Difficulty: Introductory | Prerequisites: Atomic Structure, Ions, and Isotopes notes
Chemistry has a naming system: ionic compounds (metal + non-metal) use the metal name followed by the non-metal with an "-ide" ending, while covalent compounds (two non-metals) use Greek prefixes. Polyatomic ions like sulfate and nitrate have names you need to memorise. The periodic table is organised into groups that tell you an element's charge, classification, and behaviour.
Ionic compound
A compound formed by the electrostatic attraction between a metal cation and a non-metal anion (or a polyatomic ion). Example: NaCl, K₂S.
In simple terms: a metal hands electrons to a non-metal and the opposite charges hold them together.
Covalent (molecular) compound
A compound formed when two or more non-metals share electrons. Named using Greek prefixes (mono-, di-, tri-, tetra-, penta-, etc.). Example: P₂O₄ is diphosphorus tetroxide.
In simple terms: non-metals share rather than transfer electrons, and you count the atoms with prefixes.
Polyatomic ion
A group of covalently bonded atoms that together carry a net charge and act as a single unit in ionic compounds. You need to memorise the common ones.
Examples: sulfate (SO₄²⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), bicarbonate/hydrogen carbonate (HCO₃⁻).
Halogen
An element in Group VIIA (Group 17) of the periodic table: F, Cl, Br, I, At. Halogens form 1⁻ ions.
In simple terms: the column one step left of the noble gases. They need one electron to fill their outer shell.
Alkali metal
An element in Group IA (Group 1), excluding hydrogen: Li, Na, K, Rb, Cs, Fr. They form 1⁺ ions.
Alkaline earth metal
An element in Group IIA (Group 2): Be, Mg, Ca, Sr, Ba, Ra. They form 2⁺ ions.
Transition metal
An element in the d-block of the periodic table (Groups 3 through 12). Many transition metals can form ions with more than one charge, which is why Roman numerals appear in their names, e.g. iron(III) = Fe³⁺.
In simple terms: the big middle block of the periodic table. Note that Mg (magnesium) is not a transition metal; it sits in Group IIA.
Metal / Metalloid / Nonmetal
The periodic table divides elements into three broad classes. Metals (left and centre) conduct electricity and form cations. Non-metals (upper right) tend to form anions or share electrons. Metalloids (along the staircase line, e.g. Si, Ge, As) have properties intermediate between the two.
For carbon, sodium, and silicon: sodium is the metal, silicon is the metalloid, carbon is the nonmetal.
Identify the metal (cation) and non-metal (anion)
The cation name stays the same as the element. The anion name takes an "-ide" ending (chlorine becomes chloride, sulfur becomes sulfide, oxygen becomes oxide)
Balance charges so the compound is neutral overall
Potassium (K⁺) and sulfide (S²⁻): two K⁺ ions balance one S²⁻, giving K₂S
Aluminium (Al³⁺) and sulfur (S²⁻): cross the charges to get Al₂S₃
Iron(III) chloride: Fe³⁺ and Cl⁻, so three chlorides balance one iron, giving FeCl₃. The Roman numeral (III) tells you iron's charge in this compound
For metals in Groups IA and IIA, and aluminium, no Roman numeral is needed because their charge is predictable
Al₂O₃ is simply "aluminium oxide" (not "aluminium(III) oxide" because aluminium always carries a 3⁺ charge)
Ionic compounds form between a metal and a non-metal
Mg and F: magnesium is a metal, fluorine is a non-metal, so this is ionic
P and Cl, C and O, N and I are all non-metal pairs, so they form covalent (molecular) compounds, not ionic ones
Use Greek prefixes to indicate the number of each atom: mono (1, usually dropped for the first element), di (2), tri (3), tetra (4), penta (5), hexa (6)
The second element gets an "-ide" ending
P₂O₄ = diphosphorus tetroxide
The prefix "mono" is not used for the first element but is used for the second (e.g. carbon monoxide, CO)
Sulfate: SO₄²⁻
Sulfite: SO₃²⁻
Nitrate: NO₃⁻
Carbonate: CO₃²⁻
Bicarbonate (hydrogen carbonate): HCO₃⁻ (note: charge is 1⁻, not 2⁻)
The "-ate" ending generally has more oxygen atoms than the "-ite" ending
NaNO₃ is made of Na⁺ and NO₃⁻ (sodium and nitrate). You do not break the nitrate apart into separate N and O atoms
Na₂CO₃ is made of two Na⁺ ions and one CO₃²⁻ ion. Again, carbonate stays as one polyatomic unit
Group IA (alkali metals): form 1⁺ ions
Group IIA (alkaline earth metals): form 2⁺ ions
Group VIIA (halogens): form 1⁻ ions
Group VIIIA (noble gases): generally unreactive
Group VIA elements have 6 valence electrons
Transition metals sit in the d-block (Groups 3-12). Mg is in Group IIA, not a transition metal. Mn (manganese), Mo (molybdenum), and W (tungsten) are all transition metals
Metals: left side and centre of the table. Sodium is a metal
Metalloids: along the staircase line (B, Si, Ge, As, Sb, Te). Silicon is a metalloid
Nonmetals: upper right of the table. Carbon is a nonmetal
Metals tend to lose electrons and become positively charged (form cations)
They do not lose protons, and they do not gain electrons
Students often think all compounds with a metal use Greek prefixes. They do not. Greek prefixes are for covalent compounds (non-metal + non-metal) only. Ionic compounds never use prefixes.
