Tamil
Nadu Matriculation Board
Class IX : Chemistry |
Chapter
1: Atomic Structure |
Essay
Answers:
|
| Ans1.
Atoms: |
| The smallest
particle of matter that take part in a chemical reaction is called atom. |
| Atoms are made of
sub atomic particles namely protons, neutrons and electrons. |
| The central part
of the atom is called as nucleus. This is made up of protons and neutrons and the nucleus is
surrounded by electrons revolving around in orbits. The protons are positively charged,
neutrons are neutral and electrons are negative in nature. |
| Mass
number: |
|
The total number
of protons and neutrons present in the atom is known as the mass number.
Number of protons
+ number of neutrons = mass number. Mass number = Atomic number + number of neutrons.
Since atomic number is equal to the number of protons.
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For eg: atomic
number is 17 number of neutrons = 18. Mass number 17 + 18 =35.
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| Atomic
number: |
Atomic number of
an element is the number of protons or electrons present in an atom.
:.Atomic number = number of protons or electrons. |
Eg:Atomic
number of sodium = 11.
Therefore number of protons or electrons = 11.
This does not change even during ion formation.
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| Ans2.
Energy levels of atom: |
| The path of
electrons which revolve around the nucleus are called as orbits. These orbits are
also known as
energy levels, each orbit having different energy level. |
| According to Neil
Bohr, the orbits close to the nucleus have low energy levels, while those away from the
nucleus have lower energy levels. |
| Bohr also said that
when the electron revolve in a particular orbit or energy level, they do not radiate
energy. But when they jump from lower level to higher level or higher level to lower level,
they radiate energy. |
| The fixed circular paths
or orbits, also known as energy levels are represented either as numbers
or by
letters. |
| The energy level
nearest to the nucleus is given the number 1, nest 2,3,4 etc…. The 1st
orbit represented as K then l,m,n,o, etc….. each having its own energy level,
electrons are arranged depending on the energy level. The number of electrons in an energy
level is determined by the formula 2n2 where n is the energy level. |
:.
K shell can have 2´1´1 = 2 electrons.
L shell can have 2´2´2 = 8 electrons.
M shell can have 2´3´3 = 18 electrons etc. |
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Ans3.
a) Argon -Atomic Number =18.
Configuration - 1s2 2s2 2p6 3s2 3p6 |
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b) Sodium
- Atomic number -11
1s2 2s2 2p6 3s1 |
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| c) Helium -
1s2 Atomic number - 2. |
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