Rationale of trends in the Periodic table
Basis of periodic trends:
1) zeff = z - s
2) F = k Zeff Qe / r2 ; attractive force between the nucleus & outermost electron; r = distance between the nucleus & outermost electron
3) F = k Qe Qe / r2 ; repulsive force between electron - electron
4) electron energy level profile & basis of Eminimum to eject electron in the photoelectric effect; IE = Δ energy between n = ∞ & occupied highest energy electron level
n = ∞ ___ ← electron is ejected
.
.
___ ___ ___ 2p
n = 2 ___ 2s
n = 1 ____ 1s
atomic size
* left → right in the periodic table (ignore transition metals):
± s & ↑ z → ↑ zeff → ↑ Fnucleus-electron attraction → ↓ size
* top → bottom in the periodic table
↑ s & ↑ z → ± zeff
but ↑ # electron shell → ↑ size
atom / ion size
* anion versus (corresponding) atom
add electron (to atom to form anion) → ↑ electron - electron repulsion→ ↑ size (of anion relative to atom); i.e. size of anion > size of corresponding atom
*cation versus (corresponding) atom
remove electron (from atom to form cation) → ↓ electron - electron repulsion→ ↓ size (of cation relative to atom); i.e. size of cation < size of corresponding atom
* ions (cations or anions)
[left → right] or [top → bottom] of periodic table
have the same trend as atomic size for similar reasons
Ionization energy (IE) trends
* 1st IE < 2nd IE < 3rd IE < . . .
remove electron →
↓ electron - electron repulsion→ ↓ size → ↑ Fnucleus-electron attraction → ↑ energy to remove electron = ↑ IE
* there is a "big" jump in IE between electron subshells, e.g. 2nd IE of Na & 3rd IE of Mg (Re. table 7.2 in the textbook); because, to remove the electron from the next lower subshell, there is a decrease in shielding
i.e. ↓ s & ± z → ↑ zeff → ↑ Fnucleus- electron attraction → ↑ energy to remove electron = ↑ IE
* left → right in periodic table
↑ z & ± s → ↑ zeff → ↓ size [ & ↑ zeff ] → ↑ Fnucleus-electron attraction → ↑ energy to remove electron = ↑ IE
* while going left → right in the periodic table, IE drops between Be & B; Mg & Al
electron configuration in above pair of atoms corresponds to going from a full s-orbital to a higher energy p-orbital
i.e. ↑ energy of electron orbital → ↓ energy needed to eject electron → ↓ IE
* while going left → right in the periodic table, IE drops between N & O; P & S; As& Se
electron configuration of these atoms: ↑ ↑ ↑ → ↑ ↓ ↑ ↑
np3 orbital electrons are in separate orbitals, while an electron in np4 orbital is paired with another electron in the same orbital → there is electron - electron repulsion in np4 orbital (while there is no such interaction in np3 orbital) → becomes easier to remove electron = ↓ IE
* top → bottom in the periodic table
↑ r & ± zeff → ↓ Fnucleus-electron attraction → ↓ energy to remove electron → ↓ IE
EA trends
* left → right in the periodic table (ignore Noble gases)
↑ z & ± s → ↑ zeff (& ↓ size) → ↑ F nucleus-electron attraction [ → easier for electron to be attracted to the atom] → ↑ [magnitude of] EA (for process, where EA < 0).
* while going left → right in the periodic table, the magnitude of EA drops between C & N; Si & P; Ge & As; Sn & Sb
electron configuration: n p2 versus n p3
when add electron to above atom, its's easier to add an electron to np2 [since there is an empty p-orbital] than np3 [since it means that the electron would be added to an occupied p-orbital] → there is electron - electron repulsion→ "harder" to add electron → ↓ energy release due to adding electron to the atom → ↓ magnitude of EA (for process, where EA < 0)
* top → bottom in the periodic table
no clear trend: there are opposing factors / influences that cancel each others' effect
e.g. ± zeff → ± Fattraction . . . → ± EA
[coulomb's law consideration]: ↑ size → ↓ Fattraction . . . → ↓ EA
[geometry consideration]: ↑ size → ↓ electron - electron repulsion→ ↑ EA