Atoms

History of the model of the atom

- John Dalton

• Suggested that atoms were tiny, hard balls
• Each chemical element has its own atoms that differed from others in mass
• Atoms were the fundamental building blocks of nature and could not be split
• In chemical reaction, the atoms would rearrange themselves and combine with other atoms in new ways

- J.J Thomson

• Discovered electron
• Called negatively charged particles electrons
• Suggested that electrons come from inside atoms
• Proposed that tiny negatively charge electrons must be embedded in a cloud of positive charge

- Ernest Rutherford

• Proposed that Thomson's model was not right : he said that the positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source based on his experiment.

- Niels Bohr

• Suggested that the electrons must be orbiting the nucleus of the atom in certain fixed energy levels (explaining why light was given out when atoms were heated always had specific amounts of energy)

Summary: 

John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteen century.

In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.

In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.

In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.

We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).

Structure of the atom as nucleus with electrons surrounding it (Bohr's Model)
- Atoms are made out of 3 different particles

• Protons
• Electrons
• Neutrons

Isotopes

- Isotopes are atoms of the same element with the same number of protons but different number of neutrons.

- Isotopes have the same chemical properties but slightly different physical properties

Eg.

They are similar except that Hydrogen 1 has no neutrons, hydrogen-2 has one neutron and hydrogen-3 has two neutrons. These hydrogens atoms are known as isotopes.

Uses of Isotopes

- Isotopes that emit high-energy radiation are called radioisotopes. They are classified as radioactive substances. Radiation emitted by radioisotopes is dangerous because it can damage living cells and cause cancer. However, radioisotopes can have important applications and can be safely used if they are handled properly.

Electronic Configuration

The electrons in an atom move around the nucleus in regions known as electron shells. Each electron shell can only hold a certain number of electrons.

Above is a magnesium atom. It has 2 electrons in the first shell, eight electrons in the second shell and 2 electrons in the third shell. Thus, its electronic structure or electronic configuration can also be represented as (2,8,2).