Question

Calculate the mass of a helium nucleus.

[Mass of proton is $1.0078u$ and mass of a neutron is $1.0087u$]

• A. $4.0330u$
• B. $2.0330u$
• C. $1.0330u$
• D. $5.0330u$

Answer 1

The correct option is A $4.0330u$

Consider a helium atom ${}_2{He}^4$.
It has 2 electrons, 2 protons and 2 neutrons.
The mass of a proton $=1.0078u$; The mass of a neutron $=1.0087u$.
$\therefore$ The total mass of the nucleus $=2\times 1.0078+2\times 1.0087=4.0330u$