Atoms, Molecules & Quantitative Chemistry – Core Vocabulary

Atom

The smallest part of an element that retains the chemical properties of that element.

Element

A pure substance made of atoms that all have the same number of protons; cannot be broken down chemically into simpler substances.

Molecule

A particle formed when two or more atoms are covalently bonded together.

Compound

A substance containing atoms of two or more different elements chemically combined in fixed proportions.

Ion

An atom or group of atoms that carries an overall positive or negative charge.

Cation

A positively charged ion.

Anion

A negatively charged ion.

Monatomic

Describes an element or ion that exists as single, un-bonded atoms (e.g. He, Cl⁻).

Diatomic

Describes a substance whose molecules or ions contain exactly two atoms (e.g. O₂, OH⁻).

Polyatomic

Describes a molecule or ion that contains three or more atoms bonded together (e.g. SO₄²⁻, CH₄).

Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons.

Coefficient

The whole number written in front of a formula when balancing a chemical equation.

Spectator Ion

An ion that appears unchanged on both sides of an ionic equation and does not take part in the reaction.

Relative Atomic Mass (Aᵣ)

The weighted mean mass of an atom compared with 1⁄12 of the mass of a ¹²C atom; no units.

Relative Molecular Mass (Mᵣ)

The sum of the relative atomic masses of all atoms in a molecule; used for covalent substances.

Relative Formula Mass (Mᵣ)

The sum of the relative atomic masses of the species shown in a formula, used for ionic or giant structures.

Molar Mass (M)

The mass per mole of a substance, expressed in g mol⁻¹.

Avogadro Constant (L)

6.02 × 10²³ mol⁻¹, the number of particles in one mole of any substance.

Mole

The amount of substance that contains exactly Avogadro’s number of specified particles.

Empirical Formula

The simplest whole-number ratio of atoms of each element in a compound.

Molecular Formula

The actual number of atoms of each element in one molecule of a compound.

Molar Volume

The volume occupied by one mole of a gas; 24 dm³ mol⁻¹ (≈ 24 000 cm³ mol⁻¹) at room temperature and pressure.

Solute

The substance that is dissolved in a solvent to form a solution.

Solvent

The liquid that dissolves a solute to produce a solution (often water).

Solution

A homogeneous mixture formed when a solute is dissolved in a solvent.

Mass Concentration

The mass of solute per volume of solution, expressed in g dm⁻³.

Molar Concentration

The amount of solute per volume of solution, expressed in mol dm⁻³; often denoted by square brackets, e.g. [HCl].

Theoretical Yield

The maximum possible mass of product expected from a reaction, assuming complete conversion and no losses.

Actual Yield

The mass of product actually obtained from a reaction.

Percentage Yield

(Actual yield ÷ Theoretical yield) × 100 %; a measure of the efficiency of a reaction.

Atom Economy

(Molar mass of desired product ÷ Sum of molar masses of all products) × 100 %; indicates how many reactant atoms end up in the desired product.

Hydrate

A compound that contains water of crystallisation, shown in its formula, e.g. CuSO₄·5H₂O.

Displacement Reaction

A reaction in which a more reactive element displaces a less reactive element from one of its compounds.

Precipitation Reaction

A reaction in which mixing two solutions produces an insoluble solid (precipitate).

Neutralisation Reaction

A reaction in which H⁺ ions from an acid react with OH⁻ ions (or O²⁻, CO₃²⁻, HCO₃⁻ etc.) to form water (and often a salt).

Oxidation

Loss of electrons or increase in oxidation number during a reaction.

Reduction

Gain of electrons or decrease in oxidation number during a reaction.

Ionic Equation

An equation that shows only the species that actually change in a reaction, written as ions where appropriate.

I

What is the defining characteristic of an Atom in relation to its element?

The smallest part of an element that retains the chemical properties of that element.

What fundamental property distinguishes an Element as a pure substance?

A pure substance made of atoms that all have the same number of protons; cannot be broken down chemically into simpler substances.

How is a Molecule structurally formed from atoms?

A particle formed when two or more atoms are covalently bonded together.

What structural characteristic defines a Compound regarding its elemental composition?

A substance containing atoms of two or more different elements chemically combined in fixed proportions.

What causes an atom or group of atoms to become an Ion?

An atom or group of atoms that carries an overall positive or negative charge.

What type of charge characterizes a Cation?

A positively charged ion.

What type of charge characterizes an Anion?

A negatively charged ion.

In what structural form do Monatomic elements or ions exist?

Describes an element or ion that exists as single, un-bonded atoms (e.g. He, Cl⁻).

How many atoms are present in a Diatomic substance, and can you provide an example?

