Which element has the highest first ionization energy Sodium aluminum calcium Phosphorus

Ionization energies reported in unites of kilojoules per mole (kJ/mol).

Data taken from John Emsley, The Elements, 3rd edition. Oxford: Clarendon Press, 1998.
The ionization energy of an atom is the amount of energy that is required to remove an electron from a mole of atoms in the gas phase:
M(g) � M+(g) + e-
It is possible to remove more electrons from most elements, so this quantity is more precisely known as the first ionization energy, the energy to go from neutral atoms to cations with a 1+ charge. The second ionization energy is the energy that is required to remove a second electron, to form 2+ cations from 1+ cations:
M+(g) � M2+(g) + e-

The third ionization energy is the energy required to form 3+ cations:
M2+(g) � M3+(g) + e-

and so on. Ionization energies are always positive numbers, because energy must be supplied (an endothermic energy change) to separate electrons from atoms. The second ionization energy is always larger than the first ionization energy, because it requires even more energy to remove an electron from a cation than it is from a neutral atom.
The first ionization energy varies in a predictable way across the periodic table. The ionization energy decreases from top to bottom in groups, and increases from left to right across a period. Thus, helium has the largest first ionization energy, while francium has one of the lowest.
From top to bottom in a group, orbitals corresponding to higher values of the principal quantum number (n) are being added, which are on average further away from the nucleus. Since the outermost electrons are further away, they are less strongly attracted by the nucleus, and are easier to remove, corresponding to a lower value for the first ionization energy.
From left to right across a period, more protons are being added to the nucleus, but the number of electrons in the inner, lower-energy shells remains the same. The valence electrons feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons. The valence electrons are therefore held more tightly, the atom decreases in size (see atomic radius), and it becomes increasingly difficult to remove them, corresponding to a higher value for the first ionization energy.
The following charts illustrate the general trends in the first ionization energy:

Chemical elements listed by ionization energy

click on any element's name for further information on chemical properties, environmental data or health effects.

This list contains the 118 elements of chemistry.

The chemical elements of the periodic chart sorted by:	Ionization Energy	Name chemical element	Symbol	Atomic number
- Name alphabetically	3,8939	Cesium	Cs	55
- Atomic number	4,0727	Francium	Fr	87
- Symbol	4,1771	Rubidium	Rb	37
- Atomic Mass	4,3407	Potassium	K	19
- Electronegativity	4,9	Lawrencium	Lr	103
- Density	5,1391	Sodium	Na	11
- Melting point	5,17	Actinium	Ac	89
- Boiling point	5,2117	Barium	Ba	56
- Vanderwaals radius	5,2784	Radium	Ra	88
- Year of discovery	5,3917	Lithium	Li	3
- Inventor surname	5,4259	Lutetium	Lu	71
- Elements in earthcrust	5,473	Praseodymium	Pr	59
- Elements in human body	5,525	Neodymium	Nd	60
- Covalenz radius	5,5387	Cerium	Ce	58
- Ionization energy	5,5769	Lanthanum	La	57
For chemistry students and teachers: The tabular chart on the right is arranged by Ionization energy. The first chemical element is Cesium and the last one is Helium. The unity for ionization energy is eV. Please note that the elements do not show their natural relation towards each other as in the Periodic system. There you can find the metals, semi-conductor(s), non-metal(s), inert noble gas(ses), Halogens, Lanthanoides, Actinoids (rare earth elements) and transition metals.	5,582	Promethium	Pm	61
5,6437	Samarium	Sm	62
5,6704	Europium	Eu	63
5,6949	Strontium	Sr	38
5,7864	Indium	In	49
5,8638	Terbium	Tb	65
5,89	Protactinium	Pa	91
5,9389	Dysprosium	Dy	66
5,9738	Americium	Am	95
5,9858	Aluminum	Al	13
5,9915	Curium	Cm	96
5,9993	Gallium	Ga	31
6,0215	Holmium	Ho	67
6,0262	Plutonium	Pu	94
6,1077	Erbium	Er	68
6,1082	Thallium	Tl	81
6,1132	Calcium	Ca	20
6,1501	Gadolinium	Gd	64
6,1843	Thulium	Tm	69
6,1941	Uranium	U	92
6,1979	Berkelium	Bk	97
6,2173	Yttrium	Y	39
6,2542	Ytterbium	Yb	70
6,2657	Neptunium	Np	93
6,2817	Californium	Cf	98
6,3067	Thorium	Th	90
6,42	Einsteinium	Es	99
6,5	Fermium	Fm	100
6,5615	Scandium	Sc	21
6,58	Mendelevium	Md	101
6,6339	Zirconium	Zr	40
6,65	Nobelium	No	102
	6,7462	Vanadium	V	23
	6,7589	Niobium	Nb	41
	6,7665	Chromium	Cr	24
	6,8251	Hafnium	Hf	72
	6,8281	Titanium	Ti	22
	7,0924	Molybdenum	Mo	42
	7,28	Technetium	Tc	43
	7,2856	Bismuth	Bi	83
	7,3439	Tin	Sn	50
	7,3605	Ruthenium	Ru	44
	7,4167	Lead	Pb	82
	7,434	Manganese	Mn	25
	7,4589	Rhodium	Rh	45
	7,5496	Tantalum	Ta	73
	7,5762	Silver	Ag	47
	7,6398	Nickel	Ni	28
	7,6462	Magnesium	Mg	12
	7,7264	Copper	Cu	29
	7,8335	Rhenium	Re	75
	7,864	Tungsten	W	74
	7,881	Cobalt	Co	27
	7,8994	Germanium	Ge	32
	7,9024	Iron	Fe	26
	8,1517	Silicon	Si	14
	8,298	Boron	B	5
	8,3369	Palladium	Pd	46
	8,417	Polonium	Po	84
	8,4382	Osmium	Os	76
	8,6084	Antimony	Sb	51
	8,9587	Platinum	Pt	78
	8,967	Iridium	Ir	77
	8,9938	Cadmium	Cd	48
	9,0096	Tellurium	Te	52
	9,2255	Gold	Au	79
	9,3	Astatine	At	85
	9,3227	Beryllium	Be	4
	9,3942	Zinc	Zn	30
	9,7524	Selenium	Se	34
	9,7886	Arsenic	As	33
	10,36	Sulfur	S	16
	10,4375	Mercury	Hg	80
	10,4513	Iodine	I	53
	10,4867	Phosphorus	P	15
	10,7485	Radon	Rn	86
	11,2603	Carbon	C	6
	11,8138	Bromine	Br	35
	12,1298	Xenon	Xe	54
	12,9676	Chlorine	Cl	17
	13,5984	Hydrogen	H	1
	13,6181	Oxygen	O	8
	13,9996	Krypton	Kr	36
	14,5341	Nitrogen	N	7
	15,7596	Argon	Ar	18
	17,4228	Fluorine	F	9
	21,5645	Neon	Ne	10
	24,5874	Helium	He	2
		Rutherfordium	Rf	104
		Dubnium	Db	105
		Seaborgium	Sg	106
		Bohrium	Bh	107
		Hassium	Hs	108
		Meitnerium	Mt	109
		Darmstadtium	Ds	110
		Roentgenium	Rg	111
		Ununbium	Uub	112
		Ununtrium	Uut	113
		Ununquadium	Uuq	114
		Ununpentium	Uup	115
		Ununhexium	Uuh	116
		Ununseptium	Uus	117
		Ununoctium	Uuo	118

Click here: for a schematic overview of the periodic table of elements in chart form

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