| Representation
of an Atom |
| Atom is
made up of three basic particles-positively
charged protons, negatively charged electrons and neutral neutrons. Neutrons and protons
make up the nucleus of an atom. The electrons
circulate around the nucleus. Mass of
a neutron is nearly equal to a mass of
a proton. But an electron mass is 1800
times smaller than the proton mass. In
an atom, the number of electrons is equal
to the number of protons. Thus an atom
as such is always electrically neutral.
|
| Number of
protons in an atom is known as the atomic
number of the atom. Letter Z
represents atomic number. Z
= number of protons. Each element in the
periodic table of elements has different
Z. For example : Hydrogen
(H) has 1 proton. Therefore its atomic
number is Z = 1. Calcium
(Ca) has 20 protons. Therefore its atomic
number Z = 20. An atom
of Gold (Au) has 79 protons. Therefore
its atomic number Z =
79. Number of neutrons in an atom is denoted
by N. N
= number of neutrons. |
| Mass number
is defined as the total number of protons
and neutrons present in an atom. Mass
number is denoted as A. |
| A
= Z + N |
| For example
: Hydrogen (H) has 1 proton and no neutrons.
Therefore its atomic number Z
= 1, N = 0, A
= 1. |
| Calcium
(Ca) has 20 protons and 20 neutrons. Therefore
its atomic number Z =
20, N=20 and A
= 40. |
| An atom
of Gold (Au) has 79 protons and 118 neutrons.
Therefore its atomic number Z
= 79, N = 118 and A
= 197. |
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