Atoms & Molecules Video Lecture - Class 9

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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 the
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 names BA 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 planet just
like planets in the solar system each
electron revolves around the nucleus in
its own different orbit the orbits of
shells are represented by the letters K
L M etc the maximum number of electrons
present in a shell is given by the
formula - N squared where n is the orbit
number of energy cells the first orbit
or K shell can have two the second orbit
or L shell can have eight 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 forces such as heat and
light this leads to the formation of
molecules during the formation of a
molecule atoms lose the 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 a
chemical factor which is responsible for
joining the molecules of wax pieces
there are two main types of bonds
covalent and ionic 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 the
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 - n square 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
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you