Q: What is the difference between atomic number and mass number?
A: The atomic number of an atom is the number of protons present in its nucleus. It defines the identity of the element. For example, hydrogen has an atomic number of 1 because it has one proton.
The mass number, on the other hand, is the total number of protons and neutrons in the nucleus. Since neutrons add mass but not charge, the mass number helps us understand the isotope of an element. For instance, carbon-12 and carbon-14 both have the same atomic number (6 protons), but different mass numbers (12 and 14) because of the number of neutrons.
In short:
The atomic number of an atom is the number of protons in its nucleus, while the mass number is the total number of protons and neutrons. That is:
- Atomic number (Z) = number of protons
- Mass number (A) = protons + neutrons
Do You Know?
Did you know that all atoms of gold have 79 protons? No matter how many neutrons they contain, if an atom has 79 protons, it will always be gold. That’s why the atomic number is considered the “fingerprint” of an element.
Frequently Asked Questions (FAQ)
Q1. Can two elements have the same atomic number?
No. The atomic number is unique for each element and defines its identity.
Q2. Why is the mass number not unique like the atomic number?
Because atoms of the same element can have different numbers of neutrons, leading to different isotopes with varying mass numbers.
Q3. Which is more important in the periodic table: atomic number or mass number?
The periodic table is arranged according to atomic number, not mass number.
Q4. Can two different elements have the same mass number?
Yes, this can happen. Such atoms are called isobars (e.g., Argon-40 and Calcium-40).