up another electron. And each electron is held by both charges in the He nucleus, so they are much more tightly held than the electron in H, so He won't give up an electron either - its inert.

• The second shell is larger (further from the nucleus) so 2 electrons in a 2s orbital, and an additional three 2p orbitals, 8 electrons can be accommodated in the second shell.

  • for lithium (Li) the inner two electrons of the 1s shell cancel the attraction of two of the three protons, so the outer 2s electron "sees" only a single charge. But its out further than the electrons in the 1s shell were, so Li loses its outer electron more readily than H and is more reactive.

  • for Fluorine on the other side of the chart we can think of the outer shell electrons being attracted to the nucleus by 9 - 2 = 7 charges, so the last open space in an orbital will be super attractive to an outside electron, so F will be be very reactive, but in an opposite way to Li - it wants to steal electrons instead of giving them up.

  • for neon all of the orbitals will be filled, and the electrons will be strongly attracted to the nucleus and there is no room for additional electron in the ground state, so Ne will again be inert like He above it.

• The third shell is larger yet (further from the nucleus), but still crowded, so initially it can only accommodate another eight electrons.

  • Of course the electrons in the 3s orbitals are even farther out from the nucleus, so we would expect Na to be even more reactive than Li, and so on for K, Rb, etc. each giving up its outermost electron more readily than the element above it in the Periodic table.

  • On the other hand Cl will also attract electrons less than F, so it will be less reactive etc. for the halogens.

• So we will expect the most reactive elements to be on the opposite corners of the table - lower left and upper right.

© R A Paselk

Last modified 27 March 2015