Difficulty: Introductory | Prerequisites: Basic understanding of atomic structure (protons, neutrons, electrons), familiarity with the periodic table layout.
This is the chapter where you learn the language chemists actually use to talk about compounds. Before you can write formulas, balance equations, or do stoichiometry, you need to know how substances are named and why. Chapter 6 covers the naming system (nomenclature) for inorganic compounds, those that generally do not contain carbon. If you skipped the periodic table or atomic structure material, go back to that first; you will need to know where elements sit on the table and what ions they form.
Chemists use two kinds of names: common names (arbitrary, everyday labels like "lime" or "laughing gas") and systematic names (which tell you the actual chemical composition). Ions are charged particles formed when atoms gain or lose electrons, and their charges follow predictable patterns based on position on the periodic table.
Chemical nomenclature
The system of names that chemists use to identify compounds. In simple terms, it is the "language" of chemistry for labelling substances.
Inorganic compounds
Compounds that do not generally contain carbon. Think of it as everything outside the carbon-based (organic) world: salts, metals, oxides, acids, and so on.
Common names
Arbitrary names for chemical substances that are not based on chemical composition. In simple terms, these are nicknames, like "quicksilver" for mercury or "laughing gas" for nitrous oxide (N₂O). They are convenient but tell you nothing about what the substance is made of.
Systematic names
Names that precisely identify the chemical composition of a compound. In simple terms, if you know the systematic name, you can write the formula, and vice versa.
Diatomic molecules
Molecules made of exactly two atoms bonded together. There are seven elements that naturally exist this way at room temperature: H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂.
Polyatomic elements
Elements whose natural form consists of more than two atoms bonded together. Sulfur (S₈), phosphorus (P₄), and selenium (Se₈) are the key examples.
Ion
A charged particle produced by adding or removing one or more electrons from a neutral atom. Think of it as an atom that is no longer electrically balanced.
Cation
A positive ion, formed when a neutral atom loses one or more electrons. Named the same as the parent atom (e.g., K⁺ is "potassium ion"). In simple terms, losing negative charges leaves you with a net positive charge.
Anion
A negative ion, formed when a neutral atom gains one or more electrons. Named by taking the stem of the parent element's name and adding the suffix "-ide" (e.g., Cl⁻ is "chloride ion").
Common names are traditional labels with no connection to chemical makeup.
"Lime" = calcium oxide (CaO)
"Hypo" = sodium thiosulfate (Na₂S₂O₃)
"Laughing gas" = nitrous oxide (N₂O)
"Quicksilver" = mercury (Hg)
Common names persist because systematic names can be long and unwieldy, but they have clear limitations: they do not tell you the composition, and the same substance may have several common names.
Systematic names eliminate ambiguity. If you know the rules, you can go from name to formula and from formula to name with no guesswork.
Common Name | Formula | Chemical Name |
|---|---|---|
Acetylene | C₂H₂ | ethyne |
Lime | CaO | calcium oxide |
Slaked lime | Ca(OH)₂ | calcium hydroxide |
Water | H₂O | water |
Galena | PbS | lead(II) sulfide |
Alumina | Al₂O₃ | aluminum oxide |
Baking soda | NaHCO₃ | sodium hydrogen carbonate |
Brimstone | S | sulfur |
Table salt | NaCl | sodium chloride |
Vinegar | HC₂H₃O₂ | acetic acid |
Grain alcohol | C₂H₅OH | ethanol, ethyl alcohol |
Muriatic acid | HCl | hydrochloric acid |
Quicksilver | Hg | mercury |
Epsom salts | MgSO₄·7 H₂O | magnesium sulfate heptahydrate |
Washing soda | Na₂CO₃·10 H₂O | sodium carbonate decahydrate |
Milk of magnesia | Mg(OH)₂ | magnesium hydroxide |
You do not need to memorise this entire table, but recognising the most common entries (water, table salt, baking soda, lime, vinegar) is useful and comes up in exams.
All atoms are composed of protons, electrons, and neutrons.
An element is defined by its number of protons (atomic number).
Atoms are electrically neutral: equal numbers of protons and electrons.
For most elements, the formula is simply the element's symbol (e.g., Fe, Na, Cu).
A small number of elements exist as multi-atom units at room temperature:
The seven diatomic molecules (must memorise):
H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂
A helpful mnemonic: the "seven up" pattern on the periodic table, or simply "Have No Fear Of Ice Cold Beer" (Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine).
Polyatomic elements:
Sulfur: S₈
Phosphorus: P₄
Selenium: Se₈
How ions form:
Ions are produced when atoms gain or lose electrons. This happens most often when metals combine with nonmetals.
Example: a potassium atom has 19 protons and 19 electrons. Remove one electron and you get K⁺, with 19 protons and 18 electrons, giving a net +1 charge.
Cations (positive ions):
Formed when atoms lose electrons.
