Mass Number Formula

Mass number is the total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify.

The Formula

A=Z+NA = Z + N (protons + neutrons)

When to use: How heavy the nucleus is — each proton and neutron contributes about 1 atomic mass unit.

Quick Example

Carbon-12: 6 protons + 6 neutrons = mass number 12. Carbon-14: 6 + 8 = 14.

Notation

AA is the mass number (superscript in isotope notation). ZZ is the atomic number (subscript). In the notation 614C^{14}_6\text{C}, 14 is the mass number and 6 is the atomic number.

What This Formula Means

The total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify specific isotopes of an element.

How heavy the nucleus is — each proton and neutron contributes about 1 atomic mass unit.

Formal View

The mass number AA is defined as the total number of nucleons (protons + neutrons) in the nucleus: A=Z+NA = Z + N, where ZZ is the atomic number (protons) and NN is the neutron number. Isotope notation is ZAX^A_Z X.

Worked Examples

Example 1

easy
An atom has 17 protons and 18 neutrons. What is its mass number and what element is it?

Answer

A=35, Chlorine-35A = 35,\text{ Chlorine-35}

First step

1
Mass number A=protons+neutrons=17+18=35A = \text{protons} + \text{neutrons} = 17 + 18 = 35.

Full solution

  1. 2
    Atomic number Z=17Z = 17 corresponds to chlorine (Cl).
  2. 3
    This isotope is written as 1735Cl{}^{35}_{17}\text{Cl}.
The mass number is the total number of nucleons (protons + neutrons) in the nucleus. It is always a whole number and appears as a superscript in isotope notation.

Example 2

medium
Why is the atomic mass of chlorine listed as 35.4535.45 amu on the periodic table, rather than a whole number like 3535 or 3737?

Example 3

medium
An isotope has 26 protons and 30 neutrons. What is its mass number? Identify the element.

Common Mistakes

  • Confusing mass number (AA, whole number of nucleons) with atomic mass (weighted average of isotopes, usually a decimal) - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement.
  • Including electrons in the mass number — electrons have negligible mass and are not counted - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement.
  • Thinking mass number is unique to an element — different isotopes of the same element have different mass numbers - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement.
  • Using mass number from a keyword alone - Signal words like atom, proton, neutron only point to a possible model; the substances and evidence must match too. - Fix this by naming the substances or sample, checking "Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?", and attaching units, formulas, states, or evidence to the final statement.

Why This Formula Matters

Mass Number gives students the particle inventory needed for nearly every later chemistry idea. It makes periodic table entries, ions, isotopes, bonding, and formulas easier because the atom is described by evidence instead of by a vague picture.

Frequently Asked Questions

What is the Mass Number formula?

The total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify specific isotopes of an element.

How do you use the Mass Number formula?

How heavy the nucleus is — each proton and neutron contributes about 1 atomic mass unit.

What do the symbols mean in the Mass Number formula?

AA is the mass number (superscript in isotope notation). ZZ is the atomic number (subscript). In the notation 614C^{14}_6\text{C}, 14 is the mass number and 6 is the atomic number.

Why is the Mass Number formula important in Chemistry?

Mass Number gives students the particle inventory needed for nearly every later chemistry idea. It makes periodic table entries, ions, isotopes, bonding, and formulas easier because the atom is described by evidence instead of by a vague picture.

What do students get wrong about Mass Number?

Students often know a formula related to mass number but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.

What should I learn before the Mass Number formula?

Before studying the Mass Number formula, you should understand: proton, neutron.

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