Basic Terminology

Atom: An atom is the smallest particles of an element that can exist and take part in a chemical reaction. Atoms are so small that they cannot be seen even under the most powerful optical microscope. However, it has been possible to photograph images of atoms using the scanning tunneling microscope. Hydrogen atom is the smallest atom known. Its atomic radius is 0.037nm. Atoms of noble gases (such as helium, neon, argon, krypton etc.) are chemically un reactive and exist in the free state, as single atoms. Atoms of most elements are very reactive and do not exist as single atoms. Instead, they exist as molecules or ions.

Molecule: A molecule is an electrically neutral group of two or more atoms chemically bonded together. It is the smallest particle of a substance (element or compound) that can exist in the free state, and has the properties of that substance.

Atomicity: The number of atoms present in one molecule of an element is called its atomicity. The atomicity of hydrogen (H₂), nitrogen (N₂), oxygen (O₂), and chlorine (Cl₂) is 2, i.e., these molecules are diatomic, Ozone (O₃) is triatomic, phosphorus (P₄) is tetra-atomic, and sulphur (S₆) is octa-atomic. All these are homoatomic molecules, i.e., molecules containing same kind of atoms.

Heteronuclear molecules: The molecule  of a compound consists of two or more different kinds of atoms chemically combined together. The molecule of ammonia (NH₃), contains one atom of nitrogen (N) and three atoms of hydrogen (H). Water (H₂O), carbon dioxide (CO₂), hydrogen chloride (HCl) are some examples of molecular compounds (compounds which consist of molecules). All these are heteroatomic molecules, i.e. molecules in which two or more kinds of atoms are present.

Chemical symbol: They are a shorthand notation for the name of elements. The symbol consists of either a single letter or two letters (the first one being a "capital" letter and the second, a 'small' letter). This was proposed by JJ Berzelius. For example the symbol for nitrogen is 'N' and that for chlorine is 'Cl'.

Chemical formulae: They are used to denote compound composition in a concise manner. They consist of the symbols of the elements present in the compound and numerical subscripts (locate to the right of each symbol) that indicate the number of atoms of each element present in a molecule of the compound. A molecule of sulphur dioxide contains one atom of sulphur and two atoms of oxygen. Therefore, the formula of sulphur dioxide is SO₂. Similarly, ammonia is NH₃, water is H₂O, methane is CH₄, and so on.

Atomic mass\molecular mass: The atomic mass of an element is the relative mass of its atom as compared with the mass of a Carbon-12 atom taken as 12 units. It indicates the number of times one atom of the substance is heavier than 1/12 (one-twelfth) of a Carbon -12 atom.

Molecular mass: The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a Carbon-12 atom taken as 12 units. It indicates the number of times one atom of the substance is heavier than 1/12 (one-twelfth) of a Carbon-12 atom. It indicates the number of times one molecule of the substance is heavier than 1/12 (one-twelfth) of a Carbon-12 atom.

Atomic mass unit: Atomic mass unit (1 u) = 1/12 the mass of a Carbon-12 atom. 1u = 1.66x 10^-24 g. The atomic mass of sulphur is 32u, i.e., it is 32 times heavier than 1/12 of a Carbon-12 atom. Similarly, the atomic mass of oxygen is 16u.

 Atomic number: The number of protons (or the number of electrons) is one atom of an element is known as the atomic number of that element and is represented by the letter 'Z'. For example, the number of protons (and electrons) in carbon is 6, so the atomic number (Z) of carbon is 6.

Mass number: The total number of protons and neutrons present in one atom of an element is known as its mass number and is denoted by the letter 'A'. A carbon atom has 6 protons and 6 neutron, so the mass number (A) of carbon is 6+6 = 12. The atomic mass of an atom is numerically equal to its mass number. For example, if the mass number of an atom is 16, then its atomic mass will be 16u.