ATOMS AND MOLECULES
Frequently Asked Questions
An atom is the smallest particle of an element that retains its chemical properties.
A molecule is a group of two or more atoms chemically bonded together.
Dalton’s theory states matter is made of indivisible atoms, identical for each element, combining in fixed ratios.
A chemical formula represents a compound using symbols and shows elements’ proportions.
Atomic mass unit is a standard mass unit equal to 1/12 the mass of one carbon-12 atom.
Atomicity is the number of atoms in a molecule of an element or compound.
A mole is the amount of substance containing Avogadro’s number of particles (6.022 × 10²³).
Mass is neither created nor destroyed during a chemical reaction.
Elements combine in a fixed mass ratio to form a compound.
It is the ratio of the average atomic mass of an element to 1/12 of a carbon-12 atom’s mass.
Avogadro’s number is 6.022 × 10²³, the number of particles in one mole of substance.
An ion is an atom or molecule with a net electric charge due to electron gain or loss.
Atom is a single particle; molecule consists of two or more atoms bonded together.
The symbol for sodium is Na.
Polyatomic ions are ions formed by a group of atoms with an overall charge.
The chemical formula of water is H2O.
The subscript shows the number of atoms of that element in the molecule.
Diatomic molecules are molecules made up of two atoms, e.g., O2, N2, H2.
An empirical formula shows the simplest whole-number ratio of elements in a compound.
Monatomic: single atom (e.g., He); diatomic: two atoms (e.g., O2).
Indivisible atoms, identical mass/size for an element, compounds in fixed ratios, atoms cannot be created/destroyed.
It is the mass of one mole of a molecule in grams.
Law of conservation of mass, law of constant proportions, law of multiple proportions.
Molecular mass is the sum of the atomic masses of all atoms in a molecule.
It is the sum of atomic masses of ions in one formula unit of an ionic compound.
If two elements form more than one compound, mass ratios of elements with fixed mass are simple whole numbers.
Valency is the combining capacity of an atom to form molecules.
Cation: positively charged ion; Anion: negatively charged ion.
A chemical reaction involves the transformation of reactants into products with new properties.
Example: Ozone (O3), Sulphur trioxide (SO3).
The chemical formula for ammonia is NH3.
Stoichiometry is the calculation of reactants and products in chemical reactions.
Because hydrogen exists as a diatomic molecule, H2, in nature.
Molecular mass is for molecules; formula mass is for ionic compounds.
Number of moles = given mass (g) ÷ molar mass (g/mol).
Cross-multiply the valencies, write them as subscripts, and reduce to lowest terms.
Isotopes: same element, different masses; isobars: different elements, same mass number.
