Each element, be it oxygen, lead, sodium or any other is made up of atoms. The atoms of any particular element are specific to that element. For example oxygen behaves and reacts the way it does because of the particular structure of its atoms. It therefore follows that each element will have it's own specific ionization energy. To explain ionization energy we first look at structure.
This article will assume a basic explaination as to the structure of atoms. That they are made up of a nucleus of protons and neutrons, each proton having a positive electrical charge, and that the nucleus has a over all positive electrical charge dependent on the number of protons. For the purposes of this explanation, the neutrons, which have no electrical charge,will play no further part. The nucleus is surrounded by electrons each of which occupies a specific orbit around the atom. The electrons each have a negative charge and the amount of electrons is dependent on the amount of protons in the nucleus and the thus the two sets of charges are balanced and overall the atom is electrically neutral.
It is the electrical attraction between the positively charged nucleus and the negatively charged electrons which prevent the electrons from shooting off into space. Indeed to remove an electron from an atom energy is required. To remove an electron from an atom is to create an positively charged ion. In other words the atom is now ionized. For each particular element , the energy required to remove an electron will be different and can be measured.
To measure this energy we must set certain conditions. Elements exist in different states, hydrogen is a gas, carbon a solid, mercury a liquid etc. To compare energies the atoms of each element must be in the same state. And so the ionization energy of an element is defined as the energy required to remove an electron from a free atom of that element in the gaseous state. It should be noted that more than one electron can be removed from most atoms apart from hydrogen which only has one electron to lose. Thus values can be obtained for the first ionization energy of an element, the second ionization energy of an element and so on.
The units ionization energies are measured in are usually kilojoules per mole (kJ/mol) or electron volts (eV). They are sometimes also known as ionization potentials. Once we have values for each element we can compare ionization energies and discover any patterns or trends. For example ionization energies associated with the noble gases are high compared to the ionization energies of the alkali metals. It is far harder and requires more energy to remove an electron from an argon atom (15.75 eV), than a sodium atom (5.14 eV), although they are in the same row of the periodic table. We also know that argon is an inert gas and does not readily form compounds, whereas sodium is highly reactive. Clearly there is a link between ionization energy and reactivity.
Hopefully this article has given a fairly basic definition of the ionization energy of an element, and the usefulness of this information.