# Mass Number: Why Can It Be a Fraction?

• kooyewyoung
In summary, the conversation discusses the concept of mass number and its relation to the number of protons and neutrons in an atom. It is explained that although the mass number is the total number of nucleons, it does not necessarily mean that the mass itself is a whole number. The suggestion to explore isotopes is given in order to gain a better understanding of this concept.

## Homework Statement

My instructor give me a question:
Mass number (number of protons and number of neutrons in an atom) is supposed to be a whole number for a particular atom, in reality IT IS NOT. Why is it so?

## The Attempt at a Solution

Here, I am very sure that this question ask for mass number (nucleon number) not atomic weight. However, how can only a part of a proton or neutron exist in an atom if mass number are not a whole number?

Last edited:
Hint: Check out isotopes.

you are absolutely correct. Mass number is the NUMBER of neutrons and protons (nucleons)
This does not mean that the MASS is a whole number.
Like steamking I suggest you look at isotopes to get the idea of mass

## What is mass number?

Mass number is the total number of protons and neutrons in an atom's nucleus.

## Why can mass number be a fraction?

Mass number can be a fraction because some atoms have a different number of neutrons than protons, resulting in a non-integer value for the mass number. This is known as an isotope.

## How is mass number different from atomic mass?

Mass number only takes into account the number of protons and neutrons in an atom's nucleus, whereas atomic mass is an average of all the different isotopes of an element, taking into account their relative abundance.

## What is the significance of mass number in chemistry?

Mass number is important in chemistry because it determines the stability and properties of an atom. It also helps in identifying different isotopes of an element.

## Can mass number change?

Yes, mass number can change through nuclear reactions such as radioactive decay or nuclear fusion. However, the number of protons in an atom remains constant, as this determines the element's identity.