Topic 3: Periodicity

Nuclear charge felt by an electron in the atom Feel charge of nucleus but often shielded by other electrons Z (atomic number) - S (shielding constant)

Increase Electrons added to same shell at roughly same distance from nucleus Don’t shield each other perfectly

s, p, d, f

Increase Increased number of protons Atomic orbitals less effective at shielding

half the distance between the centres of neighbouring atoms in a solid or, for non-metals, in a homonuclear molecule (e.g. O2, P4, S8)

Large increase Noble gas only have Van Der Waals Inert and monatomic

Decrease as Z eff increases Outer electrons feeling more effect of nuclear charge due to Z eff, pulling in electrons tightly and decreasing atomic radii

Increase This shows that the valence shell (n) which the outer electrons occupy is more important than Z eff here

derived from the distance between the centres of adjacent ions in a crystal, as determined using X-ray diffraction studies.

2 halves (as you move from positively to negatively charged elements Decrease for increasing atomic number in each half (Li, Be, B) (C, N, O, F) Large increase from B to C e.g. For Li+ (Na+), Be2+ (Mg2+) & B3+ (Al3+) The nuclear charge increases; at the same time the positive charge carried by the element increases [from +1 to +3] Remaining electrons feel increasing Z eff + drawn closer to nucleus Ions decrease in size For C4- (Si4-), N3- (P3-), O2- (S2-), & F- (Cl-) The nuclear charge increases; at the same time the negative charge carried by the element decreases [from -4 to -1] The effect is that the electrons feel an increasing Z eff, and are drawn closer to the nucleus Ions decrease in size Same for period 3

the energy required to remove one mole of electrons from one mole of gaseous species in their ground (or standard) state

General increase Decrease Be (2s2) to B (2s2 2p1) and P (2s2 2p3) to S (2s2 2p4) (period 3 follows same pattern) B readily looses p electron, have to break into Be full 2s sub shell which requires more energy S has a paired electron in 2p sub shell, paired electrons repel each other so are one more easily, P has unpaired sp electrons

Decrease due to increasing quantum number n 13 Al to Ga: increase due to increased Z eff due to less effective shielding from 3d orbitals In to Tl: increase due to increased Z eff due to less effective shielding by 4f orbitals

The power [ability] of anatom in a molecule to attract electrons to itself. 0 → 4

χ increases across a group due to increasing Z off χ decreases down a group due to increasing principle quantum number, n

the energy released when 1 mole of electrons is added to 1 mole of gaseous atoms in their ground (or standard) state Positive Negative (Opposite to usual)

the change in standard molar enthalpy when a gaseous atom gains an electron

ΔegHo = –Ea –(5/2)RT

Increases due to increasing Z eff and decreasing atomic radii

Decrease Be 2s2 Mg 3s2 s sub shell full so added electron needs to occupy empty np shell

Decrease have a full shell so the added electron needs to enter the (n+1) shell

Decrease N: 2s2 2p3 configuration P: 3s2 3p3 configuration Adding electron must pair unpaired p electron which causes repulsion of electrons and reduces stability

Kinetics O from O2 would need to attack central C but C is well shielded Si is larger so has more room to attack so is sterically hindered Si has ability too expand its octet as its empty d-orbitals (3d) closer in energy to p-orbitals (3p) than for C as its 3d orbitals a lot higher in energy than 2p

General trend of increasing atomic radii as a group is descended is due to increasing values of n * C→ Si follows trend in increasing due the higher n. * Si→ Ge shows only a very slight increase as the Ge valence containing poorly shielding d-orbital. This increases Zeff to almost overcome the increasing n. * Ge→ Sn again shows return to trend with increasing n . * Sn → Pb shows strong reversal by increasing radii. This is due to Pb valence now containing poorly shielding f-orbitals and this is enough to allow Zeff to overcome increasing n