Question

The mass number of an atom is 15 and its atomic number is 7. Now, this atom absorbs an \( \alpha \)-particle and emits a proton. What will be the mass number of changed atom?

• A. 16
• B. 18
• C. 17
• D. 15

Hint:

Track mass number carefully: \(+4\) for \(\alpha\)-particle, \(-1\) for \(\beta^-\)-particle.

Answer 1

The Correct Option is B

To determine the mass number of the changed atom, let's analyze each step carefully:

Original Atom Details:

• Mass Number = 15 
• Atomic Number = 7

Absorption of an \( \alpha \)-particle:

An \( \alpha \)-particle, which is a helium nucleus, consists of 2 protons and 2 neutrons. Therefore, when an atom absorbs an \( \alpha \)-particle, its mass number increases by 4 (due to the 2 protons and 2 neutrons), and its atomic number increases by 2 (due to the 2 protons).

• New Mass Number = 15 + 4 = 19
• New Atomic Number = 7 + 2 = 9

Emission of a Proton:

When the atom emits a proton, its atomic number decreases by 1 (due to the loss of 1 proton), and its mass number decreases by 1.

• New Mass Number = 19 - 1 = 18
• New Atomic Number = 9 - 1 = 8

Conclusion:

Therefore, the mass number of the changed atom, after absorbing an \( \alpha \)-particle and emitting a proton, is 18.

The correct answer is 18.