3.2.1 Periodicity

Question 1

What is periodicity?

Answer 1

Periodicity is the repeating pattern of physical or chemical properties going across the periods

Question 2

Describe the classification of elements in s, p, d blocks.

Answer 2

Classification of elements in s, p, d blocks Elements are classified as s, p or d block, according to which orbitals the highest energy electrons are in.

s block - groups 1-2
p block - groups 3-0
d block - transition metals

Question 3

Describe the trend in atomic radius across period 2 & 3.

Answer 3

Atomic radii decrease from left to right across a period, because the increased number of protons create more positive charge attraction for electrons which are in the same shell with similar shielding.
- same outer shell of electrons - same shielding - greater attraction between nucleus and outer electrons = pulled further in

Question 4

What is the trend in 1st ionisation energy across period 2 & 3?

Answer 4

General trend - increases.
This is due to increasing number of protons (greater nuclear charge) as the electrons are being added to the same shell (same shielding) = (smaller atomic radius) - harder to remove outer electrons.

Small drop at Al - Easier to remove an electron from Al than Mg (Al has lower first IE)
(Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s sub shell. The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons.)

Easier to remove an electron from S than P (S has lower first IE)
(With sulfur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly fill the first 3p orbital. When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove.)

(Exactly the same trend in period 2 with drops between Be & B and N to O for same reasons- make sure change 3s and 3p to 2s and 2p in explanation)

Question 5

Describe the melting and boiling points across period 2 & 3.
Period 2 = Li, Be, B, C, N, O, F, Ne
Period 3 = Na, Mg, Al, Si, P, S, Cl, Ar

Answer 5

Na, Mg, Al - Metallic bonding: strong bonding - gets stronger the more electrons there are in the outer shell that are released to the sea of electrons. A smaller sized ion with a greater positive charge also makes the bonding stronger. Higher energy is needed to break bonds.

Si - Macromolecular: many strong covalent bonds between atoms, high energy needed to break covalent bonds - very high mp + bp.

Cl2(g), S8(s), P4(s) - Simple molecular: weak van der waals between molecules, so little energy is needed to break them - low mp + bp.

S8 has a higher mp than P4 because it has more electrons (S8 = 128)(P4 = 60) so has stronger v der w between molecules.

Ar - monoatomic - weak van der waals between atoms

Similar trend in period 2
Li, Be metallic bonding (high mp)
B, C macromolecular (very high mp)
N2, O2 molecular (gases - Low mp as small v der w)
Ne monoatomic gas (very low mp)