5 – Inside the Atom
Q 1
Rewrite the sentences after filling the blanks.
Ans.
(1) Electron, proton, neutron are
the types of subatomic
particles in an atom.
(2) An electron carries a negative charge.
(3) The electron shell K is nearest to the nucleus.
(4) The electronic configuration of magnesium is 2, 8, 2. From this it
is understood that the valence shell of
Magnesium is M.
(5) The valency of hydrogen is 'one' as per the molecular
formula H,O. Therefore valency of ‘Fe' turns out to be 3 as per the
formula Fe,O3.
Q. 2
Match the columns :
Ans.
(1) Proton - Positively charged
(2)
Electron - Negatively charged
(3)
Neutron – Neutral
Q 3. Define the terms:
Ans. (1) Atom : An atom is the smallest particle of an
element which retains its chemical identity in all physical and chemical
changes.
(2) Atomic number (Z): The number of electrons or protons in
an atom is called the atomic number. It is denoted by Z.
(3) Atomic mass number (A): The total number of protons and
neutrons in the nucleus of the atom is called the atomic mass number. It is
denoted by A.
(4) Isotopes.
Ans. Atoms of the same element having the same atomic
number, but different atomic mass numbers are called isotopes.
(5) Moderator.
Ans. The substance which reduces the speed of fast moving
neutrons produced in a fission is called a moderator.
Q 4. Deduce from the datum provided.
Datum To deduce
(1) Na Neutron number
(2) C Mass number
(3) Cl Proton number
23
Ans. (1) There are 12 neutrons in the sodium (11Na).
(N = A - Z) 23 - 11 = 12
(2) Atomic mass
number of C is 14.
(3) There are 17
protons in chlorine (CI)
Q. 5 Give scientific reasons :
(1) All the mass of an atom is concentrated in the nucleus.
Ans. (1) The nucleus of an atom contains protons and
neutrons.
(2) The electrons revolve around the nucleus.
(3) The mass of an electron is negligible compared to that
of a proton or a neutron.
(4) Hence, the mass of an atom depends mainly on the number
of protons and neutrons. Therefore, practically all the mass of an atom is
concentrated in the nucleus.
(2) Atom is
electrically neutral.
Ans. (1) An atom is made of two parts, viz. the nucleus and
the extranuclear part.
(2) The nucleus is positively charged. The positive charge
on the nucleus is due to protons.
(3) The extranuclear part of an atom is made of negatively
charged electrons.
(4) In an atom, the number of protons is equal to the number
of extranuclear electrons.
(5) The magnitude of the positive charge on the nucleus
equals the magnitude of the negative charge on the electrons. As the opposite
charges are balanced, the atom is electrically neutral.
(3) Atomic mass
number is a whole number.
Ans. (1) The total number of protons and neutrons in the
nucleus of the atom is called the atomic mass number (A).
(2) As protons and neutrons are whole numbers, the atomic
mass number is also a whole number.
(4) Atoms are
stable though negatively charged electron are revolving within it.
Ans. (1) The entire mass of the atom is concentrated in the
nucleus and the positively charged nucleus at centre of an atom.
(2) The negatively charged electrons revolve around the
nucleus.
(3) The total negative charge on all the electron is equal
to positive charge on the nucleus. As the opposite charges are balanced, the
atom is stable.
Q 5. Answer the following.
(1 What is the
difference in the atomic models of Thomson and Rutherford?
Ans. Thomson's atomic model Rutherford's atomic model
(1) According to Thomson's atomic model, the (1) According to Rutherford's atomic model
negatively charged electrons are embedded the negatively charged electrons
revolve
in a gel of positive charge. around the nucleus.
(2) Atom is homogenous sphere of positive (2) The positive charge is in the
nucleus of charge.
the atom.
(2) What is meant by
valency of an element? What is the relationship between the number of valence
electron and valency?
Ans. Valency: The
capacity of an element to combine with another element is known as valency.
Valence electrons : The electrons in the outermost shell of an atom of
an element are called valence electrons.
Helium and neon, atoms of both these gaseous element do not
combine with any other atom. These elements are chemically inert, i.e. their
valency is zero. Helium atom contains two electrons, indicates that the
outermost shell of helium has an electron duplet. The valence shell of neon is
completely filled, i.e. neon has an electron octet. Similarly argon contains
eight electrons in the valence shell, i.e. argon has an electron octet. It is
confirmed that the valency is zero when electron octet (or duplet) is complete.
Atoms of all the elements except inert gases have tendency
to combine with other atoms, i.e. they have a non zero valency. The molecules
formed by combination with hydrogen (E.g. H2, HCl) that valency of hydrogen is
one. The electronic configuration of hydrogen shows that there is one electron
less than the complete duplet state. This number 'one' matches with the valency
of hydrogen which is also one. It means that there is relationship between the
valency of an element and the number of electrons in its valence shell.
(3) What is
meant by atomic mass number? Explain how the atomic number and mass number of
carbon are 6 and 12 respectively.
Ans. The total number of protons and neutrons in the nucleus
of the atom is called the atomic mass number. The atomic number, i.e. the
proton number of carbon is 6 and the mass number is total number of protons and
neutrons in the carbon, i.e. 6 protons + 6 neutrons = 12. Therefore, the atomic
number and mass number of carbon are 6 and 12 respectively.
(4) What is meant by subatomic particle? Give brief
information of three subatomic particles with reference to electrical charge,
mass and location.
Ans. A particle which is a constituent of an atom hence
smaller than the atom is called subatomic particle.
An atom is formed from the nucleus and the extranuclear
part. These contain three types of subatomic particles.
The nucleus contains two types of subatomic particles
together called nucleons. Protons and neutrons are the two types of nucleons or
subatomic particles and electrons are subatomic particles in the extra nuclear
part.
(1) Proton (p): Proton is a positively charged subatomic
particle in the atomic nucleus. The positive charge on the nucleus is due to
the proton in it. A proton is represented by the symbol 'p'. Each proton
carries a positive charge of +le. (le = 1.6 x 10-19 coulomb). When total
positive charge on the nucleus is expressed in the unit 'e', its magnitude is
equal to the number of proton in the nucleus.
The mass of one proton is
approximately lu (1 Dalton).
(lu = 1.66 x 10-27g) (The mass of
one hydrogen atom is also approximately lu.)
(2) Neutron (n): Neutron is an electrically neutral
subatomic particle and is denoted by the symbol 'n'. The number of neutron in
the nucleus is denoted by the symbol 'N'. Atomic nuclei of all the elements
except hydrogen with atomic mass lu, contain neutrons. The mass of a neutron is
approximately lu, which is almost equal to that of a proton.
(3) Electron (e-) : Electron is a negatively charged
subatomic particle and is denoted by the symbol 'e-. Each electron carries one
unit of negative charge (-1e). Mass of an electron is 1800 times less than that
of a hydrogen atom. Therefore the mass of an electron can be treated as
negligible.
Electron in the extranuclear part revolve in the discrete
orbits around the nucleus. The energy of an electron is determined by the shell
in which it is present.
(1) Diagrammatic sketch of electronic configuration of argon
(Atomic number 18).
Ans.
(2) Diagrammatic sketch of electronic configuration of megnesium (Atomic number 12).
Ans
3. Ruththrford’s scattering experiment
Ans
4. Thomson’s atomic model
