Species with the same atomic number but a different mass number are called isotopes, e.g. 1H1, 1H2, 1H3.
Species with the same mass number but a different atomic number are called isobars, e.g. 18Ar40, 19K40, 20Ca40 all of them contain the same mass number 40 but different atomic number i.e 18, 19, and 20.
Example of Isobars
Difference between Isotopes and Isobars
Species having the same number of neutrons are called isotones, e.g. 1H3 and 2He4 are isotones.
Species with the same isotopic number are called isodiaphers, e.g. 19K39, 19F9.
Isotopic number = mass number − × [2 atomic number]
Species with the same number of electrons are called isoelectronic species, e.g. Na+, Mg2+.
Q1: Isobars have the same number of ______?
Solution: The answer is option 4 (Nucleons). The number of protons and neutrons alone will vary in isobars but the number of nucleons or the sum of protons and neutrons will always be the same.
Q2: Which of the following statements about Isobars is false?
(A) Their atomic numbers vary from each other.
(B) Chemically they are the same element but their forms are different.
(C) Physical properties can be similar to each other.
(D) They are different chemical elements having the same atomic mass.
Solution: Statement 2 is false. Isobars are different elements altogether.
Q3: If the atomic number of sodium is 11. Find out how many electrons and protons are present in a sodium atom.
Solution: We know that “Atomic Number = No. of Protons = No. of Electrons”
Thus, number of electrons=11 and number of protons =11
Q4: What is the atomic number of chlorine?
Solution: The correct answer is “C”. The atomic number of chlorine is 17 because the number of protons in a chlorine atom is 17.
Q5: Isotopes of a single element vary in the number of
(D) All of the above
Solution: The answer is "A" (Neutrons).
The number of protons will always constant in a single element. However, the number of neutrons can change. The number of neutrons varies in isotopes of an element but the number of protons always remains the same.