Atoms – Structure of Atoms – Isotopes and Neutron Numbers

An atom is the smallest possible particle of an element that retains the characteristics of the elements. The radius of an atom is 10 8 cm.

Structure of an Atom
Atoms of different elements are distinguished by their characteristic number of sub-atomic particles. The electrons protons and neutrons are the three sub-atomic particles. Protons and neutrons form a centralized mass called the nucleus. The electrons surround the nucleus.

Electrons carry a negative charge, protons carry a positive charge and neutrons are electrically neutral. Within an atom, the charges on protons and electrons cancel each other out. An electron has about 1/2000 the mass of a proton or neutron.

All matter can be broken down into pure substances, called elements. The periodic table depicts in short hand the structures of the atoms of each element. Each element has a symbol, which can come from the English word for that element (for example, He for Helium), or from the word in another language (eg. Na for sodium, which is Natrium in Latin) The number in the upper left-hand corner of each element symbol is the mass number, which refers to the total number of protons and neutrons in the nucleus of an atom. The mass number estimates atomic mass because it does not count the tiny contribution of the electrons.

The atomic number, in the lower left-hand corner, shows the number of protons in the atom. For example, hydrogen (IH) has atomic number one and mass number also one.

The elements are arranged sequentially by atomic number in the periodic table. We can calculate the number of neutrons by substracting the atomic number (the number of proton) from the mass number (the total number of protons and neutrons). Atoms of different elements may vary greatly in size.

Isotopes vary in neutron numbers. Elements can exist in different forms
called isotopes, that have different number of neutrons. All isotopes of an element have the same charge, but different masses. Often an isotope of an element is very abundant, and others are rare; this is the case for oxygen. Similarly, about 99% of carbon isotopes have six neutrons, while only 1% are isotopes with seven or eight neutrons.

Some isotopes are unstable, which means that they have extra neutrons and tend to breakdown into more stable forms of the elements. When these unstable isotopes break down, they emit radioactive energy. Every type of radioactive isotope has a characteristic half-life.