ionisation energy and properties

Ionisation energy (enthalpy) is the energy required to remove an electron from a gas phase atom.

E(g)

E⁺(g) + e^–

The graph below shows the periodic nature of a plot of ionisation enthalpy against atomic number. The number of elements in the periods is the same as the number of elements in the periods of the periodic table.

big ionisation enthalpy bar

1	2
							1H										2He
3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
11 Na	12 Mg											13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
55 Cs	56 Ba	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At

In general ionisation enthalpies

increase across a period of the periodic table
decrease down a group of the periodic table

The ease with which an atom loses an electron can be correlated with chemical reactivity which, for elements, always involves change in electron ownership.

Elements that have lower ionisation enthalpies have a greater tendency to

exist in their compounds as cations
be electrical conductors

These elements are known as metals.

Elements that do not have these properties are known as non-metals.
As the periodic table above shows, three-quarters of the elements are metals