Ionization energy of elements
Hovering above a point will show the atomic number and the ionization energy of the element.
Definition
Ionization energy (IE) is the amount of energy required to remove an electron from an isolated atom or molecule. The first ionization energy refers to removing the first electron, the second ionization energy removes the second, and so on.
IE is measured in units of kilojoules per mole (kJ/mol) or electron volts (eV).
Ionization energy (IE) Trends in the Periodic Table
Across a period (Left to Right)
IE increases across a period (Left to Right). In each period, the element of Group 1 has the lowest IE, and the element of Group 18 has the highest IE.
Thus, overall, Group 1 elements (H, Li, Na, K, Rb, Cs, Fr) have the lowest IE's (see Green line in the Graph), and
the Noble gas elements in Group 18 (He, Ne, Ar, Kr, Xe, Rn) have the highest IE (see Red line in the Graph).
Down a group
IE decreases down a group. The decrease for Group I can be seen in the Green line in the Graph, and the decrease for Group I8 can be seen in the Red line.
Reasons
Moving across a period, the atomic number increases, meaning there are more protons in the nucleus. This increases the nuclear charge, pulling the electrons closer to the nucleus. Consequently, it becomes more difficult to remove an electron, and ionization energy rises.