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Matter has mass and occupies space. Atoms are basic building blocks of matter, and cannot be chemically subdivided by ordinary means.

The word 'atom ' is derived from the Greek word atom which means 'indivisible'. The Greeks concluded that matter could be broken down into particles too small to be seen. These particles were called atoms.

Atoms are composed of three types of particles: protons, neutrons, and electrons. Protons and neutrons are responsible for most of the atomic mass.

Both the protons and neutrons reside in the nucleus. Protons have a positive (+) charge, neutrons have no charge and they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).

It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may also vary.

The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form.


Example : Oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons.


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