Question

the atomic mass of helium - 4 is 4.0026 amu. how many of each particle (protons, electrons, neutrons) is present in an atom of helium - 4?
a 4 protons, 4 electrons, 2 neutrons
b 2 protons, 2 electrons, 4 neutrons
c 2 protons, 1 electron, 1 neutron
d 2 protons, 2 electrons, 2 neutrons

Explanation:

Step1: Recall atomic number of helium

Helium has an atomic number of 2. The atomic number is equal to the number of protons and in a neutral atom, the number of electrons is equal to the number of protons. So, number of protons = 2 and number of electrons = 2.

Step2: Calculate number of neutrons

The mass - number of helium - 4 is 4. Mass number = number of protons+number of neutrons. So, number of neutrons = mass number - number of protons = 4 - 2=2.

Answer:

D. 2 protons, 2 electrons, 2 neutrons