Isobars – Definition, Examples, Uses, Practice Problems and FAQ

Have you ever observed in international cricket generally balls of two different colours are used and both have the same mass?

Despite having the same mass, international cricket often uses red and white balls for various game forms.

For One-Day Internationals and Twenty-20 Internationals, the white ball is typically utilised. Since these shorter game types are played during the day and at night, white balls are used for improved visibility. For the Test format, which is exclusively played during the day, the red ball is used. Both balls differ from one another in a few ways. Red balls are more durable than white balls, however, the white ball swings more than red balls do.

Similar to how the two different types of balls used in cricket have the same mass but different characteristics, there are species that have the same mass number but different atomic numbers. These species are called ‘Isobars’.

We will get to know more about isobars on this concept page.

TABLE OF CONTENTS

Isobars – Definition
Isobars – Examples
Isobars – Uses
Practice Problems
Frequently Asked Questions – FAQ

Isobars – Definition

Elements that have the same mass number i.e. number of nucleons (neutrons and protons) but a different atomic number (number of protons or number of electrons) are called isobars. Isobars have the same physical properties as their mass numbers are the same. Isobars have the same total number of nucleons (protons and neutrons) but a different number of protons. In a neutral species, the number of protons is equal to the number of electrons and the electrons decide the chemical properties of any species. So, isobars have different chemical properties.

Isobars – Examples

${A r}_{18}^{40}$ , $K_{19}^{40}$ and ${C a}_{20}^{40}$ are isobars, having the same mass number (number of nucleons) but different atomic number (number of protons).

The number of neutrons in ${A r}_{18}^{40}$ = 40-18 =22

The number of neutrons in $K_{19}^{40}$ = 40-19 =21

The number of neutrons in ${C a}_{20}^{40}$ = 40-20 =20

The numbers 18, 19 and 20 represent the atomic number of the elements and the number 40 represents the mass number of the elements.

As the atomic number of the given three species differ, they exhibit different chemical properties.

Isobars – Uses

The following are some of the uses of isobars.

Uranium isobars can be used in nuclear reactors.

The isobars of iodine are used to treat goitre.

Cobalt isobars can be used to treat cancer.

It is possible to cure blood cancer with phosphorus isobars.

Practice Problems

Q1. Potassium and calcium both have different atomic numbers but the same mass number. The name given to such pairs is

A. Isotopes
B. Isobars
C. Isotones
D. Isodiaphers

Answer: B

Solution: Elements that have the same mass number (number of nucleons) but different atomic numbers (number of protons) are called isobars.

Element	Symbol	Atomic number	Mass number
Potassium	K	19	40
Calcium	Ca	20	40

So, option B is the correct answer.

Q2. Select the incorrect statements among the following.

A. Isobars are atoms of different elements
B. Isobars have different atomic numbers
C. Isobars have different chemical properties
D. All the above

Answer: D

Solution: Elements that have the same mass number (number of nucleons) but different atomic numbers (number of protons) are called isobars. Pairs showing isobaric phenomena have different chemical properties due to their different nuclear charge. As the atomic numbers of isobars are different, they are atoms of different elements.

So, option D is the correct answer.

Q3. Which of the following pairs are isobars?

$A . {A r}_{18}^{40} & K_{19}^{40} B . N_{7}^{14} & C_{6}^{12} C . {A r}_{18}^{40} & K_{19}^{41} D . N o n e o f t h e s e$

Answer: A

Solution: Out of the given pairs of atoms, Ar1840 & K1940 are isobars because they have different atomic number but the same mass number.

So, option A is the correct answer.

Q4. Isobars exhibit which of the following properties?

A. They have different numbers of protons
B. They have a different number of neutrons
C. They have different nuclear charges
D. All of these

Answer: D

Solution: Isobars have a different number of protons, neutrons and nuclear charges. For example, ${A r}_{18}^{40} & K_{19}^{40}$ are isobars because they have different atomic numbers but the same mass number.

Element	Symbol	Atomic number	Mass number	Number of protons	Number of neutrons
Argon	Ar	18	40	18	40 - 18 = 22
Potassium	K	19	40	19	40 - 19 = 21

Nucleus of an atom contains protons and neutrons. Neutrons are neutral in nature only protons are positive in charge. Isobars have different protons, so they have different nuclear charges.

So, option D is the correct answer.

Frequently Asked Questions – FAQ

Q1. Why is the atomic mass of some elements, like $C l^{35.5}$ , fractional?
Answer: Chlorine has two major isotopes having atomic masses 35 and 37. The natural abundance of isotopes of chlorine is Cl37 (25 %) and Cl37 (75 %). So, chlorine has fractional atomic mass.

Q2. What is the half-life of elements?
Answer: Half-life is the time span over which half of the constituent atoms in a particular sample convert from an unstable isotope into another through nuclear decay.

For example, the half-life of C¹⁴ is 5730 years, the half-life of Zn⁶⁹ is 57 minutes, and the half-life of O15 is 122.2 seconds.

Q3. What is the unit of mass number?
Answer: The mass number is unitless because it is the total number of nucleons present in the atomic nucleus, but the atomic mass is represented in amu or ‘u’.

Q4. Why is the mass of electrons not considered during the calculation of atomic mass?
Answer: The mass of a single electron is approximately 11837 times lighter than the mass of the proton. The mass of a neutron is higher than the mass of a proton but for numerical simplicity, we treat the mass of a proton as equal to the mass of the neutron.