Difficulty: Introductory | Prerequisites: Atomic Structure and the Periodic Table notes
This covers how to write correct chemical formulas for ionic compounds, how to name molecular (covalent) compounds using prefixes, and how ionic size relates to charge and electron configuration. If you understand ion charges and the periodic table groups, this material follows naturally.
Ionic compound formulas are built by balancing cation and anion charges so the overall charge is zero. Molecular (covalent) compounds are named with Greek prefixes that tell you how many atoms of each element are present, and the second element always ends in "-ide." Ionic radius follows a clear pattern: anions are larger than their parent atoms, cations are smaller, and among isoelectronic species the one with fewer protons is the largest.
Ionic compound
A compound formed by the electrostatic attraction between positively charged cations and negatively charged anions. Think of it as: a metal hands electrons to a nonmetal, and the opposite charges hold them together.
Cation
A positively charged ion, formed when an atom loses one or more electrons. In simple terms, fewer electrons than protons means a net positive charge.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. More electrons than protons gives a net negative charge.
Molecular (covalent) compound
A compound formed by nonmetals sharing electrons rather than transferring them. Named using Greek prefixes (mono-, di-, tri-, etc.) to indicate atom counts.
Formula unit
The simplest whole-number ratio of ions in an ionic compound. Think of it as the ionic equivalent of a molecular formula.
Polyatomic ion
An ion made of two or more atoms bonded together that carries an overall charge. For example, NH₄⁺ (ammonium) or CO₃²⁻ (carbonate).
Ionic radius
The effective size of an ion in a crystal lattice. Cations are smaller than their parent atoms; anions are larger.
Charge neutrality
The principle that the total positive charge must equal the total negative charge in an ionic compound's formula. This is why you cross-multiply charges to find the subscripts.
Identify the cation (metal or polyatomic ion) and anion (nonmetal or polyatomic ion).
Balance charges so the compound is electrically neutral.
Magnesium chloride: Mg²⁺ needs two Cl⁻ ions to balance, giving MgCl₂.
Cesium sulfide: Cs⁺ and S²⁻ combine as Cs₂S (two Cs⁺ ions balance one S²⁻).
Ammonium carbonate: NH₄⁺ and CO₃²⁻ combine as (NH₄)₂CO₃.
The cation is always written first in the formula.
When a polyatomic ion appears more than once, enclose it in parentheses before adding the subscript.
Molecular compounds consist of nonmetals sharing electrons.
Name using Greek prefixes to indicate atom counts:
1 = mono- (often omitted for the first element)
2 = di-
3 = tri-
4 = tetra-
5 = penta-
6 = hexa-
7 = hepta-
8 = octa-
9 = nona-
10 = deca-
The second element always ends in "-ide."
N₂O₅ = dinitrogen pentoxide ("di-" for 2 nitrogen, "penta-" for 5 oxygen).
NCl₃ = nitrogen trichloride (no prefix on nitrogen because there is only one, "tri-" for 3 chlorine).
Anions are larger than their parent atoms. Gaining electrons increases electron-electron repulsion, which pushes the electron cloud outward.
Cations are smaller than their parent atoms. Losing electrons reduces repulsion and lets the nucleus pull the remaining electrons closer.
Among isoelectronic species (same number of electrons), the ion with fewer protons is the largest because there is less nuclear pull per electron.
Isoelectronic series example, all with 18 electrons: S²⁻ > Cl⁻ > K⁺ > Ca²⁺.
S²⁻ has 16 protons pulling on 18 electrons (least pull per electron, so largest).
Ca²⁺ has 20 protons pulling on 18 electrons (most pull per electron, so smallest).
This pattern shows up consistently: more protons with the same electron count means a smaller radius.
Charge-balancing method for ionic formulas
Write the charges of the cation and anion. Cross the absolute values as subscripts (then simplify to the lowest ratio).
Example: Al³⁺ and O²⁻ → Al₂O₃ (the 3 becomes oxygen's subscript, the 2 becomes aluminium's).
Naming prefix table for molecular compounds
Number of atoms | Prefix |
|---|---|
1 | mono- |
2 | di- |
3 | tri- |
4 | tetra- |
5 | penta- |
6 | hexa- |
7 | hepta- |
8 | octa- |
9 | nona- |
10 | deca- |
Ionic compounds are everywhere: table salt (NaCl), limestone (CaCO₃), and the magnesium chloride used to de-ice roads in winter. The naming system for molecular compounds is how chemists unambiguously communicate formulas, which matters when, for example, dinitrogen monoxide (N₂O, laughing gas) and nitrogen dioxide (NO₂, a toxic pollutant) contain the same two elements but behave entirely differently.
Students often think ionic compound formulas show the number of molecules. They do not. The formula MgCl₂ is a ratio (one Mg²⁺ for every two Cl⁻), not a discrete molecule.
Students sometimes apply Greek prefixes to ionic compound names. Prefixes are for molecular (covalent) compounds only. You say "magnesium chloride," not "magnesium dichloride."
Students often assume all anions are larger than all cations. This holds within an isoelectronic series, but a cation from period 5 (like Rb⁺) can be larger than an anion from period 2.
Students sometimes forget to enclose polyatomic ions in parentheses when the subscript is greater than 1. (NH₄)₂CO₃ is correct; NH₄₂CO₃ is not.
⚠️ Writing ionic formulas from ion charges is tested heavily. Practise the charge-balancing (cross-multiplication) method until it is automatic.
⚠️ Naming molecular compounds with the correct prefix is a common question. Remember: "mono-" is dropped from the first element, never from the second.
⚠️ Ordering ions by size (isoelectronic series) appears regularly. The key: same electron count, more protons = smaller ion.
⚠️ Know the difference between ionic naming and covalent naming. Ionic: metal name + nonmetal root + "-ide" (no prefixes). Covalent: prefix + element name + prefix + element root + "-ide."
Fill in the blank: The formula for calcium fluoride is ______. (CaF₂.)
True or false: N₂O₅ is called "nitrogen pentoxide." (False: it is dinitrogen pentoxide.)
Fill in the blank: Among S²⁻, Cl⁻, K⁺, and Ca²⁺, the largest ion is ______. (S²⁻.)
True or false: You should use Greek prefixes when naming ionic compounds. (False: prefixes are for molecular/covalent compounds only.)
Fill in the blank: Polyatomic ions with subscripts greater than 1 must be enclosed in ______. (Parentheses.)
Q: Write the formula for ammonium carbonate.
A: (NH₄)₂CO₃. Two ammonium ions (NH₄⁺, each 1+) balance one carbonate ion (CO₃²⁻, 2–).
Q: Name the compound N₂O₅.
A: Dinitrogen pentoxide.
Q: Name the compound NCl₃.
A: Nitrogen trichloride. ("Mono-" is omitted for the first element.)
Q: Arrange the following ions from largest to smallest: S²⁻, Cl⁻, K⁺, Ca²⁺.
A: S²⁻ > Cl⁻ > K⁺ > Ca²⁺. All have 18 electrons, so the one with fewest protons (S, 16) is largest and the one with most protons (Ca, 20) is smallest.
Q: Why is Rb⁺ larger than Al³⁺?
A: Rb⁺ is in period 5 with a much larger principal quantum number than Al³⁺ (period 3). Even after losing an electron, rubidium's electron cloud extends over more shells.
Q: Write the formula for cesium sulfide.
A: Cs₂S. Two Cs⁺ ions (each 1+) balance one S²⁻ ion (2–).
This connects directly to atomic structure: you need to know how many valence electrons an element has (and therefore what charge its ion will carry) before you can write any ionic formula. It also connects to electronegativity and bonding: the difference in electronegativity between two elements determines whether the bond is ionic or covalent, which in turn determines whether you use ionic naming rules or prefix-based naming rules.
ionic compounds, covalent compounds, molecular compounds, naming compounds, chemical nomenclature, Greek prefixes, dinitrogen pentoxide, nitrogen trichloride, formula unit, charge balancing, cross-multiplication method, polyatomic ions, ammonium, carbonate, ionic radius, isoelectronic series, cation size, anion size, ionic size trends, MgCl₂, Cs₂S, CHEM 101, general chemistry