Atoms and Molecules

Class 09 Science

Ancient Greek philosophers, Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms.

By the end of the eighteenth century, scientists recognised the difference between elements and compounds and naturally became interested in finding out how and why elements combine and what happens when they combine.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Laws of Chemical Combination

Law of Conservation of Mass

Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

Law of Constant Proportions

The law of constant proportions, which is also known as the law of definite proportions, states that in a chemical substance the elements are always present in definite proportions by mass.

In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.

Dalton’s Atomic Theory

British chemist John Dalton provided the basic theory about the nature of matter. Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. He took the name ‘atoms’ as given by the Greeks and said that the smallest particles of matter are atoms. His theory was based on the laws of chemical combination.

Dalton’s atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions.

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Postulates

  1. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
  2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  3. Atoms of a given element are identical in mass and chemical properties.
  4. Atoms of different elements have different masses and chemical properties.
  5. Atoms combine in the ratio of small whole numbers to form compounds.
  6. The relative number and kinds of atoms are constant in a given compound.

Atom

The building blocks of all matter are atoms.

Atomic Mass

According to Dalton, each element had a characteristic atomic mass. Since determining the mass of an individual atom was a difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

For example, carbon monoxide (CO) is formed by carbon and oxygen. It was observed experimentally that 3 g of carbon combines with 4 g of oxygen to form CO. In other words, carbon combines with 4/3 times its mass of oxygen. Suppose we define the atomic mass unit as equal to the mass of one carbon atom, then we would assign carbon an atomic mass of 1.0 u and oxygen an atomic mass of 1.33 u.

However, it is more convenient to have these numbers as whole numbers. In 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Molecule

A molecule is a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.

Molecules of Elements

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.

But this is not the case with most of the non-metals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone, O3. The number of atoms constituting a molecule is known as its atomicity.

Molecules of Compounds

Atoms of different elements join together in definite proportions to form molecules of compounds.

Ions

Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. An ion can be negatively or positively charged.

A negatively charged ion is called an anion and the positively charged ion, a cation. For example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–).

Chemical Formulae

The chemical formula of a compound is a symbolic representation of its composition.

The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atoms of another element to form a chemical compound.

When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO), etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

In compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example, Mg(OH)2. In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH.

Formulae of Simple Compounds

The simplest compounds, which are made up of two different elements are called binary compounds.

  1. hydrogen chloride: HCl
  2. hydrogen sulphide: H2S
  3. carbon tetrachloride: CCl4
  4. magnesium chloride: MgCl2
  5. aluminium oxide: Al2O3
  6. calcium oxide: CaO
  7. sodium nitrate: NaNO3
  8. calcium hydroxide: Ca(OH)2
  9. sodium carbonate: Na2CO3
  10. ammonium sulphate: (NH4)2SO4

Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is the relative mass of a molecule expressed in atomic mass units (u).

Example: Calculate the relative molecular mass of water (H2O).

Atomic mass of hydrogen = 1u, oxygen = 16 u

So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is

= (2  x 1) + (1 x 16) = 18 u

Example: Calculate the molecular mass of HNO3.

The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N + 3 x the atomic mass of O

= 1 + 14 + 48 = 63 u