Question

An isotope of helium is represented by the symbol $_2^3He$. How many protons, neutrons and electrons are there in an atom of the isotope?
A. 1 proton, 2 neutrons, 1 electron
B. 1 proton, 3 neutrons, 2 electrons
C. 2 protons, 1 neutron, 2 electrons
D. 2 protons, 3 neutrons, 2 electrons

Answer 1

Isotopes are basically the elements with the same atomic number and different mass number. Different mass numbers are due to the different neutron numbers. For any atom, ion, element atomic weight = proton number + neutron number. This means the total number of neutrons and protons is equal to its atomic weight or mass number. Now the number of protons is equal to the number of electrons of a neutral atom.

Complete step by step answer:
For the isotope of $_2^3He$ the total number of protons is 2. Therefore, the neutron number can be calculated as follows,
Atomic weight = proton number + neutron number
or, 3=2 + neutron number
or, 3-2=2 neutron number
or, neutron number = 1
Now the number of electrons is equal to the proton number of that atom. So, the number of electrons is 2.

So, the correct option is C, 2 protons, 1 neutron, 2 electrons .

Additional information:
Nucleus of alpha ray is very similar to the Helium-4 nucleus. Its mass is due to the presence of two protons and two neutrons tightly bound together with no electron. It is a radiation emitted from the decay of various radioactive substances.
Alpha particles are produced during standard radioactive decay of radioactive elements. These are emitted in the form of radiation, having an average kinetic energy of 5MeV and velocity $5\%$ as compared to the speed of light.
Mass of an alpha particle is 4.00 amu, which is the mass of a doubly ionized helium atom. Thus, they are also known as doubly ionized helium atoms i.e., helium atoms with both the electrons removed.
Alpha decay can be written as:
$_{92}^{238}U \to _{90}^{234}Th + _2^4He$
Where, $_2^4He$is the alpha particle, denoted by Greek alphabet $\alpha$ .

Note:
Now, if electrons get removed from the atom a positive charge is developed. But the number of protons remains the same. The number of positive charges is equivalent to the number of electrons removed from the atom. . Electronic distribution of an atom/ion is an arrangement in a space of the electrons around the center of the mass(nucleus) of the atom or ion among the orbits or shells.