An atom becomes a cation when it loses one or more electrons. This can happen when the atom is involved in a chemical reaction, such as when it reacts with another atom or molecule that has a stronger electronegativity. The electronegativity of an atom is its ability to attract electrons. When an atom loses an electron, it becomes positively charged, and is therefore called a cation.

For example, when sodium reacts with chlorine gas, the sodium atom loses an electron to the chlorine atom, forming a sodium cation (Na+) and a chloride anion (Cl-). This process is known as ionization.

The ionization energy of an atom is the amount of energy required to remove one electron from the atom. The ionization energy of an atom increases as the number of protons in the nucleus increases. This is because the protons in the nucleus have a positive charge, and they repel the electrons in the atom.

The ionization energy of an atom also increases as the number of electrons in the atom increases. This is because the electrons in an atom are negatively charged, and they repel each other.

The ionization energy of an atom is a measure of how strongly the electrons are bound to the atom.