Isotopes, Isobars, Isotones & Isodiaphers - Definitions, Examples, Practice Problems & FAQs

Have you ever heard of the term “heavy water”?

Well, the chemical formula of heavy water is exactly the same as normal water. It's just that two different types of isotopes of hydrogen are present in heavy water and normal water.

What happens if we drink heavy water (D₂O) instead of normal water (H₂O)?

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Up to a certain level it is fine but a high amount of D₂O intake can be dangerous for life. D₂O and H₂O both have different physical properties like density, viscosity, melting point, boiling point etc.

Let’s dive deeper into some of the concepts related to isotopes, isotones, isobars and isodiaphers.

Table of Contents

Isotopes
Isobars
Isotones
Isodiaphers
Practice Problems
Frequently Asked Questions-FAQs

Isotopes

Isotopes are the elements that have the same atomic number (number of protons) but different mass number (number of nucleons). Atomic numbers of two species are the same but differ only in terms of mass (mass number) so we can say that they differ only in terms of the number of neutrons because the number of protons and electrons (for neutral species) are the same.

$C_{6}^{12}, C_{6}^{13} & C_{6}^{14}$ are isotopes, having the same atomic number (number of protons) but different mass numbers.

Mass number of $C_{6}^{12}$ = 12, Number of neutrons in $C_{6}^{12}$ = 12 - 6 = 6
Mass number of $C_{6}^{13}$ = 13, Number of neutrons in $C_{6}^{13}$ = 13 - 6 = 7
Mass number of $C_{6}^{14}$ = 14, Number of neutrons in $C_{6}^{14}$ = 14 - 6 = 8

Other example of isotopes is $H_{1}^{1}$ (hydrogen or protium), $H_{1}^{2}$ (deuterium) & $H_{1}^{3}$ (tritium). These are the three isotopes of hydrogen element.

Isobars

Elements that have the same mass number (number of nucleons) but different atomic numbers (number of protons).

For example:

$K_{19}^{40} & c a_{20}^{40}$ & Ca2040; Both are isobars, having the same mass number (number of nucleons) but different atomic number (number of protons).

Number of neutrons in $K_{19}^{40}$ = 40-19 =21
Number of neutrons in $c a_{20}^{40}$ = 40-20 =20

Isotones

Isotones are elements that have the same number of neutrons but can have different atomic numbers (number of protons) and mass numbers (number of nucleons).

$o \frac{16}{8}$ & $c \frac{14}{6}$ ; Both are isotones, having the same number of neutrons.

Number of neutrons in $o \frac{16}{8}$ = 16-8 =8

Number of neutrons in $c \frac{14}{6}$ = 14-6 = 8

Isodiaphers

Species have different numbers of protons and neutrons, but the difference between the neutrons and protons is the same (same neutron excess) are called isodiaphers.

For example:

${T h}_{90}^{234}$ & $u_{92}^{238}$ ; are isodiaphers.

${T h}_{90}^{234}$ ; number of protons = 90 and number of neutrons = 234-90 =144

Difference between neutrons and protons (neutron excess ) = 144-90=54

$u_{92}^{238}$ ; number of protons = 92 and number of neutrons = 238-92 =146

Difference between neutrons and protons (neutron excess ) = 146-92=54

We can represent isodiaphers having same |n-p| or |m-2z| value.

Where, n = number of neutrons, p = number of protons, m = mass number, z = atomic number

Practice Problems

Q 1. The total number of protons present in one CO32- ion is:

Answer: (A)

Solution:

Number of protons in one atom of C (C612) = 6

Number of protons in one atom of O (O816) = 8

Total number of protons in 1 molecule of CO32- = 6 + (38) = 30

So, option A is the correct answer.

Q 2. The number of neutrons in cuprous ion with mass number 63 is:

Answer: (A)

Solution:

Cu+ ioncontains 29 protons and (63 - 29) = 34 neutrons.

(Cuprous ion is Cu+ and cupric ion is Cu2+).

Cu2963 contains 29 electrons but Cu+ ionhas 28 electrons and Cu2+ ionhas 27 electrons.

So, option A is the correct answer.

Q 3. Which pair represents isotopes?

O816 & C614
O816 & N714
C612 & C614
None of these

Answer: (C)

Solution:

Isotopes are species having same atomic number (number of protons) but different mass number.

C614 & C614 have the same atomic number but different mass number.

So, option C is the correct answer.

Q 4. The pair of nuclei that is isotonic:

N715 & F917
C612 & F919
C614 & F919
N715 & F919

Answer: (A)

Solution:

Isotones are the elements having the same number of neutrons.

N715; number of neutrons = 15-7 = 8

F917number of neutrons = 17-9 = 8

So, option A is the correct answer.

Q 5. Calculate the total number of neutrons present in 1 mole of hydrogen gas.

Answer: (A)

Solution:

Number of neutrons in one atom of H (H11) = 0

Total number of neutrons in 1 mole of H2 = 0

So, option A is the correct answer.

Frequently Asked Questions-FAQs

Q. Why atomic mass of different elements are fractions like Cl35.5?

Answer: Chlorine has two major isotopes having atomic masses 35 𝜇 and 37 𝜇.

The natural percentage abundance of isotopes of chlorine = Cl37 (25 %) and Cl37 (75 %)

So, if the atomic mass of chlorine is to be calculated, it will come out to be fractional.

Atomic mass of chlorine =25 × 37 + 75 × 35 25 + 75=35.5 𝜇

Q. Write the formula used for the calculation of the weighted average atomic mass of any element?

Answer:

weighted average atomic mass = [(% of 1st isotopes mass number) + (% of 2nd isotopes mass number) + ...........]100

Using the above formula, the average atomic mass of elements can be calculated. So, while calculating the atomic mass of elements, all the isotopes of a particular element are considered.

Q. Protium and Deuterium both have the same electronic configurations. Write common and different properties of H2 & D2?

Answer: Since hydrogen and deuterium both have the same electronic configuration, they show similar

chemical properties but differ in their rates of reaction (due to different bond dissociation energy). Both have different physical properties (like melting point, boiling point, density, etc) due to their different mass.

Q. What are the percentage abundances of isotopes of hydrogen?

Answer: There are three isotopes of hydrogen: H11 (hydrogen or protium), H12 (deuterium) & H13 (tritium).

The percentage abundances of these isotopes are: