Ionization energy is the amount of energy that an isolated atom in its ground electronic state must absorb to release an electron, resulting in an ion. The closer the electron is to the nucleus, the more difficult it is to tear it off, which means that more energy needs to be transferred. This value is expressed in kJ/mol: the amount of energy required to lose one electron.

Element	Title	Ionization energy, kJ/mol
H	Hydrogen	1312
He	Helium	2373
Li	Lithium	520
Be	Beryllium	899.5
B	Bor	801
C	Carbon	1086
N	Nitrogen	1402
O	Oxygen	1314
F	Fluorine	1681
Ne	Neon	2080.7
Na	Sodium	495
Mg	Magnesium	738
Al	Aluminum	578
Si	Silicon	787
P	Phosphorus	1012
S	Sulfur	1000
Cl	Chlorine	1251
Ar	Argon	1520.6
K	Potassium	418.8
Ca	Calcium	590
Sc	Scandium	633.1
Ti	Titanium	658.8
V	Vanadium	650.9
Cr	Chrome	652.9
Mn	Manganese	717.3
Fe	Iron	762.5
Co	Cobalt	760.4
Ni	Nickel	737.1
Cu	Copper	745.5
Zn	Zinc	906.4
Ga	Gallium	578.8
Ge	Germanium	762
As	Arsenic	947
Se	Selenium	941
Br	Brom	1142
Kr	Krypton	1350.8
Rb	Rubidium	403
Sr	Strontium	549
Y	Yttrium	600
Zr	Zirconium	640.1
Nb	Niobium	652.1
Mo	Molybdenum	684.3
Tc	Technetium	702
Ru	Ruthenium	710.2
Rh	Rhodium	719.7
Pd	Palladium	804.4
Ag	Silver	731
Cd	Cadmium	867.8
In	Indium	558.3 ??
Sn	Tin	709
Sb	Antimony	834
Te	Tellurium	869
I	Iodine	1008
Xe	Xenon	1170.4
Cs	Cesium	375.7
Ba	Barium	503
La	Lanthanum	538.1
Ce	Cerium	534.4
Pr	Praseodymium	527
Nd	Neodymium	533.1
Pm	Promethium	540
Sm	Samarium	544.5
Eu	Europium	547.1
Gd	Gadolinium	593.4
Tb	Terbium	565.8
Dy	Dysprosium	573
Ho	Holmium	581
Er	Erbium	589.3
Tm	Tulium	596.7
Yb	Ytterbium	603.4
Lu	Lutetium	523.5
Hf	Hafnium	658.5
Ta	Tantalum	761
W	Tungsten	770
Re	Rhenium	760
Os	Osmium	840
Ir	Iridium	880
Pt	Platinum	870
Au	Gold	890.1
Hg	Mercury	1007.1
Tl	Thallium	589.4
Pb	Lead	715.6
Bi	Bismuth	703
Po	Polonium	812.1
At	Astatine	890
Rn	Radon	1037
Fr	France	380
Ra	Radium	509.3
Ac	Anemones	499
Th	Thorium	587
Pa	Protactinium	568
U	Uranus	597.6
Np	Neptunium	604.5
Pu	Plutonium	584.7
Am	Americium	578
Cm	Curium	581
Bk	Berkelium	601
Cf	California	608
Es	Einsteinium	619
Fm	Fermi	627
Md	Mendelevium	635
No	Nobelium	642
Lr	Lawrence	470
Rf	Rutherfordium	580

Also, electrons can attach to an atom. In the process of such attachment, the electron releases energy, which is called the electron affinity energy, for each electron of a particular atom, the affinity energy is numerically equal and opposite in sign to the ionization energy. For example, element Cl: in order to remove the 17th electron from the chlorine atom, it is necessary to inform it of 13 eV. Any other electron that joins in the place of the 17th electron will also release 13 eV.