Classwork Exercise and Series (Chemistry-SS 1): Particulate Nature Of Matter

Definition Of Matter

Matter is defined as anything that occupies space and has mass. The particles of matter could be atoms, molecules or ions.

Atoms

The ancient Greeks were the first to use the word atom, which means indivisible. An atom is defined as the smallest particle of an element which can take part in a chemical reaction. In other word, atom is the smallest part of an element that can ever exist and still possess the chemical properties of that element.

Molecules

A molecule is the smallest particle of a substance that can normally exist alone and still retain the chemical properties of that substance, be it an element or compound.

Most atoms cannot exist alone. They generally bond with other atoms to form molecules. Molecules may be made-up of atoms of the same element or of different elements. The number of atoms in each molecule of an element is called the atomicity of the element.

Ions

An ion is any atom or group of atoms which possesses an electric charge. There are two types of ions:

1. The positively charged ions or cations, examples are Na⁺, Ca⁺, Fe³⁺, NH₄⁺, etc.
2. The negatively charged ions or anions, examples are Cl^–, NO₃^–, OH^–, etc.

Dalton’s Atomic Theory

In 1808, John Dalton proposed the Atomic Theory which can be summarized as follows:

1. All elements are made up small, indivisible particles called atoms.
2. Atoms can neither be created nor destroyed.
3. Atoms of the same element are alike in every aspect, and differ from atoms of all other elements.
4. When atoms combine with other atoms, they do so in simple ratios.
5. All chemical changes result from the combination or the separation of atoms.

Modifications of Dalton’s Atomic Theory

1. All elements are made up of small indivisible particles called atoms. This statement has been proven wrong by Rutherford’s discovery – the atom is built up of three main types of sub-particles: the proton, the electron and the neutron. It is not an indivisible solid piece.
2. The atom can neither be created nor destroyed. This statement still holds good for ordinary chemical reactions. During a nuclear reaction, such as the fission of uranium-235, the nucleus is broken up into smaller units which form simpler atoms while a tremendous amount of heat energy is released. These changes that occur during nuclear fission destroy the atoms of the element involved.
3. The atoms of the same element are alike in every aspect and differ from atoms of all other elements. The discovery of isotopes makes this statement unacceptable. Chlorine, for instance, has two different atoms or isotopes which differ in their neutron content and hence in their relative atomic masses although they have the same atomic number and chemical activities.
4. During a chemical reaction there is a combination of atoms in small whole numbers. This statement is generally true only for inorganic compounds which usually contain a few atoms per molecule. Carbon, however, forms very large organic molecules, such as proteins, fats, and starch, which contain thousands of atoms.