Named the same as the parent element.
K → K⁺ (potassium ion)
Mg → Mg²⁺ (magnesium ion)
Al → Al³⁺ (aluminum ion)
Anions (negative ions):
Formed when atoms gain electrons.
Named differently from cations: take the stem of the element name and add "-ide."
F → F⁻ (fluoride ion)
Cl → Cl⁻ (chloride ion)
Br → Br⁻ (bromide ion)
I → I⁻ (iodide ion)
O → O²⁻ (oxide ion)
N → N³⁻ (nitride ion)
S → S²⁻ (sulfide ion)
Stems for common anion-forming elements:
Element | Stem | Anion Name |
|---|---|---|
Bromine | brom | bromide |
Chlorine | chlor | chloride |
Fluorine | fluor | fluoride |
Hydrogen | hydr | hydride |
Iodine | iod | iodide |
Nitrogen | nitr | nitride |
Oxygen | ox | oxide |
Phosphorus | phosph | phosphide |
Sulfur | sulf | sulfide |
Carbon | carb | carbide |
Boron | bor | boride |
Nontransition metals (Groups 1A, 2A, 3A): the charge on the cation equals the group number.
Group 1A metals form +1 ions (Na⁺, K⁺, Li⁺)
Group 2A metals form +2 ions (Mg²⁺, Ca²⁺, Ba²⁺)
Aluminum (Group 3A) forms +3 ions (Al³⁺)
Nonmetals (Groups 5A, 6A, 7A): the charge on the anion equals the group number minus 8.
Group 7A nonmetals form −1 ions (F⁻, Cl⁻, Br⁻, I⁻)
Group 6A nonmetals form −2 ions (O²⁻, S²⁻)
Group 5A nonmetals form −3 ions (N³⁻, P³⁻)
Transition metals: many can form more than one type of cation (covered in Part 2).
Common names survive in industry and everyday life. A builder buys "lime" (CaO) and "slaked lime" (Ca(OH)₂) without thinking about formulas. A photographer once needed "hypo" (sodium thiosulfate) to fix prints. Learning both naming systems lets you move between the lab bench and the real world.
Students often confuse atoms and ions. A neutral potassium atom (K) is not the same as a potassium ion (K⁺). The ion has one fewer electron.
Students sometimes think that losing electrons makes an atom negative (because you "lost" something). The opposite is true: losing negative electrons leaves a positive charge.
Anion naming trips people up. It is not "oxygen ion," it is "oxide ion." Always apply the stem + "-ide" rule.
The seven diatomic elements are frequently forgotten on exams. Hydrogen, oxygen, and nitrogen exist as H₂, O₂, and N₂ in their natural state, not as single atoms.
⚠️ You will almost certainly be asked to name ions given a symbol, or write the symbol given the name. Know your stems cold.
⚠️ Predicting ion charge from position on the periodic table is a recurring exam question. Group number → cation charge for metals; group number minus 8 → anion charge for nonmetals.
⚠️ Diatomic elements appear in balancing equations and stoichiometry problems later. If you write O instead of O₂, every calculation downstream is wrong.
⚠️ The difference between common and systematic names is a low-effort, high-reward exam topic: easy marks if you have reviewed the table.
True or False: A cation is formed when an atom gains electrons.
Fill in the blank: The anion formed from sulfur is called __________.
True or False: The formula for elemental nitrogen at room temperature is N.
Fill in the blank: Group 2A metals form ions with a charge of __________.
True or False: "Lime" is the systematic name for CaO.
Answers: 1. False (cations form by losing electrons). 2. Sulfide (S²⁻). 3. False (it is N₂). 4. +2. 5. False ("lime" is the common name; the systematic name is calcium oxide).
Q: What is the difference between a common name and a systematic name?
A: A common name is an arbitrary, traditional label (e.g., "laughing gas") that does not reveal chemical composition. A systematic name precisely identifies the elements and their ratios in the compound (e.g., "dinitrogen monoxide" for N₂O).
Q: Write the symbol and name for the ion formed by each: (a) potassium, (b) oxygen, (c) aluminum.
A: (a) K⁺, potassium ion. (b) O²⁻, oxide ion. (c) Al³⁺, aluminum ion.
Q: Name the seven diatomic elements.
A: Hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂).
Q: A nonmetal in Group 6A forms an ion. What is its charge, and why?
A: The charge is −2. Group number (6) minus 8 = −2. The atom gains two electrons to achieve a stable electron configuration.
Q: Give the stem and anion name for phosphorus.
A: Stem: phosph. Anion name: phosphide (P³⁻).
This material connects directly to Sections 6.3 and 6.4 (covered in Part 2), where you use ion charges to write formulas and name binary compounds. It also underpins later work in stoichiometry, where correct formulas are essential for mole calculations. If you go on to study electrochemistry, the concept of cations and anions moving in solution is central to how batteries and electrolytic cells work.
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