Consult a periodic Table for a list of elements.

The concept of an atom is atributed to Democritus around 350BC.

Atoms contain protons, neutrons and electrons.

Atoms are electrically neutral

Atomic number and mass number help identification of atoms

A - Z = N

The number of protons in the nucleus determines the behavior of an atom.

An electron is a tiny part of an atom, which is the tiny building block of our world.

What is an atom?
An atom is the smallest part of an element that still retains the properties of that element.
The atom is our basic building block. Everything is made of atoms. The word derives from the Greek meaning "indivisible". When radioactivity was discovered it led to a major reappraisal of the concept of an atom. There are essentially, only 92 different types of atom, corresponding to the 92 naturally occurring elements.

Elements
An element is a substance that cannot be broken down into simpler substances by chemical means. Amongst the vast array of materials in our world, Chemists have identified about 90 elements, from which all the materials are ultimately made
All other materials like; metal, plastics, cloth, timber, bone, glass, leaves, etc. is made up of combinations of the 92 atoms that are found in nature. The Periodic Table enables elements to be arranged according to their similarities.

What does an atom consist of?
Although an atom was once thought to be the smallest possible solid particle, its true structure began to emerge as a result of the Rutherford alpha particle scattering experiment.

An atom is mostly empty space, with a compact core of mass called a nucleus and a number of tiny particles called electrons, which move in orbits around the nucleus. Thus an atom resembles a mini solar system.
  Electrons in orbit around a nucleus.

Inside the nucleus there are two kinds of particle, protons and neutrons. Protons have positive charge and repel each other. Neutrons have no charge and help to bind the nucleus together.

How are different atoms identified?
An element is represented by symbols like that shown below.

 





A




Z

N

 Z stands for the number of protons.
The value of Z determines the element in question. Z is referred to as the atomic number. In a neutral atom the number of electrons equals the number of protons.
A stands for the number of nucleons (protons and neutrons combined). Thus A – Z = the number of neutrons.
A is referred to as the mass number.

Since the mass of each nucleon is one atomic mass unit and electrons have an almost negligible mass by comparison, then the number of nucleons and the mass of an atom are both given by the value A.