Chapter 5 – Inside the Atom
1. Answer the following.
a. What is the difference in the atomic models of Thomson and Rutherford?
Ans:
| Atomic model of Thomson | Atomic model of Rutherford |
|---|---|
(a) According to Thomson's atomic model, the model of an atom was proposed to be similar to that of a Christmas pudding wherein the electrons are studded like currants in a positively charged sphere of the pudding. | (a) According to Rutherford's atomic model, an atom consists of a positively charged centre called the nucleus while the negatively charged electrons revolve around the nucleus in circular orbits. |
(b) He also proposed that charge and mass are uniformly distributed over the atom and that the atoms are electrically neutral. | (b) The total positive charge on the nucleus is equal in magnitude to the total negative charge on the electrons and since the opposite charges are balanced, the atom is electrically neutral. |
b. What is meant by valency of an element? What is the relationship between the number of valence electron and valency?
Ans:
(i) The number of chemical bonds formed by an atom in order to complete the duplet or octet is called the valency of that element.
(ii) The valency of an atom is determined by the configuration of its outermost valence shell, and the electrons present in this valence shell are known as valence electrons.
(iii) This means that the number of valence electrons determines the valency of an atom of an element.
(iv) However, if an element has more than four valence electrons, the number of electrons by which the octet is short of completion is the valency of that element.
c. What is meant by atomic mass number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.
Ans:
(i) The atomic mass number (A) is defined as the sum of the number of protons and neutrons present in the nucleus of an atom of an element.
(ii) Carbon has 6 protons and 6 neutrons in its atomic nucleus. This means that the atomic mass number of carbon is 12, since
Atomic mass number
= number of protons + number of neutrons
= 6 + 6 = 12
(iii) The mass of an electron, being negligible, is not considered while calculating the atomic mass of an element.
(iv) Also, we know that the atomic number (Z) of an element is equal to the number of protons present in the nucleus of the atom of that element.
(v) Thus, the atomic number of carbon = the number of protons = 6.
d. What is meant by subatomic particle ? Give brief information of three subatomic particles with reference to electrical charge, mass and location.
Ans:
(i) The particles present inside an atom, i.e., inside the nucleus and extranuclear part of an atom, are called the subatomic particles.
(ii) There are three types of subatomic particles in an atom: the proton, neutron, and electron.
(iii) The properties of electrons, protons, and neutrons can be compared as follows:
| Property | Electrical charge | Mass | Location/Position |
|---|---|---|---|
Electron | It is negatively charged and carries a negative charge of -1e. | Its mass is about 1/1800 times that of a hydrogen atom. | It is present outside the nucleus, i.e., in orbits. |
Proton | Its mass is about 1/1800 times that of a hydrogen atom. | Its mass is approximated or taken strictly as 1 u or Dalton. | It is present outside the nucleus, i.e., in orbits. |
Neutron | It is neutral. | Its mass is approximated or taken strictly as 1 u or Dalton. | It is present inside the nucleus. |
2. Give scientific reasons.
a. All the mass of an atom is concentrated in the nucleus.
Ans:
(i) The mass of an atom is due to the sum of the numbers of two subatomic particles, namely, protons and neutrons.
(ii) The mass of an electron is very small (1800 times less than the mass of a hydrogen atom), and hence, it can be ignored.
(iii) Both protons and neutrons are present in the nucleus of an atom.
(iv) Hence, all the mass of an atom is concentrated in the nucleus of the atom.
b. Atom is electrically neutral.
Ans:
(i) An atom consists of a nucleus and an extranuclear part.
(ii) The nucleus of an atom consists of protons, which are positively charged, and neutrons, which do not carry any charge.
(iii) The negatively charged electrons in the extranuclear part revolve in discrete orbits around the nucleus.
(iv) In an atom, the number of electrons in the extranuclear part is equal to the number of protons inside the nucleus, thus balancing the opposite charges.
(v) Hence, an atom is always electrically neutral.
c. Atomic mass number is a whole number.
Ans:
(i) The atomic mass number (A) is defined as the sum of the number of protons and neutrons present in the nucleus of an atom of an element.
(ii) The atomic mass number is always a whole number because protons and neutrons are always present in whole numbers and never in fractions.
d. Atoms are stable though negatively charged electron are revolving within it.
Ans:
(i) An atom consists of a nucleus and a broad extranuclear part.
(ii) According to Rutherford’s atomic model, the nucleus consists of positively charged protons and neutrons, which are neutral, while the extranuclear part consists of negatively charged electrons, which revolve in discrete circular orbits around the nucleus.
(iii) Any particle in a circular orbit would radiate energy, resulting in the instability of the atom.
(iv) However, while revolving in discrete orbits, the electrons do not radiate energy, as each electron has a specific constant energy depending upon the orbit in which it revolves.
(v) Hence, atoms are stable even though negatively charged electrons are revolving around them.
3. Define the following terms
a. Atom
Ans: An atom is defined as the smallest particle of an element which retains its chemical identity in all the physical and chemical changes.
b. Isotope
Ans: Atoms of the same element having same atomic number but different mass numbers, are called isotopes.
c. Atomic number
Ans: Atomic number (Z) is defined as the number of protons present in the nucleus of an atom of an element.
d. Atomic mass number
Ans: Atomic mass number (A) is defined as the sum of the number of protons and neutrons present in the nucleus of an atom of an element.
e. Moderator in nuclear reactor
Ans: A moderator is a medium used in a nuclear reactor to slow down the fast moving neutrons emitted during a nuclear fission reaction.
4. Draw a neat labelled diagram.
a. Rutherford’s scattering experiment
Ans:
b. Thomson’s atomic model
Ans:
c. Diagramatic sketch of electronic configuration of Magnesium (Atomic number 12)
Ans:
d. Diagramatic sketch of electronic configuration of Argon (Atomic number 18)
Ans:
5. Fill in the blanks.
a. Electron, proton, neutron are the types of ________ in an atom.
Ans: subatomic particles
b. An electron carries a ________ charge.
Ans: negative
c. The electron shell ________ is nearest to the nucleus.
Ans: K
d. The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is ________.
Ans: M-shell
e. The valency of hydrogen is ‘one’ as per the molecular formula H2O. Therefore valency of ‘Fe’ turns out to be as per the formula Fe₂O₃
Ans: three
6. Match the pairs.
| Group ‘A’ | Group ‘B’ |
|---|---|
a. Proton | i. Negatively charged |
b. Electron | ii. Neutral |
c. Neutron | iii. Positively Charged |
Ans:
| Group ‘A’ | Group ‘B’ |
|---|---|
a. Proton | iii. Positively Charged |
b. Electron | i. Negatively charged |
c. Neutron | ii. Neutral |
7. Deduce from the datum provided.
| Datum | To deduce |
|---|---|
\({}_{11}^{23}\mathrm{Na}\) | Neutron number |
\({}_{14}^{6}\mathrm{C}\) | Mass number |
\({}_{37}^{17}\mathrm{Cl}\) | Proton number |
Ans:
(i) To deduce neutron in \({}_{11}^{23}\mathrm{Na}\)
Mass number = 23
Atomic number = 11
Number of protons = Atomic number = 11
Mass number = Number of protons + Number of neutrons
∴ Number of neutrons = Mass number – Number of protons
∴ Number of neutrons = 23 – 11
∴ Number of neutrons = 12
(ii) To deduce mass number in \({}_{14}^{6}\mathrm{C}\)
Mass number = 14
(ii) To deduce proton number in \({}_{37}^{17}\mathrm{Cl}\)
Atomic number = 17
Number of protons = Atomic number = 17