Chemistry (Part-1)

Isobars are atoms of different elements that have different atomic numbers (number of protons) but the same mass number (total number of protons and neutrons).

Isobars are atoms of different elements that have different atomic numbers (number of protons) but the same mass number (total number of protons and neutrons).

Calcium (Ca) and Argon (Ar) are isobars because their atomic masses are the same i.e. 40. The atomic number of Calcium is 20, while that of Argon is 18.

Chemical properties of an element are primarily determined by the number and arrangement of its electrons. Isotopes of the same element have the same number of protons, and in a neutral atom, they also have the same number of electrons.

This identical electron configuration leads to virtually identical chemical behavior.

While there can be very subtle differences in reaction rates due to the mass difference (kinetic isotope effects), for most practical purposes and in general chemistry, isotopes of the same element are considered to have the same chemical properties.

Atoms with the same atomic number have similar chemical properties, while isotopes have different physical properties. Atoms that have the same atomic number are placed in the same place in the periodic table.

Isotopes of the same element have the same number of protons or electrons. Their chemical properties are also similar, but their physical properties are different.

Protium (¹$H₁$), Deuterium (²$H₁$ or D), and Tritium (​³H₁ or T) are the naturally occurring isotopes of  Hydrogen.

All three have the same atomic number (1), meaning they each have one proton. However, they differ in their number of neutrons:

Protium: 0 neutrons
Deuterium: 1 neutron
Tritium: 2 neutrons

This difference in neutron number leads to their different mass numbers (1, 2, and 3 respectively).

The mass number of an atom is equal to the sum of the number of protons and neutrons present in its nucleus.
mass number = number of protons + number of neutrons

Hydrogen (Protium - the most common isotope): 1 proton, 0 neutrons. 

Deuterium (an isotope of Hydrogen): 1 proton, 1 neutron.

Iodine (most common isotope Iodine-127): 53 protons, 74 neutrons ($127-53=74$).  

Chlorine (most common isotope Chlorine-35): 17 protons, 18 neutrons ($35-17=18$).

Therefore, (b) Deuterium has an equal number of neutrons and protons.

Scandium is a chemical element with the symbol Sc, whose atomic number is 21. It is the element of the d-block. There are 13 isotopes of scandium among the given options.