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Question 15 Marks
What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.
Answer
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 +1 e . ( $1 e =1.6 \times 10^{-19}$ 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).
$\left(1 u =1.66 \times 10^{-27} g\right.$ ) (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.
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Question 25 Marks
What is meant by atomic mass i number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.
Answer
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.
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Question 35 Marks
What is meant by valency of an element? What is the relationship between the number of valence electron and valency?
Answer
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. $H _2$, 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.
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Question 45 Marks
What is the difference in the atomic models of Thomson and Rutherford?
Answer
Thomson’s atomic modelRutherford’s atomic model
1. According to Thomson’s atomic model, the negatively charged electrons are embedded in a gel of positive charge.1. According to Rutherford’s atomic model the negatively charged electrons revolve around the nucleus.
2. Atom is homogenous sphere of positive charge.2. The positive charge is in the nucleus of the atom.
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Question 55 Marks
What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.
Answer
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 +1 e . $\left(1 e =1.6 \times 10^{-19}\right.$ 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).
( $1 u =1.66 \times 10^{-27} g$ ) (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.
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Question 65 Marks
What is meant by atomic mass i number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.
Answer
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.
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Question 75 Marks
What is meant by valency of an element? What is the relationship between the number of valence electron and valency?
Answer
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. $H _2$, 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.
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Question 85 Marks
What is the difference in the atomic models of Thomson and Rutherford?
Answer
Thomson’s atomic modelRutherford’s atomic model
1. According to Thomson’s atomic model, the negatively charged electrons are embedded in a gel of positive charge.1. According to Rutherford’s atomic model the negatively charged electrons revolve around the nucleus.
2. Atom is homogenous sphere of positive charge.2. The positive charge is in the nucleus of the atom.
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Answer the following question. - Science STD 8 Questions - Vidyadip