Ions
Ions
An ion is an atom or group ofatoms that carries an electrical charge because its number of electrons does not equal its number of protons. Atoms are neutral when proton count equals electron count. When electrons are gained or lost, the atom becomes charged —and charged atoms are called ions.
Cations and Anions
Ions fall into two categories based on their charge:
Cations (positive ions):
Cations that are formed when an atom loses one or more electrons. Since protons outnumber electrons, the net charge is positive. Metals almost always form cations. For example, sodium (Na, 11 protons) loses 1 electron to become Na⁺ (11 protons, 10 electrons).
Anions (negative ions):
Anions are formed when an atom gains one or more electrons. Since electrons outnumber protons, the net charge is negative. Non-metals almost always form anions. For example, chlorine (Cl, 17 protons) gains 1 electron to become Cl⁻ (17 protons, 18 electrons).
Predicting Ion Charges from the Periodic Table
The periodic table is your best tool for predicting what charge an ion will carry. Elements in the same group(column) have the same number of valence electrons and form ions of the same charge. The charge an element forms is driven by the tendency to achieve a full outer electron shell — the stable electron configuration of the nearest noble gas.
Table 1. Typical ion charges by group. Transition metals are the main exception to predictable charges.
Group | Ion Charge | Elements | Example |
|---|---|---|---|
1 (Alkali metals) | +1 | Li, Na, K, Rb, Cs | Na → Na⁺ (loses 1e⁻) |
2 (Alkaline earth metals) | +2 | Be, Mg, Ca, Sr, Ba | Ca → Ca²⁺ (loses 2e⁻) |
13 | +3 | Al, Ga | Al → Al³⁺ (loses 3e⁻) |
15 (Pnictogens) | −3 | N, P, As | N → N³⁻ (gains 3e⁻) |
16 (Chalcogens) | −2 | O, S, Se, Te | S → S²⁻ (gains 2e⁻) |
17 (Halogens) | −1 | F, Cl, Br, I | Cl → Cl⁻ (gains 1e⁻) |
18 (Noble gases) | 0 (no ion) | He, Ne, Ar… | Do not form ions under normal conditions |
3–12 (Transition metals) | Variable (+1, +2, +3…) | Fe, Cu, Mn, Cr… | Fe → Fe²⁺ or Fe³⁺ |
Figure 1: Ion Charge by Group in the Periodic Table
The periodic table highlights ion charges by group, showing common charges for elements in the same group, with metals on the left forming positive ions and nonmetals on the right forming negative ions.
Transition Metals: Variable Charges
Unlike main-group metals, most transition metals can form more than one stable ion. This is because they have electrons in d orbitals (covered later this semester) that can be removed at relatively similar energy costs. For example, iron (Fe) commonly forms both Fe²⁺ (ferrous) and Fe³⁺ (ferric); copper (Cu) forms both Cu⁺ (cuprous) and Cu²⁺ (cupric). The charge on a transition metal ion in a compound can be determined by the charges of the other ions present, which is covered in the Ionic Nomenclature lecture.
Ionic Compounds vs. Covalent Compounds
Knowing where elements sit in the periodic table allows you to predict what type of compound they will form when they combine:
Metal + non-metal → ionic compound: The metal loses electrons (forming a cation) and the non-metal gains them (forming an anion). The oppositely charged ions attract each other electrostatically to form an ionic compound. Example: Na + Cl → NaCl (sodium chloride).
Non-metal + non-metal → covalent (molecular)compound: Both atoms want to gain electrons, so instead of transferring electrons they share them. This sharing of electrons forms a covalent bond. Example: H₂O, CO₂, NH₃.
This distinction is critical because ionic and covalent compounds have completely different naming rules (nomenclature). Before you can name a compound, you must first identify whether it is ionic or covalent. The presence of a metal is your primary clue that a compound is ionic.
Polyatomic Ions
So far we have discussed monoatomic ions — single atoms that carry a charge. There is an important second category called polyatomic ions: groups of two or more atoms covalently bonded together that carry an overall charge as a unit. The atoms within the polyatomic ion are held together by covalent bonds, but the ion as a whole carries a charge because the group has gained or lost electrons relative to the total proton count of all atoms in the group.
Polyatomic ions behave exactly like monoatomic ions when forming compounds — they combine in ratios that produce electrical neutrality. The key difference in writing formulas is that when more than one of the same polyatomic ion is needed, the entire ion must been closed in parentheses before the subscript is written. The subscript appliesto the whole unit, not just the last element.
The most important polyatomic ions — their names, formulas, and charges — must be memorized. They appear constantly in chemistry and there is no shortcut for learning them. Notice that all common polyatomic ions are negatively charged except for ammonium (NH₄⁺),which is positively charged. This makes ammonium the only polyatomic cation you will regularly encounter.
Figure 2: The Common Polyatomic Ions You Must Know
A comprehensive chart of common polyatomic ions with their names, formulas, charges, and memory aids, designed to help students memorize essential chemistry concepts efficiently.
Summary
Ions are charged particles formed when atoms gain or lose electrons. Positively charged ions are called cations and typically form when metals lose electrons. Negatively charged ions are called anions and typically form when nonmetals gain electrons. The periodic table provides a powerful tool for predicting common ion charges because elements in the same group tend to form ions with similar charges.
Ions combine to form ionic compounds through electrostatic attraction, while nonmetals typically form covalent compounds by sharing electrons. In addition to monoatomic ions, many important charged species are polyatomic ions, groups of covalently bonded atoms that carry an overall charge. Understanding ions is essential for predicting chemical formulas, naming compounds, and understanding chemical reactions.
Key Points
Ions are charged atoms or groups of atoms.
Cations are positively charged and form by losing electrons.
Anions are negatively charged and form by gaining electrons.
Metals typically form cations.
Nonmetals typically form anions.
Group 1 forms +1 ions.
Group 2 forms +2 ions.
Group 13 commonly forms +3 ions.
Group 15 commonly forms −3 ions.
Group 16 commonly forms −2 ions.
Group 17 commonly forms −1 ions.
Noble gases generally do not form ions.
Transition metals often form multiple ion charges.
Metal + nonmetal usually forms an ionic compound.
Nonmetal + nonmetal usually forms a covalent compound.
Polyatomic ions are charged groups of covalently bonded atoms.
Ammonium (NH₄⁺) is the most important polyatomic cation.
Common polyatomic ions must be memorized.
Parentheses are required when multiple polyatomic ions appear in a formula.