Section 1a

BASIC DEFINITIONS AND VOCABULARY ON STRUCTURE OF MATTER

Chemistry: the study of matter, its composition and properties, and the changes it undergoes.

Matter: anything that has rest mass.

The basic unit of common matter from the viewpoint of chemistry is the atom.

To develop this topic we need to define the atom, the basic unit of common matter.

Atom: a neutral particle consisting of a nucleus containing most of its mass, and electrons occupying most of its volume.

Electron: subatomic particle with a charge of –1.

Nucleus: consists of two types of subatomic particles; protons, each with an electric charge of +1, and neutrons which have no charge.

Atomic number. Symbol Z, the number of protons in the atom.

Mass number: symbol A, the sum of the number of protons and neutrons in the atom. (Nucleon is a term for either a proton or neutron. Thus A is the number of nucleons in an atom and the number of neutrons is A – Z.)

Element: a substance composed of atoms all of which have the same atomic number , i.e. the same number of protons in the nucleus, and thus the same number of electrons.

The atomic number defines the element.

Each element has a name (of varying historic origin) and shorthand symbol.

The periodic table lists the elements in order of increasing atomic number.

We often speak of elements as
metals,
non-metals
metalloids (semimetals)

Isotopes: Atoms of the same element (same atomic number) but with different numbers of neutrons and hence a different mass number. Some isotopes are unstable and emit particles or radiation over a period of time and are called radioactive

Radioactivity: The spontaneous breakdown of one type of nucleus into another.

Relative atomic mass: Symbol Ar, the mean mass of an atom of an element allowing for the relative abundance of its naturally occurring isotopes relative to (divided by) 1/12th of the mass of one atom of the isotope 12C. (This topic is covered in Section 8.)

The chemical properties (behaviour) of the different elements can be understood in terms of the number and arrangement of the electrons in their atoms. The arrangement of the elements in the periodic table is based on this electronic structure . This topic is covered in Section 3.

Molecule: a discrete neutral particle consisting of a definite number of atoms bonded (held) together by chemical bonds. (Chemical bonds is the subject of Section 4.)

Substance: a single, pure type of matter.
examples: water, sucrose (common sugar), sodium chloride (common salt), argon (gas in many light bulbs)

A pure substance may be an element
examples oxygen, aluminium
or a compound
examples: water, sodium chloride.

A pure elemental substance may consist of

  • individual atoms,
    examples He, Ne, Ar;
  • molecules,
    examples nitrogen, N2; oxygen, O2; sulfur, S8 where the number of atoms in the molecule is given by a subscript number following the symbol for the element;
  • large aggregates of atoms
    examples all metals, carbon in the form of diamond or graphite.

Allotrope: Alternative form of an element differing in the way the atoms are linked
examples: carbon - diamond and graphite; oxygen - O2 (dioxygen) and O3 (ozone).
Each form is an allotrope.

Compound: a pure substance consisting of atoms, or ions of two or more elements in a definite number ratio.
It may consist of

  1. discrete molecules
  2. large aggregates of atoms
  3. ions.

Ion: a charged particle.
A cation is a positively charged particle, one containing fewer electrons than protons.
An anion is a negatively charged particle, one containing more electrons than protons.

An ion may be

  • monatomic, i.e. just one atom,
  • polyatomic, i.e. a discrete number of atoms jointed together,
    examples: sulfate, SO42− nitrate, NO3¯; ammonium, NH4+
  • polymeric, i.e. large aggregate of atoms joined together,
    example: silicate (SiO32−)x. The overall charge is given as a superscript after the formula as shown.

Ionic compound: a compound consisting of ions. The total charge on the cations equals the total charge on the anions, as substances are neutral. An ionic compound is an aggregate of those ions.

Matter exists normally in three states:
gas (g),
liquid (l) or
solid (s).

Phase: a particular state of matter.
Examples: gas, liquid, solid

Crystalline solid: A solid in which there is an orderly array of its constituents (atoms, molecules or ions). A crystalline solid may exist as a single crystal or as numerous crystals.

Mixture: a type of matter consisting of more than one substance and which can be separated into its components by making use of different physical properties of the substances present. Mixtures may be homogeneous or heterogeneous.

Homogeneous mixture: a mixture in which any two samples have the same composition and properties.
examples: air (a gas); seawater (a liquid).

Heterogeneous mixture: a mixture containing entities of sufficient size such that all samples are not identical,
examples: concrete; smoke.

Solution: a homogeneous mixture (usually with one component in larger amount); normally a liquid,
examples: seawater; hydrochloric acid,
but the term may be also apply to a solid, but not to a gas.

Solvent: the component of a solution in largest amount.
example: the water of seawater.

Solute: the components of a solution in lesser amounts.
examples: the salts in seawater.

Aqueous solution: a solution with water as the solvent.

Crystallization: The process of a solute coming out of solution as crystals.

Colloid: A mixture consisting of a finely divided phase (the dispersed phase) distributed uniformally in a continuous phase (the dispersion medium). Colloids are heterogeneous mixtures.