Atoms and Molecules
This is an anthill. The ants are busy building
it. It takes many small grains of sand to
make an anthill. If you take a closer look
at a grain of sand under a scanning tunneling
microscope, you can see that even the small
grain of sand consists of millions of particles
which are miniscule in nature. These particles
are called atoms. ATOMS ARE THE BUILDING BLOCKS
OF ALL MATERIAL.
Atoms are made up of three types of particles
called protons, neutrons and electrons. An
atom has a central nucleus with protons and
neutrons in it. Protons are positively charged
and neutrons have no charge. So, the nucleus
is positively charged. Protons and neutrons
are collectively called nucleons.
Electrons are negatively charged and are located
around the nucleus. An atom has the same number
of negatively charged electrons as it has
positively charged protons in the nucleus.
Therefore, atoms are electrically neutral.
Electrons are distributed in a definite path
around the nucleus and are called shells or
orbits. This concept was introduced by Neils
Bohr. According to Bohr, different electrons
revolve around the nucleus in their own different
orbits.
The atom has been compared to the solar system.
In an atom, the nucleus is the sun and the
electrons are the planets. Just like planets
in the solar system, each electron revolves
around the nucleus in its own different orbit.
The orbits or shells are represented by the
letters K, L, M, N, etc. The maximum number
of electrons present in a shell is given by
the formula 2n2, where 'n' is the orbit
number of energy levels. The first orbit or
K-shell can have 2, the second orbit or L-shell
can have 8 and M-shell can have 18 electrons,
etc. The electrons present in the outermost
shell of an atom are called valence electrons
and the shell which valence electrons occupy
is called the valence shell.
A molecule is, in general, a group of two
or more atoms that is chemically bonded together.
Atoms combine chemically by external factors
such as heat and light. This leads to the
formation of molecules. During the formation
of a molecule, atoms lose their valence electrons
or gain electrons from other atoms.
For instance, if two pieces of wax are to
be joined, they are heated. After heating,
the two pieces are pressed together to form
a single structure. Here, heat is the chemical
factor which is responsible for joining the
molecules of the wax pieces.
If a molecule is made up of atoms of the same
element, it is called a homo-atomic molecule.
If the molecule is made up of atoms of different
elements, it is called a hetero-atomic molecule.
THINGS TO REMEMBER
Building blocks of matter that we see in the
universe are atoms.
Protons, neutrons and electrons constitute
an atom. Protons have a positive charge and
neutrons have no charge and electrons are
negatively charged.
The maximum number of electrons present in
a shell is given by the formula 2n2.
A molecule is a chemical bonding of two or
more atoms.
Bonds are classified as covalent and ionic.
There are two types of molecules: homo-atomic
and hetero-atomic.