A common mistake is writing potassium sulfide as KS instead of K₂S. Remember to balance charges: K is 1⁺, S is 2⁻, so you need two potassium ions.
Students frequently break polyatomic ions apart when identifying ions in a compound. NaNO₃ is Na⁺ and NO₃⁻, not Na⁺ + N³⁻ + O²⁻.
Bicarbonate (HCO₃⁻) has a 1⁻ charge, not 2⁻. This trips students up regularly.
⚠️ Formula-writing from names (and vice versa) is heavily tested. Practise going both directions: "potassium sulfide" to K₂S, and K₂S back to "potassium sulfide."
⚠️ Expect a question asking which combination of elements forms an ionic compound. The rule: metal + non-metal = ionic.
⚠️ Polyatomic ion names and formulas (especially sulfate vs sulfite vs sulfide, and carbonate vs bicarbonate) are tested directly.
⚠️ The periodic table group names (halogens = VIIA, alkaline earth metals = IIA) appear on the exam. Know which group forms which charge.
⚠️ The metal/metalloid/nonmetal classification question is a standard exam item. Remember silicon is a metalloid, not a nonmetal.
True or false: Al₂O₃ should be named "aluminium(III) oxide." (Answer: false. Aluminium only forms a 3⁺ ion, so no Roman numeral is needed. It is simply "aluminium oxide.")
Fill in the blank: the formula for iron(III) chloride is ___. (Answer: FeCl₃)
True or false: P and Cl form an ionic compound. (Answer: false. Both are non-metals, so they form a covalent compound.)
Fill in the blank: SO₄²⁻ is called the ___ ion. (Answer: sulfate)
True or false: the halogens are found in Group IIA. (Answer: false. Halogens are in Group VIIA.)
Q: What is the formula for potassium sulfide?
A: K₂S. Potassium forms K⁺ and sulfur forms S²⁻. Two K⁺ ions balance one S²⁻.
Q: What is the chemical formula of iron(III) chloride?
A: FeCl₃. Iron(III) means Fe³⁺. Three Cl⁻ ions balance the 3⁺ charge.
Q: Name the ion SO₄²⁻.
A: Sulfate ion. (Sulfite is SO₃²⁻, sulfide is S²⁻.)
Q: Aluminium and sulfur react to form an ionic compound. What is the formula?
A: Al₂S₃. Al is 3⁺, S is 2⁻. Cross the charges: 2 aluminium atoms and 3 sulfur atoms.
Q: Which combination of elements produces an ionic compound: P and Cl, C and O, N and I, or Mg and F?
A: Mg and F. Magnesium is a metal, fluorine is a non-metal. The other pairs are all non-metal combinations and form covalent compounds.
Q: Identify the formula for bicarbonate ion (hydrogen carbonate ion).
A: HCO₃⁻. Note the charge is 1⁻. CO₃²⁻ is plain carbonate.
Q: What are the ions that make up Na₂CO₃?
A: Two Na⁺ ions and one CO₃²⁻ ion. Carbonate is a polyatomic ion and stays together as a unit.
Q: What are the ions that make up NaNO₃?
A: Na⁺ and NO₃⁻. Nitrate is a polyatomic ion; do not break it into individual N and O atoms.
Q: Name the compound Al₂O₃.
A: Aluminium oxide. No Roman numeral needed because aluminium only forms one type of ion (3⁺).
Q: Write the formula for diphosphorus tetroxide.
A: P₂O₄. "Di" = 2 phosphorus, "tetra" = 4 oxygen. The "-oxide" ending confirms oxygen.
Q: Which of the following lists the correct classification for carbon, sodium, and silicon as metal, metalloid, nonmetal?
A: Na (metal), Si (metalloid), C (nonmetal). Sodium is an alkali metal, silicon sits on the metalloid staircase, carbon is a non-metal.
Q: To form an ion, metals tend to do what?
A: Lose electrons and become positively charged.
Naming compounds and writing formulas builds directly on your understanding of ion charges from atomic structure and electron configuration. Periodic trends (covered in the next set of notes) rely on knowing where elements sit in the table, which you practise here by identifying groups and classifications.
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