Atomic Number (Z)
Atomic number of an element corresponds to the total number of protons present in the nucleus or the total number of electrons present in the neutral atom.
Mass Number (A)
The mass of the nucleus is due to protons and neutrons, thus they are collectively called nucleons. The total number of nucleons is termed as the mass number of the atom.
The mass number of an element = number of protons + number of neutrons
Representation of an Atom
Different Types of Atomic Species
(a) Isotopes Species with the same atomic number but different mass number are called isotopes, e.g. 1H1, 1H2.
Fig: Hydrogen isotopes
USES OF ISOTOPES:
Fig: Isotopes of Carbon
(b) Isobars Species with same mass number but different atomic number are called isobars, e.g. 18Ar40, 19K40.
(c) Isotones Species having same number of neutrons are called isotones, e.g. 1H3 and 2He4 are isotones.
(d) Isodiaphers Species with same isotopic number are called isodiaphers, e.g. 19K39, 19F9.
Isotopic number = mass number − × [2 atomic number]
(e) Isoelectronic Species with same number of electrons are called isoelectronic speices, e.g. Na+, Mg2+.
Question 1: 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 same.
Question 2: 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 same atomic mass.
Solution: The statement 2 is false. Isobars are different elements altogether.
Question 3: 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
Question 4: 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.
Question 5: Isotopes of a single element vary in the number of
(D) All of the above
Solution: The answer is 1 (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 same.