Atomic Structure and Periodicity - SS2 Chemistry Past Questions and Answers - page 2

Dalton's atomic theory consists of four main postulates:

A.    All matter is made up of indivisible particles called atoms.

B.    Atoms of the same element are identical in their properties, such as size, mass, and chemical behaviour.

C.   Atoms combine in simple whole-number ratios to form compounds.

D.   Chemical reactions involve the rearrangement of atoms, but no atoms are created, destroyed, or changed into atoms of another element.

Dalton's theory laid the foundation for modern atomic theory by introducing the concept of atoms as fundamental building blocks of matter. It provided a theoretical framework that explained chemical reactions and the behaviour of elements. However, subsequent discoveries, such as the existence of subatomic particles, revealed limitations in Dalton's theory.

J.J. Thomson conducted experiments with cathode rays and made significant contributions to our understanding of atomic structure. He discovered the existence of electrons, which are negatively charged subatomic particles, through his cathode ray tube experiments. Thomson proposed the "plum pudding" model of the atom, where electrons were embedded in a positively charged sphere.

Thomson's experiments demonstrated that atoms were not indivisible, as Dalton had initially suggested. Instead, they consisted of smaller, negatively charged particles. This discovery laid the groundwork for further investigations into the structure of atoms.

The Rutherford gold foil experiment involved firing alpha particles at a thin sheet of gold foil and observing their behaviour. Rutherford expected the alpha particles to pass through the gold foil with minimal deflection, according to Thomson's "plum pudding" model. However, he observed unexpected results.

Some alpha particles deflected at large angles, while a few even bounced back in the opposite direction. This led Rutherford to conclude that the atom has a small, dense, positively charged nucleus at its centre. The majority of the atom's volume was in space, with electrons orbiting the nucleus at a significant distance.

Rutherford's experiment revolutionised the understanding of the atomic structure, disproving the prevailing model at the time and introducing the concept of a nuclear model of the atom.

Atomic number refers to the number of protons in the nucleus of an atom. It is a fundamental property of an element that determines its position in the periodic table. Each element has a unique atomic number.

The periodic table is organised based on the increasing atomic number of elements. Elements with similar chemical properties and behaviours are grouped in columns called groups or families. The atomic number provides a systematic way of arranging elements in the periodic table, allowing scientists to understand trends in properties as they move across periods and down groups.