Which of the following pairs of 'number–composition' is correct?
I. Atomic number – Number of protons
II. Mass number – Sum of the number of neutrons and protons
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. Both I and II.
I. Atomic Number – Number of Protons:
- The atomic number of an element is defined as the number of protons present in the nucleus of an atom.
- It is unique to each element and determines its identity and position in the periodic table.
- Since atoms are electrically neutral, the atomic number also equals the number of electrons in a neutral atom.
II. Mass Number – Sum of Protons and Neutrons:
- The mass number is the total count of protons and neutrons in an atom’s nucleus. These two subatomic particles contribute almost all of the atom’s mass.
- For example, if an atom has 6 protons and 6 neutrons, its mass number is 12.
- Electrons are not included in the mass number because their mass is negligible compared to protons and neutrons.
Thus, both statements accurately describe important aspects of atomic structure essential to understanding chemical and physical properties of elements.