Atoms are the fundamental building blocks of matter, composed of protons (positive charge), neutrons (no charge) found in the nucleus, and electrons (negative charge) orbiting the nucleus. In a neutral atom, the number of protons and electrons is identical, meaning the total positive charge cancels out the total negative charge. This balance gives the atom an overall neutral electrical standing. However, this stable state can be disrupted, causing the atom to become electrically charged by the movement of electrons.
Defining the Ion The Result of Electron Transfer
The specific term for an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge, is an ion. The process that creates this charged particle is known as ionization. An ion is distinguished from a neutral atom because it possesses an imbalance between the number of protons and electrons. Since the number of protons is fixed and defines the element, the net charge results solely from a change in the electron count. Losing electrons creates a positive charge, while gaining electrons creates a negative charge.
Cations Losing Electrons and Anions Gaining Electrons
Ions are categorized into two types based on the sign of their electrical charge.
Cations
A positively charged ion is called a cation. Cations form when a neutral atom loses one or more electrons, leaving it with more protons than electrons. For example, when a sodium atom (Na) loses one electron, it becomes a sodium cation (\(\text{Na}^+\)). Metals, such as sodium and calcium, typically form cations.
Anions
Conversely, a negatively charged ion is called an anion. Anions are created when a neutral atom gains one or more electrons, resulting in a greater number of electrons than protons. For example, a chlorine atom (Cl) becomes a chloride anion (\(\text{Cl}^-\)) by gaining one electron. Nonmetals, like chlorine and oxygen, generally form anions.
Calculating Ion Charge
The specific charge of any ion is determined by the calculation: (Number of Protons) – (Number of Electrons) = Ion Charge. For the chloride ion with 17 protons and 18 electrons, the charge is \(17 – 18 = -1\).
Why Atoms Seek Charge
Atoms gain or lose electrons to achieve a state of greater stability. Atoms are most chemically stable when their outermost electron shell, the valence shell, is completely filled with electrons. For most elements, this stable configuration requires eight electrons in the valence shell, a principle known as the Octet Rule. This arrangement mimics the electron configuration of noble gases, which are chemically unreactive.
An atom like sodium, with only one valence electron, finds it easier to lose that electron than to gain seven others. Losing the electron makes the next full shell the new stable outermost shell, resulting in the \(\text{Na}^+\) cation. Conversely, chlorine, which has seven valence electrons, readily accepts one electron to complete its octet and become the stable \(\text{Cl}^-\) anion. This drive to achieve stability by forming charged particles governs their chemical interactions and leads to the formation of ionic bonds.