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Practice each question, then open the answer or worked solution.
The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes 8X16 and 8X18 in the sample?
Let the percentage of 8X16 be ‘a’ and that of 8X18 be ‘100-a’.
As per the given data,
16.2u = 16 a / 100 + 18 (100-a) /100
1620 = 16a + 1800 – 18a
1620 = 1800 – 2a
a = 90%
Hence, the percentage of the isotope in the sample 8X16 is 90% and that of
8X18 = 100-a = 100- 90=10%
If Z=3, what would be the valency of the element? Also, name the element.
Given: Atomic number, Z = 3
The electronic configuration of the element = K-2; L-1, hence its valency = 1
The element with atomic number 3 is Lithium.
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?
Mass number of X: Protons + neutrons = 6+6 = 12
Mass number of Y: Protons + neutrons = 6+8 = 14
They are the same element, and their atomic numbers are the same.
They are isotopes, as they differ in the number of neutrons and hence their mass numbers.
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.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of a proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
(a) The statement is False.
(b) The statement is False.
(c) The statement is True.
(d) The statement is False.
Put a tick(✓) against the correct choice and cross(x) against the wrong choice in questions 15, 16 and 17.
Rutherford’s alpha–particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
Isotopes of an element have
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers
(c) Different number of neutrons
Number of valence electrons in Cl– ion are
(a) 16
(b) 8
(c) 17
(d) 18
(b) 8
The electronic distribution of Cl is K-2, L-8, M-7. Valence electrons are 7; hence, chlorine gains one electron for the formation of Cl–. Therefore, its valency is 8.
Which one of the following is a correct electronic configuration of Sodium?
(d) 2, 8, 1
Complete the following table.
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of the Atomic Species |
| 9
16 – – – |
–
32 24 2 1 |
10
– – – 0 |
–
– 12 1 1 |
–
– – – 0 |
–
Sulphur – – – |
The following table depicts the missing data:
Atomic number(Z) = Number of protons
Mass number = Number of neutrons + atomic number
(or)
Mass number(A) = Number of neutrons + number of neutrons
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of the Atomic Species |
| 9
16 12 1 1 |
19
32 24 2 1 |
10
16 12 1 0 |
9
16 12 1 1 |
9
16 12 1 0 |
Fluorine
Sulphur Magnesium Deuterium Hydrogen |