Not long after the discovery of the atom, it was found that the atom
contained two regions: The center of the atom, which contains the nucleus,
and the surrounding area. Electrons were already known about (particles
with a negative charge) and soon after were found 'protons' and 'neutrons;
in the nucleus.
Protons are particles in the nucleus which give a positive charge and
neutrons have no charge. Together with electrons, these particles cause
the atom to be stable and be electrically neutral. Since Protons and Neutrons
are very similar in size and weight, together they are known as Nucleons.
Atoms are classified according to the number of protons in their nuclei.
For example, any atom with six protons in its nucleus is a carbon atom.
Any atom with one proton is a hydrogen atom. 26 protons defines an iron,
92 protons a uranium etc. The number of neutrons in an atom can vary, but
it is usually close to the number of protons. For example, the most common
variety of carbon has six neutrons and 6 protons. 6 protons + 6 neutrons
= 12 nucleons, and so this variety is called Carbon-12. Many carbon atoms,
however, have 8 neutrons. 6 + 8 = 14, so this type of carbon is Carbon-14,
some you may have heard in the context of chemistry. Different varieties
of the same element with different numbers of neutrons are called different
"isotopes."
Surrounding the nucleus of an atom are its electrons. Electrons are
described as revolving around about the nucleus but this is misleading
because the quantum theory shows that particles have certain wave properties.
From this the electrons should not be viewed as planets orbiting the sun
but rather as clouds of electrons in different shells around the nucleus.
An Atom can, if the conditions are right, lose or gain electrons from
other atoms. These atoms with an excess or deficit of electrons are called
Ions. Ions have very different properties than the neutral atom and are
very important in reactions. It is easier to remove the electrons in the
outher shell than the inner electrons. As the number of electrons increase
in larger atoms we find that they arrange themselves in progressively larger
shells. An atom that has lost electrons will then be more positively charged
and is called a cation. It is somewhat smaller than the neutral atom as
the remaining electrons are drawn closer to the positive nucleus. An atom
that has gained extra electrons has become more negatively charged an is
called an anion. Both of these Ions, cations and anions, are present in
our daily lives. For example a crystal of ordinary table salt consists
of positive sodium ions mixed with negative chloride ions. A sample of
the crystal itself is neutral as it contains equal amounts of positive
and negative ions. to be more precise, a crystal fragment may have a net
charge at the surface. This excess or deficit of electrons is minuscule
compared to the huge number of balanced charges in the fragment. On the
other hand such surface charges become significant when we consider very
small particles such as occur in colloidal suspensions
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