Compare the properties of electrons, protons and neutrons.

More Exercises Questions

2. What are the limitations of J.J. Thomson’s model of the atom?

It couldn’t explain how negatively charged electrons were held within a positively charged sphere. According to electrostatics, they should repel each other.

It didn’t explain about the positively charged nucleus in the center of the atom.

The model couldn’t explain the results of Ernest Rutherford’s gold foil experiment, where some alpha particles deflected significantly, indicating a very small, dense core

3. What are the limitations of Rutherford’s model of the atom?

Electron orbits: It didn’t specify how electrons moved around the nucleus. Classical physics predicted they would lose energy and spiral into the nucleus.

Light emission: It couldn’t explain how atoms emit specific frequencies of light.

4. Describe Bohr’s model of the atom.

Quantized energy levels: Electrons occupy specific energy levels or shells around the nucleus, not random orbits.

Stable orbits: Electrons in these allowed energy levels are stable and don’t radiate energy.

Light emission: When an electron jumps from a higher to a lower energy level, it emits a specific frequency of light, explaining atomic spectra.

5. Compare all the proposed models of an atom given in this chapter.

Dalton’s Model (1803):

Dalton’s model proposed that atoms were indivisible and identical spheres. According to this model, atoms of different elements had different masses and properties. He also suggested that atoms combine in simple whole-number ratios to form compounds.

Key Points:

Atoms are indivisible and cannot be broken down into smaller particles.

All atoms of a given element are identical in mass and properties.

Atoms combine in fixed ratios to form compounds.

Chemical reactions involve the rearrangement of atoms.

Thomson’s Model (1897):

DescriptionThomson’s model, also known as the “plum pudding” model or the “raisin bun” model, proposed that atoms consist of a positively charged sphere with negatively charged electrons embedded within it, much like raisins in a pudding or plums in a bun.

Key Points:

The atom is mostly positively charged.

Electrons are embedded within this positively charged sphere.

The model does not specify the arrangement of electrons within the atom.

Rutherford’s Model (1911):

Rutherford’s model arose from the famous gold foil experiment. He discovered that atoms have a dense, positively charged nucleus at their center, surrounded by negatively charged electrons. He proposed that electrons orbit the nucleus like planets orbit the sun.

Key Points:

Most of the atom’s mass and positive charge is concentrated in the nucleus.

Electrons orbit the nucleus in a manner similar to planets orbiting the sun.

The nucleus is very small compared to the overall size of the atom.

Bohr’s Model (1913):

Bohr’s model built upon Rutherford’s nuclear model but introduced the concept of quantized electron orbits. Bohr suggested that electrons revolve around the nucleus in fixed energy levels or shells. Electrons could jump between these energy levels by absorbing or emitting specific amounts of energy, leading to the explanation of atomic spectra.

Key Points:

Electrons move in quantized orbits around the nucleus.

Each energy level or shell corresponds to a specific energy value.

Electrons can absorb or emit energy to move between energy levels.

The model successfully explained the spectral lines observed in hydrogen.

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Electrons fill the lowest energy levels or shells first before occupying higher energy levels.

The maximum number of electrons in a shell is given by the formula 2n², where ‘n’ is the principal quantum number of that shell.

The electron distribution for the first eighteen elements follows the pattern: 2, 8, 8 for the first, second, and third energy levels (or shells) respectively.

7. Define valency by taking examples of silicon and oxygen.

Valency is the combining capacity of an atom, expressed by the number of electrons it gains, loses, or shares to complete its valence shell and achieve stability.

Silicon (Si): Silicon has four valence electrons, so its valency is 4. It can either lose four electrons to form Si⁴⁺ ions or gain four electrons to form Si⁴⁻ ions.

Oxygen (O): Oxygen has six valence electrons, so its valency is 2. It tends to gain two electrons to complete its valence shell and form O²⁻ ions.

8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes.

Atomic Number: The atomic number of an element is the number of protons in its nucleus. For example, the atomic number of oxygen (O) is 8, indicating it has 8 protons.

Mass Number: The mass number is the sum of the number of protons and neutrons in the nucleus of an atom. For example, for carbon (C), with 6 protons and 6 neutrons, the mass number is 12.

Isotopes: Isotopes are atoms of the same element having the same atomic number but different mass numbers due to varying numbers of neutrons. For example, carbon-12 (¹²C) and carbon-14 (¹⁴C) are isotopes of carbon.

Isobars: Isobars are atoms of different elements that have the same mass number but different atomic numbers. For example, carbon-14 (¹⁴C) and nitrogen-14 (¹⁴N) are isobars.

Uses of Isotopes:

Radioactive Dating: Isotopes such as carbon-14 are used to determine the age of ancient materials through radioactive decay.

