Atoms and Molecules

The One Thing

Atoms combine in fixed whole-number ratios to form molecules and compounds. Atomic mass, molecular mass, and the mole concept provide the framework for comparing particles and calculating their quantities while obeying conservation of atoms and mass.

Reactions, Processes and Experiments

Key Terms

• Atom: The smallest particle of an element that takes part in a chemical reaction and retains the chemical properties of that element.
• Molecule: The smallest independent particle of a substance that can exist freely and shows all the properties of that substance.
• Element: A pure substance made up of only one kind of atom, such as hydrogen, oxygen, or iron.
• Compound: A pure substance formed when atoms of two or more different elements combine chemically in a fixed ratio.
• Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
• Law of Constant Proportions: A pure chemical compound always contains the same elements combined in the same fixed proportion by mass.
• Dalton's Atomic Theory: Matter is made of atoms; atoms of the same element are similar in chemical properties; atoms combine in whole-number ratios; and chemical reactions involve rearrangement of atoms.
• Atomic Mass: The relative mass of an atom compared with one-twelfth of the mass of a carbon-12 atom, expressed in atomic mass units (u).
• Atomic Mass Unit: One atomic mass unit, written as 1 u, is equal to one-twelfth of the mass of one carbon-12 atom.
• Molecular Mass: The sum of the atomic masses of all atoms present in one molecule, expressed in u.
• Formula Unit: The simplest whole-number ratio of ions in an ionic compound, such as NaCl or CaCl2.
• Mole: The amount of substance containing 6.022 x 10^23 particles, such as atoms, molecules, or ions.
• Avogadro Constant: The number of particles in one mole of a substance: 6.022 x 10^23 mol^-1.
• Molar Mass: The mass of one mole of a substance, expressed in grams per mole (g mol^-1); its numerical value is equal to the atomic, molecular, or formula-unit mass in u.
• Chemical Formula: A symbolic representation showing which elements are present in a substance and the number or ratio of their atoms or ions.
• Valency: The combining capacity of an atom, usually determined by the number of electrons lost, gained, or shared during bonding.

Easily Confused

• Atom and molecule: An atom is the smallest particle of an element that participates in reactions, whereas a molecule is the smallest independent particle of a substance that can exist freely.
• Element and compound: An element contains only one kind of atom, whereas a compound contains atoms of two or more different elements chemically combined in a fixed ratio.
• Molecular mass and formula-unit mass: Molecular mass applies to molecules, whereas formula-unit mass applies to the simplest ionic ratio, such as NaCl or CaCl2.
• Subscript and coefficient: A subscript shows the number of atoms or ions within one molecule or formula unit, whereas a coefficient shows the number of molecules or formula units.
• Atomic mass unit and molar mass: Atomic mass unit measures the relative mass of a particle in u, whereas molar mass is the mass of one mole in g mol^-1.
• Law of Conservation of Mass and Law of Constant Proportions: Conservation of mass concerns equality of total reactant and product masses in a reaction, whereas constant proportions concerns the fixed mass ratio of elements within a pure compound.
• Number of moles and number of particles: Number of moles measures amount in units of mol, whereas number of particles gives the corresponding count of atoms, molecules, or ions.

What Gets Asked

• State or apply the laws of chemical combination: Use Total mass of reactants = Total mass of products for the Law of Conservation of Mass, and remember that hydrogen and oxygen in water have the fixed mass ratio 1:8 for the Law of Constant Proportions. The common error is confusing conservation of total mass with the fixed composition of one compound.
• Interpret molecular formulae: Identify that H2O contains two hydrogen atoms and one oxygen atom, CO2 contains one carbon atom and two oxygen atoms, and NH3 represents a simple whole-number ratio. The common error is overlooking subscripts.
• Calculate molecular or formula-unit mass: Use Molecular mass = Sum of the atomic masses of all atoms in the molecule; for H2O, (2 x 1) + 16 = 18 u, and for CO2, 12 + (2 x 16) = 44 u. For NaCl, calculate formula-unit mass as 23 + 35.5 = 58.5 u. The common error is failing to multiply an atomic mass by its subscript.
• Calculate quantities using the mole concept: Apply Number of moles = Given mass / Molar mass, Number of particles = Number of moles x 6.022 x 10^23, Number of moles = Number of particles / 6.022 x 10^23, and Mass of a substance = Number of moles x Molar mass. The common error is using the wrong relationship or confusing particles with moles.
• Write chemical formulae using valency: Ensure that the total positive charge and total negative charge balance, and that the symbols and formulae obey the valencies of the elements or ions involved. The common error is writing subscripts that do not produce charge balance.
• Distinguish symbols, formulae, subscripts, and coefficients: Recognise symbols such as H, O, Na, and Cl; use formulae to show composition; use subscripts for atoms or ions within a unit; and use coefficients for numbers of molecules or formula units. The common error is treating a coefficient as part of the composition of a single molecule.