Source: The Atom lecture, Purdue University General Chemistry
Difficulty: Beginner | Prerequisites: Basic understanding of atomic structure (protons, neutrons, electrons)
Tags: ions, cations, anions, ion formation, transition metals, monatomic ions, electron loss, electron gain, charged atoms, general chemistry
Atoms are electrically neutral when they have equal numbers of protons and electrons. When that balance shifts, atoms become ions, which carry a net positive or negative charge. Ion formation is central to understanding chemical bonding, electrolyte behaviour, and the reactivity patterns you will see across the periodic table. If you are comfortable with the basic structure of an atom (protons in the nucleus, electrons in shells), you are ready for this material.
Removing electrons from a neutral atom produces a positively charged cation. Adding electrons produces a negatively charged anion. Transition metals are unusual because they can form cations with several different charges, which is why you will see Roman numerals in their compound names.
Cation
A positively charged ion formed when one or more electrons are removed from a neutral atom. Think of it as an atom that has lost electrons and now has more protons than electrons.
Anion
A negatively charged ion formed when one or more electrons are added to a neutral atom. In simple terms, the atom has gained extra electrons and carries a negative charge.
Monatomic ion
An ion consisting of only one type of atom, as opposed to polyatomic ions (which contain multiple atoms bonded together). Think of it as a single atom that has become charged.
Transition metals
The elements found in the d-block of the periodic table (groups 3 through 12). These metals can form cations of various charges because they can lose different numbers of electrons.
A neutral atom loses one or more electrons.
The atom now has more protons than electrons, giving it a net positive charge.
Example: sodium (Na) loses one electron to become Na+. It goes from 11 protons and 11 electrons to 11 protons and 10 electrons.
A neutral atom gains one or more electrons.
The atom now has more electrons than protons, giving it a net negative charge.
Example: chlorine (Cl) gains one electron to become Cl-. It goes from 17 protons and 17 electrons to 17 protons and 18 electrons.
Transition metals do not follow the simple "group number equals charge" rule that main-group metals do.
They can form cations of different charges depending on the compound.
Iron, for instance, can exist as Fe2+ or Fe3+.
This is why Roman numerals appear in naming: iron(II) chloride vs iron(III) chloride.
Ions formed from a single atom.
Distinguished from polyatomic ions, which are clusters of atoms bonded together that carry a collective charge (e.g. sulfate, SO4 2-).
Students often think that gaining electrons makes an atom a cation (positive). It is the opposite: gaining electrons makes an anion (negative). A useful mnemonic is that "cation" has a "t" that looks like a plus sign.
Students sometimes assume that the charge of a transition metal ion is always the same. It is not. Transition metals can form multiple different cations.
Losing electrons does not mean losing protons. The number of protons stays fixed; it is only the electron count that changes when ions form.
⚠️ You will almost certainly be asked to identify whether an atom forms a cation or anion based on electron loss or gain. Know the definitions cold.
⚠️ Transition metal variable charges are a recurring exam theme, especially in naming compounds.
⚠️ Do not confuse monatomic ions with polyatomic ions. The distinction comes up in nomenclature questions.
True or false: removing an electron from a neutral atom creates an anion. (False, it creates a cation.)
Fill in the blank: an atom that gains electrons becomes a ______ charged ion. (Negatively.)
True or false: all transition metals form cations with a single fixed charge. (False, they can form cations of various charges.)
Fill in the blank: a monatomic ion consists of ______ type(s) of atom. (One.)
Q: What happens to the charge of a neutral atom when it loses two electrons?
A: It becomes a cation with a 2+ charge, because it now has two more protons than electrons.
Q: Why do transition metals require Roman numerals in their compound names?
A: Because they can form cations of different charges, and the Roman numeral specifies which charge is present in a given compound.
Q: A neutral oxygen atom gains two electrons. What type of ion is formed, and what is its charge?
A: An anion with a 2- charge (O2-).
Q: What distinguishes a monatomic ion from a polyatomic ion?
A: A monatomic ion is formed from a single atom, while a polyatomic ion is a group of bonded atoms carrying a collective charge.
This connects to chemical bonding, because ionic bonds form when cations and anions attract each other. Understanding ion formation is also essential for writing correct chemical formulas and balancing equations. Later in the course, electron configuration will explain why certain atoms prefer to lose or gain electrons.
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