CHEMISTRY ATOMIC STRUCTURE CLASS IX
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What is the structure of atoms-
• Structure
of atom was proposed firstly by the
scientist J.J. THOMSON, he gave his own model of atom which was like a water
melon.
• After
this Rutherford, a great scientist discovered that the structure of atom under
his famous gold foil experiment changes and hence fourth his experiment lead to
the discovery of nucleus, electrons and protons and the ray which
rebounds(alpha ray).
• Although
a new objection was created by people that if there are electrons how they stay
and why don’t they lose energy while centerpetral and centerfugral forces are
acting
upon it, this too got
elucidated by BOHR’S theory of electrons.
• J.J.
THOMSON’S MODEL
• GOLD
FOIL MODEL
• BOHR’S
MODEL
• Thomson
proposed a model of an atom to be similar to that of a Christmas pudding or
like a water melon in which red edible part forms the positive charge but the
black seeds are the electrons in it. It
was only the famous discharged tube experiment that the sub- atomic particle
electron was found.
• Thomson
proposed that:-
• An
atom consists of a positively charged sphere and the electrons are embedded in
it.
• The
negative and positive charges are equal in magnitude. So, the atom as a whole
is electrically neutral.
•
Limitations of j.j. Thomson’s model
• J.J.
Thomson’s model indicated that it contained no nucleus in the atomic model.
• It
does not have an experimental evidence in its support.
Conclusion of Rutherford’s model-
• Most
of the space inside the atom is empty because most of the alpha particles
passed through the gold foil without getting deflected.
• Very
few particles were deflected from their path, indicating that the positive charge of the atom occupies very
little space.
• A
very small fraction of alpha particles, deflected by 180 degree(rebound, only
one out of 12000 ), indicating that all the positive charge and mass of atom
were concentrated in a very small volume within the atom
Limitations of Rutherford’s model of atom-
• Any
charged particle when accelerated is expected to radiate energy. To remain in a
circular orbit, the electron would need to undergo acceleration. Therefore, it
would radiate energy.
• Thus,
the revolving electron would lose energy and finally fall into the nucleus. If
this were so, the atom should be highly unstable. Therefore, matter would not
exist, but we know that the matter exists. It means that the atoms are quite
stable.
• Rutherford’s
model could not explain the distribution of electrons in the extra nuclear
portion of the atom.
BOHR’S MODEL-
In actual terms no specified model
was introduced by BHOR but a few postulates were provided by him which later on
became practically proved and gave answers of objections raised by scientist
like Maxwell etc.
His postulates are-
Atom consists of positively charge
nucleus around which electron revolve in discrete orbits(electrons move in
permeable orbits and not in any orbit.)
Each of these orbits are
associated with certain energy values known as energy shells or levels. As the
energy of an orbit is fixed this is also known as stationary state.
He represented energy levels as
k,l,m,n etc.
Energy of an electron remains as
long as it remains in the discrete orbit and it does not radiate energy while
revolving.
When energy is supplied to an
electron, it can go higher energy levels. While an electron falls to lower
energy level, it radiate energy.
DISCOVERY OF NEUTRON’S-
J. CHANDWICK discovered another sub-atomic part of atom
which had no charge and a mass nearly equal to half that of a proton. As it
seemed to be neutral in nature hence fourth termed as neutron. Neutrons are
present in the nucleus of all atoms expect in hydrogen and the mass of an atom
is therefore the sum of masses of protons and neutrons present in the nucleus.
DISTRIBUTION OF ELECTRONS
Distribution of electrons was done by BOHR and BURRY. There
are a set of rules to be followed while writing the number of electrons in
different energy levels or shells:-
The maximum no. of electrons present in a shell is given by
the formula 2n
ATOMIC NUMBER AND MASS NUMBER-
ATOMIC NUMBER AND MASS NUMBER
Every element on
the periodic table has 2 numbers associated with it
The bigger number is always the MASS NUMBER
MASS NUMBER is the number of particles in the nucleus, the protons
and neutrons
The smallest number is always the ATOMIC NUMBER
The atomic number tells us how many PROTONS in the nucleus
For an atom, number of protons is the same as number of
electrons
For example carbon, C
Has a mass number of 12 and atomic number of 6
ISOTOPES-
Isotopes are defined as the atoms of the same element,
having the same atomic number but different mass numbers. In the other words it
can be said that the isotopes have same number protons but different in number
of neutrons. Each isotope of an element os a pure substance.
ISOBARS-
Atoms of different elements with different atomic numbers,
but same mass number, are known as isobars.
In the other words, isobars are the atoms of different
elements that have same number of nucleons(proton+elctons) but different no. of
protons.
VALENCY-
The electrons present in the outermost shell of an atom are
known as the valance electrons. They govern the chemical properties of the
atoms. The atoms of elements having completely filled outermost shell eight
electrons(said to posses octet) show little chemical activity i.e. highly
stable. Such elements are called inert elements. Of these inert elements, the
helium atom has two electrons in its outermost shell and all other elements
have eight electrons in the outermost shell. The number of electrons gained or
lost or shared by an atom to become stable or achieve an octet in outermost
shell is known as valency.
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