Question

From the symbol $\text{K}_{\text{19}}^{40}$ , state the mass number of potassium.

Answer 1

The mass number of any element is defined as the sum of the number of protons and the neutrons present in the nucleus. Electrons are considered to be massless or having negligible compared to the mass of the protons and the neutrons so their number is not counted.

Complete step by step solution:
The mass number is approximately equal to the atomic mass of the atom expressed in atomic mass units. The mass number is different for the different isotopes of an element, where isotopes are referred to the atoms of the same elements with different mass numbers but the same atomic number.
As per the question, the mass number of potassium $\text{K}_{\text{19}}^{40}$ is 40. This is because in the representation of an element in the periodic table, the symbol of the element is written with its mass number on the top right or left hand corner while the atomic number at the bottom right or left hand corner. As per the given symbol, the atomic number, which is the number of protons in the nucleus of the atom is equal to 19. As the protons are positively charged particles, so to balance the charge on the atom, the number of electrons must also be equal to the number of the electrons and hence the number of the electrons is also equal to 19.

Note:
The mass number of the nucleus was discovered prior to the discovery of the charge of the electrons by the Mullikan’s oil drop experiment. Ernest Rutherford was the person who stated that the mass of the atom is concentrated in the nucleus of the atom.