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#ATOM
Atoms are the basic building blocks of ordinary matter
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Atoms are composed of particle
called protons, electrons and neutrons
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The particle has a positive
electrical charge, equal and opposite to that of the electron
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It can be either free (not attached to any
atom), or bound to the nucleus of an atom
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The larger the spherical shell, the higher the energy contained in
the electron
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Cations and anions are both ions
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Ions are atoms or molecules which have gained or lost one or
more valence electron giving the ion a net positive or negative charge
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E
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Examples: hydroxide anion: OH-, oxide anion: O2-, and sulfate anion: SO4-2

#J
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Thomson (1856-1940)- THOMSON’S MODEL OF AN ATOM
Thomson proposed thati) An atom consists of a positively charged sphere and the electrons are
embedded in it
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So, the atom as
a whole is electrically neutral
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Rutherford (1871-1937)- RUTHERFORD’S MODEL OF AN ATOM
Rutherford put forward the nuclear model of an atom, which had the
following features:
(i) There is a positively charged centre in an atom called the nucleus
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(ii) The electrons revolve around the nucleus in circular paths
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#Drawbacks of Rutherford’s model of the atom
The revolution of the electron in a circular orbit is not expected to be stable
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During
acceleration, charged particles would radiate energy
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Neils Bohr (1885-1962)- BOHR’S MODEL OF ATOM
Postulates about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are
allowed
inside the atom
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These orbits or shells are called energy levels
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#NEUTRONS
Neutrons are present in the nucleus of all atoms
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The mass of an atom is therefore given by the sum of the
masses of protons and neutrons present in the nucleus
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(ii) The maximum number of electrons that can be accommodated in the
outermost orbit is 8
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That is, the shells are filled in a step-wise manner
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Valency
The electrons present in the outermost shell of an atom are known as the valence
electrons
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If an atom’s
outermost shell is completely filled, they are inert or least reactive and their combining
capacity or valency is zero
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Atoms having 8
electrons in their outermost shell is having octet configuration and are stable
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It will try to attain stability by
losing, gaining or sharing electrons with other atoms to attain octet configuration
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Atomic structure of sodium(Na)

#Atomic Number and Mass Number
The Atomic number is defined as the total number of protons present in the nucleus of
an atom
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It is
denoted by ‘Z’
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Hence protons and neutrons are also called nucleons
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The mass number is defined as the sum of the total number of
protons and neutrons present in the nucleus of an atom
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The chemical properties of isotopes are similar but their
physical properties are different
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If an element has no isotopes, then the mass of its atom would be the same as the sum of

protons and neutrons in it
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But some isotopes have special properties which
find them useful in various fields
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(ii) An isotope of cobalt is used in the treatment of cancer
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#ISOBARS
Isobars are atoms of different elements having different atomic numbers but same mass
numbers
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Eg :- Calcium (Ca) – atomic number - 20 and Argon (Ar) – atomic number 18 have different
atomic numbers but have the same mass numbers – 40
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These rays consist of positively charged particles
known as protons
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Question 2: If an atom contains one electron and one proton, will it carry any charge or not?
An electron is a negatively charged particle, whereas a proton is a positively charged particle
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Therefore, an atom containing one electron and one proton
will not carry any charge
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Question 3: On the basis of Thomson’s model of an atom, explain how the atom is neutral as
a whole?
According to Thomson’s model of the atom, an atom consists of both negatively and positively
charged particles
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These negative and positive charges are equal in magnitude
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Question 4: On the basis of Rutherford’s model of an atom, which subatomic particle is present in the
nucleus of an atom?
On the basis of Rutherford’s model of an atom, protons (positively-charged particles) are
present in the nucleus of an atom
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Question 6: What do you think would be the observation if the α-particle scattering experiment is
carried out using a foil of a metal other than gold?
If the α-scattering experiment is carried out using a foil of a metal rather than gold, there would
be no change in the observation
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It is difficult to make such foils from
other metals
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Answer:
The three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons
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How many neutrons
does it have?
Helium atom has two neutrons
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Since helium atom has two protons, mass contributed by the two protons is
(2 × 1) u
= 2 u
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Question 9: If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the
atomic number of the atom and (ii) what is the charge on the atom?
(i) The atomic number is equal to the number of protons
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(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is
0
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J
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J
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However, it was later found that the positively charged particles
reside at the centre of the atom called the nucleus, and the electrons revolve around the nucleus
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By Z = 3, we mean that the atomic number of the element is 3
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Hence, the
valency of the element is 1 (since the outermost shell has only one electron)
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Question 12: Composition of the nuclei of two atomic species X and Y are given as under X Y
Protons = 6, 6
Neutrons = 6, 8
Give the mass numbers of X and Y
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Hence, they are isotopes

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