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.
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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. |
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.
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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 of A.
What is an
isotope?
Isotopes are atoms of the same element, which have different numbers of
neutrons.
For example, atoms of carbon have 6 protons and usually have 6 neutrons.
However two isotopes of carbon exist, one of which has 7 neutrons and
the other has 8 neutrons. Chemically these all behave the same. However
these isotopes are physically different and inherently unstable. Thus
the isotopes of carbon are radioactive.