Atomic theory

Element is a pure substance that cannot be broken down into simpler substances by any chemical methods.

It is made up of very tiny invisible particles (or species) which are called atoms

a high-resolution scanning electron microscope can generate pictures of a single heavy atom resting on a thin carbon substrate.

Democritus - a Greek philosopher who lived in the fifth century BC

everything was made of indivisible small pieces of matter - “atomos” (Greek word for indivisible)

John Dalton, in 1800

• All matters are made up of indestructible atoms. • The atoms of a given element are identical; the atoms of different elements are different in some fundamental way(s). • Chemical compounds are formed when atoms combine with each other. A given compound always has the same whole number ratio and types of atoms. • Chemical reactions involve rearrangement of the atoms. The atoms themselves are not changed in a chemical reaction.

• With the discovery of sub-atomic particles, e.g., electrons, neutrons and protons, the atom can no longer be considered indivisible. • Discovery of isotopes indicated that all atoms of the same element are not perfectly identical. At least, they differ in their masses. • Dalton’s atomic theory could not explain why certain substances, with atoms of the same element, differ in their properties. For example, graphite and diamond are made up of only carbon atoms, but their properties are quite different

J. J. Thomson in an experiment discovered that different metals produce a stream of negatively charged particles when a high voltage is applied across two electrodes. He suggested that these particles are part of the atoms.

Thomson pictured the structure of an atom as a “plum pudding”, with the negatively charged electrons scattered in a positively charged sponge-like substance.

Rutherford and his research team tested Thomson’s model by firing alpha particles, which consists of two protons and two neutrons, at a piece of gold foil.

He thought that if Thomson’s model was valid, the alpha particles should either pass straight through or get stuck in the positive “sponge”. It was observed that most of the particles did pass straight through the foil, but a very small number were repelled and bounced back.

A large number of undeflected paths imply that the atom is mainly an empty space Large extent of deflections occurs when the positively charged alpha particles collide with and are repelled by a positively charged nucleus. Only a small number of alpha particles bounce back suggests that the nucleus is very small. Based on the results of the Rutherford’s experiment, atoms are in fact made up of smaller particles or sub-atomic particles

• Thomson’s model failed to explain the results of the alpha particle scattering experiment carried out by Rutherford - the majority of the alpha particle passes through the gold foil while a small number of them bounced back. • There was no experimental evidence provided by Thomson. This was one of the major drawbacks of his model.

• One of the drawbacks of the Rutherford model was that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete. • Although the early atomic models were inaccurate and failed to explain certain experimental results, they were the base for future developments of the theories in atoms and sub-atomic particles.

Niels Bohr

It was seen as a small “solar system” with electrons moving in an orbit or energy level around the positively charged nucleus of one proton. The electrostatic force of attraction between the oppositely charged subatomic particles prevents the electrons from leaving the atom. Most of the volume of the atom is empty space.

● Given its mass and wavelike nature, the electron in the hydrogen atom could not possibly orbit the nucleus in a well-defined circular path as predicted by Bohr. ● The position (an orbit of a particular radius) and the energy of the electron could not be specified.