Classification of Elements and Periodicity in Properties

Periodic trends

Across a period (left → right)
atomic radius $↓;$ ionisation enthalpy $↑;$ electronegativity $↑;$ metallic character $↓$
Down a group
atomic radius $↑;$ ionisation enthalpy $↓;$ electronegativity $↓$
Effective nuclear charge
$Z_{eff} = Z - σ$ (screening constant)
Electron gain enthalpy
most negative: Cl $>$ F (small size of F) $;$ noble gases positive
Isoelectronic species
radius falls as nuclear charge rises: O $^{2 -} >$ F $^{-} >$ Na $^{+} >$ Mg $^{2 +}$

s-block: $n s^{1 - 2};$ p-block: $n s^{2} n p^{1 - 6};$ d-block: $(n - 1) d^{1 - 10} n s^{0 - 2};$ f-block: $(n - 2) f^{1 - 14}$
Anomalous configurations
Cr: $3 d^{5} 4 s^{1};$ Cu: $3 d^{10} 4 s^{1}$

$0$ nil $, 1$ un $, 2$ bi $, 3$ tri $, 4$ quad $, 5$ pent $, 6$ hex $, 7$ sept $, 8$ oct $, 9$ enn $+$ -ium