Describes a substance whose molecules or ions contain exactly two atoms (e.g. O₂, OH⁻).

What is the minimum number of atoms required for a molecule or ion to be classified as Polyatomic? Give an example.

Describes a molecule or ion that contains three or more atoms bonded together (e.g. SO₄²⁻, CH₄).

What distinct feature, concerning subatomic particles, differentiates Isotopes of the same element?

Atoms of the same element with the same number of protons but different numbers of neutrons.

What is the purpose of a Coefficient in the context of balancing a chemical equation?

The whole number written in front of a formula when balancing a chemical equation.

How can you identify a Spectator Ion in an ionic equation, and what is its role in the reaction?

An ion that appears unchanged on both sides of an ionic equation and does not take part in the reaction.

What is the reference standard used to determine Relative Atomic Mass (A_r), and why does it have no units?

The weighted mean mass of an atom compared with 1\/12 of the mass of a ^{12}C atom; no units.

For what type of substances is M_r referred to as Relative Molecular Mass, and how is it calculated?

The sum of the relative atomic masses of all atoms in a molecule; used for covalent substances.

When is M_r specifically termed Relative Formula Mass, and for what kinds of structures is it applicable?

The sum of the relative atomic masses of the species shown in a formula, used for ionic or giant structures.

How is Molar Mass (M) conceptually defined, and what are its standard units?

The mass per mole of a substance, expressed in g mol⁻¹.

What is the numerical value of the Avogadro Constant (L), and what does it represent in terms of particles?

6.02 \times 10^{23} mol⁻¹, the number of particles in one mole of any substance.

How does the concept of a Mole quantify the amount of a substance with respect to Avogadro’s number?

The amount of substance that contains exactly Avogadro’s number of specified particles.

What is the key difference in information conveyed by an Empirical Formula versus a Molecular Formula?

The simplest whole-number ratio of atoms of each element in a compound.

The actual number of atoms of each element in one molecule of a compound.

What is the approximate Molar Volume of any gas at room temperature and pressure?

The volume occupied by one mole of a gas; 24 dm³ mol⁻¹ (\approx 24 \text{ }000 \text{ cm}^3 \text{ mol}^{-1}) at room temperature and pressure.

In a solution, which component is the one that gets dissolved?

The substance that is dissolved in a solvent to form a solution.

Which component of a solution is responsible for dissolving the solute?

The liquid that dissolves a solute to produce a solution (often water).

How is a Solution characterized in terms of its composition and uniformity?

A homogeneous mixture formed when a solute is dissolved in a solvent.

How is Mass Concentration calculated, and what are its typical units?

The mass of solute per volume of solution, expressed in g dm⁻³.

What information does Molar Concentration provide about the amount of solute in a given volume of solution?

The amount of solute per volume of solution, expressed in mol dm⁻³; often denoted by square brackets, e.g. [HCl].

What crucial assumption is made when determining the Theoretical Yield of a reaction?

The maximum possible mass of product expected from a reaction, assuming complete conversion and no losses.

How does the Actual Yield of a product differ from its theoretical calculation?

The mass of product actually obtained from a reaction.

How is Percentage Yield calculated, and what does it indicate about the efficiency of a chemical reaction?

(Actual yield ÷ Theoretical yield) × 100 %; a measure of the efficiency of a reaction.

What does Atom Economy measure in a chemical reaction, and why is it important in sustainable chemistry?

(Molar mass of desired product ÷ Sum of molar masses of all products) × 100 %; indicates how many reactant atoms end up in the desired product.

How is the presence of water of crystallisation indicated in the chemical formula of a Hydrate?

A compound that contains water of crystallisation, shown in its formula, e.g. CuSO₄·5H₂O.

What must be true about the reactivity of elements for a Displacement Reaction to occur?

A reaction in which a more reactive element competes with a less reactive element for a compound.

What observable outcome signifies that a Precipitation Reaction has taken place when mixing solutions?

A reaction in which mixing two solutions produces an insoluble solid (precipitate).

What are the common products formed when an acid undergoes a Neutralisation Reaction with a base?

A reaction in which H⁺ ions from an acid react with OH⁻ ions (or O²⁻, CO₃²⁻, HCO₃⁻ etc.) to form water (and often a salt).

How does Oxidation affect the electron count and oxidation number of a species?

Loss of electrons or increase in oxidation number during a reaction.

How does Reduction affect the electron count and oxidation number of a species?

Gain of electrons or decrease in oxidation number during a reaction.

What is the primary objective of writing an Ionic Equation, and what does it exclude?

An equation that shows only the species that actually change in a reaction, written as ions where appropriate.