How & Why Do Two Atoms Become a Cation & an Anion?

Atomic Structure

• Atoms are composed of three separate particles: protons, neutrons and electrons. Protons and neutrally charged neutrons reside in the nucleus, which is surrounded by a cloud of electrons. Each proton imparts a positive charge for the atom, and each electron has a negative charge. When the number of protons and number of electrons are the same, the atom has a net zero, or neutral, charge.

Losing and Gaining Electrons

• Atoms of elements may gain or lose electrons, though most elements favor either gaining or losing electrons depending on their configuration. In simple terms, many elements tend to favor gaining or losing electrons to acquire an electron configuration where the outermost shell of electrons is full; this is known as a noble gas electron configuration.

Ionization Energy

• In more complex terms, the tendency of an atom to gain or lose electrons is governed by how strongly the electrons are held by the nucleus. The amount of of energy needed to strip an electron away from an atom is called the ionization energy. On the periodic table, elements with the highest ionization energy tend to be located to the top and to the right. This means these atoms are less likely to lose electrons than atoms beneath and to the left of them -- though this pattern is not absolute. Therefore, atoms on the left side of the periodic table are more likely to lose electrons and become cations.

Electron Affinity

• Electron affinity is the energy change involved in the addition of an electron to an atom. The more negative this value the greater the atom's electron affinity. Elements with the greatest electron affinity tend to be found at the right and top of the periodic table. This indicates that nonmetals have greater electron affinity than metals.

Charges

• The outermost electrons are called valence electrons and are the electrons often involved in chemical reactions. Sodium has one valence electron and tends to lose this electron to achieve the electron configuration of neon and becomes a cation with a plus-1 charge. Chlorine, on the other hand, has seven valence electrons and tends to gain one to achieve the electron configuration of argon, becoming an anion with a minus-1 charge.

Ionic Compounds

• Oppositely charged particles can form ionic compounds. The atoms of ionic compounds are held together by the opposite charges of the anions and cations involved. For example, a sodium cation with a plus-1 charge and a chlorine anion with a minus-1 charge may form sodium chloride, or table salt. The sodium chloride has a net zero charge, though the individual ions within the sodium chloride retain their positive and negative charges.