Atomic mass of an element is eal to the sum of number of

The atomic mass of an element, often used interchangeably with the mass number in basic chemistry, is defined as the total number of protons and neutrons present in the nucleus of an atom [1][2]. Protons and neutrons are the primary subatomic particles that contribute significantly to an atom's mass, each having a mass of approximately 1 atomic mass unit (amu). While electrons are also present in an atom, their mass is negligible—roughly 1/1836th the mass of a proton—and is therefore ignored when calculating the mass number. The atomic nucleus, which contains these protons and neutrons, constitutes nearly all of the atom's mass [1]. Consequently, the sum of protons and neutrons provides the integral mass number (A), which is the defining characteristic for the mass of a specific isotope.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Glossary > p. 100
2. [2] https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article