Medical Imaging: Isotopes like technetium-99m are used in medical imaging techniques like PET scans for diagnosis and treatment monitoring.

9. Na+ has completely filled K and L shells. Explain.

Sodium (Na) is an element with an atomic number of 11, meaning it has 11 protons. The electronic configuration of sodium is 2-8-1, indicating that it has two electrons in its first energy level (K shell), eight electrons in its second energy level (L shell), and one electron in its outermost energy level.

When we say that Na+ has completely filled K and L shells, we’re referring to the ion formed when sodium loses one electron to achieve a stable electron configuration. In the case of Na+, it loses its outermost electron, leaving it with a configuration of 2-8. Now, its K and L shells are completely filled. This filled configuration is more stable, as it resembles the nearest noble gas, neon (Ne), which has a stable electron configuration of 2-8.

So, the Na+ ion has completely filled K and L shells because it has lost its outermost electron to achieve a stable electron configuration.

10. If bromine atom is available in the form of, say, two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom.

11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?

Let’s denote the percentages of ¹⁶₈X and ¹⁸₈X as x% and (100 – x)%, respectively. We can set up an equation based on the weighted average of the isotopic masses:

Mass of ¹⁶₈X = 16 amu (common mass for most isotopes of Oxygen)

Mass of ¹⁸₈X = 18 amu

Average atomic mass = 16.2 amu (given in the problem)

Using: Average atomic mass = f1M1 + f2M2 +… + fnMn (where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.)

(x/100) * (mass of ¹⁶₈X) + [(100 – x)/100] * (mass of ¹⁸₈X) = Average atomic mass

(x/100) * (16 amu) + [(100 – x)/100] * (18 amu) = 16.2 amu

16x + 1800 – 18x = 1620 (combining like terms)

-2x = -180

x = 90 (solving for x, the percentage of ¹⁶X)

Therefore:

Percentage of ¹⁶₈X= 90%

Percentage of ¹⁸X = (100 – 90)% = 10% ¹⁸₈XPercentage of ¹⁸X = (100 – 90)% = 10%

The sample of element X is composed of 90% ¹⁶₈X and 10% ¹⁸₈X isotopes.

12. If Z = 3, what would be the valency of the element? Also, name the element.

Ans: If Z = 3, then the element is Lithium (Li).

The atomic number (Z) represents the number of protons in the nucleus, which equals the number of electrons in a neutral atom. In Lithium’s case, with Z = 3, it has 3 protons and 3 electrons.

13. 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. What is the relation between the two species?

The mass number (A) of an atom is the sum of its protons (Z) and neutrons (N).

For species X:

Protons (Z) = 6

Neutrons (N) = 6

Mass number (A) = Z + N = 6 + 6 = 12

For species Y:

Protons (Z) = 6

Neutrons (N) = 8

Mass number (A) = Z + N = 6 + 8 = 14

So, the mass numbers of species X and Y are 12 and 14, respectively.

The relation between the two species is that they have the same number of protons (6), indicating they are both atoms of the same element. However, they have different numbers of neutrons, which means they are isotopes of the same element. Therefore, X and Y are isotopes of the same element.

14. For the following statements, write T for True and F for False.

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.  False

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. False

(c) The mass of an electron is about 1/2000 times that of proton. True

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. True

Put tick (✓) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17

15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

(a) Atomic Nucleus (✓) (b) Electron (c) Proton (d) Neutron

16. Isotopes of an element have

(a) the same physical properties 

(b) different chemical properties

(c) different number of neutrons  (✓) 

(d) different atomic numbers.

17. Number of valence electrons in Cl– ion are:

(a) 16 (b) 8  (✓)  (c) 17 (d) 18

18. Which one of the following is a correct electronic configuration of sodium? (a) 2,8 (b) 8,2,1 (c) 2,1,8 (d) 2,8,1  (✓)

InText Questions With Answers

1. What are canal rays?

Canal rays, also known as positive rays, are positively charged ions that move towards the negative electrode (cathode) in a cathode ray tube. They were discovered by Eugen Goldstein in 1886. These rays are formed when the residual gas in the cathode ray tube is ionized by the high voltage applied across it. Canal rays helped scientists understand the existence of positively charged particles later identified as protons.

2. If an atom contains one electron and one proton, will it carry any charge or not?

An atom with one electron and one proton will not carry any charge and will be neutral. This is because an electron has a negative charge, while a proton has a positive charge, and the magnitude of each charge is equal. The two charges cancel each other out, resulting in a neutral atom.

3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Thomson’s model of the atom, known as the “plum pudding” model, proposed that the atom is a sphere of positive charge with electrons embedded within it, much like plums in a pudding. In this model, the positive charge of the atom is spread out uniformly throughout the atom, while the negatively charged electrons are scattered within it. The atom is neutral as a whole because the positive charge of the atom balances the negative charge of the electrons. Therefore, despite the presence of both positive and negative charges, the overall charge of the atom is zero.

4. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

According to Rutherford’s model of an atom, protons are present in the nucleus of an atom. Protons are positively charged particles. Rutherford discovered protons in 1919 during his gold foil experiment. He projected alpha particles (helium nuclei) at gold foil, and the positive alpha particles were deflected. This led Rutherford to conclude that protons exist in a nucleus and have a positive nuclear charge

5. Draw a sketch of Bohr’s model of an atom with three shells.

Bohr’s Model of an Atom with Three Shells

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 an alpha-particle scattering experiment is carried out using a metal foil other than gold, there would be no change in observations if the foil is as thin as the gold foil used by Rutherford. This is because the basic structure of an atom remains the same regardless of the material. However, other metals are not as malleable, so it is difficult to obtain such a thin foil. 

7. Name the three sub-atomic particles of an atom.

The three sub-atomic particles of an atom are:

Protons: Positively charged particles located in the nucleus.

Neutrons: Neutral particles located in the nucleus.

Electrons: Negatively charged particles that orbit the nucleus.

8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

To find the number of neutrons in a helium atom, you subtract the number of protons from the atomic mass.

Helium atomic mass = 4 u

Number of protons = 2

Number of neutrons = Atomic mass – Number of protons = 4 – 2 = 2

Therefore, helium atom has 2 neutrons.

9. Write the distribution of electrons in carbon and sodium atoms.

Carbon (Atomic number 6): Electron distribution – 2 electrons in the first shell (K shell) and 4 electrons in the second shell (L shell). Or 2, 4

Sodium (Atomic number 11): Electron distribution – 2 electrons in the first shell (K shell), 8 electrons in the second shell (L shell), and 1 electron in the third shell (M shell). Or 2, 8, 1

10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

If the K and L shells of an atom are full, it means that the first and second electron shells are filled. According to the electronic configuration rules, the maximum number of electrons in the K shell (first shell) is 2, and the maximum number of electrons in the L shell (second shell) is 8. Therefore, the total number of electrons in the atom would be 2 (from the K shell) + 8 (from the L shell) = 10 electrons.

11. How will you find the valency of chlorine, sulphur and magnesium?

Valency is the number of electrons that an atom can gain or lose to attain the nearest noble gas configuration. Chlorine, sulphur and magnesium are all elements in the periodic table. The valency of an element can be determined by looking at its electronic configuration.

• The electronic configuration of chlorine is 2, 8, 7. This means that chlorine has 7 electrons in its outermost shell. Chlorine needs to gain 1 electron to attain the nearest noble gas configuration, which is argon. Therefore, the valency of chlorine is 1.
• The electronic configuration of sulphur is 2, 8, 6. This means that sulphur has 6 electrons in its outermost shell. Sulphur needs to gain 2 electrons to attain the nearest noble gas configuration, which is argon. Therefore, the valency of sulphur is 2.
• The electronic configuration of magnesium is 2, 8, 2. This means that magnesium has 2 electrons in its outermost shell. Magnesium needs to lose 2 electrons to attain the nearest noble gas configuration, which is neon. Therefore, the valency of magnesium is 2.

12. 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?

If the number of electrons in an atom is 8 and the number of protons is also 8, then:

(i) The atomic number of the atom is equal to the number of protons in the nucleus. Since the number of protons is 8, the atomic number of the atom is also 8.

(ii) Since the number of electrons (negative charges) is equal to the number of protons (positive charges), the atom is electrically neutral. Therefore, the charge on the atom is zero.

13. Find out the mass number of oxygen and sulphur atom

• Oxygen (O) has an atomic number of 8 and usually has 8 neutrons in its nucleus, so its most common isotope has a mass number of 16 (8 protons + 8 neutrons).
• Sulphur (S) has an atomic number of 16. Its most common isotope has 16 neutrons, giving it a mass number of 32 (16 protons + 16 neutrons).

14. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.

Subatomic Particles in H, D, and T

15. Write the electronic configuration of any one pair of isotopes and isobars.

For electronic configuration, we only need to consider the electrons, which are not affected by the number of neutrons (isotopes) or the same mass number (isobars).

Isotopes Example:

Protium (H): 1s¹ (one electron in the first orbital)

Deuterium (D): 1s¹ (one electron in the first orbital, same as protium)

Isobars Example (Neon-20)

Neon-20 (Neon): 1s²2s²2p⁶ (ten electrons in total)

Magnesium-20 (Magnesium): 1s²2s²2p⁶ (ten electrons in total, same as Neon-20)

In both isotope and isobar examples, the number of electrons remains the same, so the electronic configuration is identical.