Notes: Atoms and the Periodic Table

Page 1

Atoms were thought to be tiny spheres that could not be divided up further in early models of the atom.

In 1898, electrons were discovered by Thompson and so the representation of the atom was forced to change.

An atom has no overall charge: it is neutral.

Thompson believed in the 'plum-pudding' model: that atoms contained negatively charged electrons surrounded by positive charge.

This model was disproved through an experiment conducted by Geiger and Marsden:

In this experiment, the scientists bombarded a thin sheet of gold with positively-charged alpha particles.

A tiny number of these particles were deflected, the rest went straight through.

The results of this experiment caused the scientist Ernest Rutherford to conclude that there must be a very small area in the centre of the atom where the positive charge is concentrated.

He named this the 'nucleus' and so the plum-pudding model was replaced by the nuclear model of the atom.

Later, another scientist, Niels Bohr, deduced that electrons must orbit the nucleus at specific distance. This was concluded because if they did not, they would spiral inwards towards the nucleus.

Approximately twenty years after the nuclear model became the accepted scientific model of the atom, James Chadwick provided evidence to support the existence of neutrons in the nucleus of the atom.

Later experiments followed that led to the notion that the positive charge in the nucleus could be divided into a whole number of positively charged smaller particles, each having the same positive charge. These particles are called protons.

Page 2

Subatomic Particles

Atoms are incredibly small, usually having an atomic radius of 0.1 nano-metres or 1 x 10-10 m.

Atoms contain three types of subatomic particle:

Nearly all of the atom's mass is contained in the nucleus.

But the radius of the atom is less than 1/10,000 of the entire atomic radius so most of an atom is empty space.

Because atoms contain an equal number of protons and electrons, they have an overall neutral charge.

Every atom of a particular element has the same number of protons.

Atoms of different elements have different numbers of protons.

The atomic number is the number of protons in an atom.

The sum of the number of protons and the number of neutrons is the relative atomic mass number.

The elements are arranged in ascending order of atomic numbers.

Key Info: number of neutrons = mass number - atomic number

Isotopes and Ions

An element's isotopes have the same number of protons but a different number of neutrons (the same atomic number but a different mass number).

In order to form ions, atoms lose or gain electrons.

Metal atoms lose electrons.

Non-metal atoms gain electrons.

Electron Configuration

An atom's electrons occupy the lowest available shell, or energy level.

In the first twenty elements, two electrons can occupy the first shell and eight electrons can occupy each of the next two shells.

Electron configurations of atoms show how electrons are arranged in shells around the nucleus.