# Atomic Number and Mass

## The atomic number is the number of protons in an element while the number of protons plus the number of neutrons determines the mass number.

#### Key Points

• Atoms of each element contain an equal number of protons and electrons.

• The number of protons determines an element’s atomic number and is used to distinguish one element from another.

• The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.

• Together, the number of protons and the number of neutrons determine an element’s mass number.

• Since an element’s isotopes have slightly different mass numbers, the atomic mass is calculated by obtaining the mean of the mass numbers for its isotopes.

#### Terms

• The number of protons in an atom.

• the quantity of matter in an atomic particle, sub-atomic particle, or molecule

#### Figures

1. ##### Atomic number, Chemical Symbol, and Mass Number

Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively. Its atomic mass is 12.11.

## Atomic Number and Mass

Atoms of each element contain a characteristic number of protons and electrons, which are equal. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. The number of neutrons can vary and result in isotopes, which are different atoms of the same element that are identified by the number of neutrons in their nucleus.

An element's mass number (A) is the sum of the number of protons and the number of neutrons (Figure 1). The small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Isotopes of the same element will have the same atomic number but different mass numbers.

Since an element’s isotopes have slightly different mass numbers, scientists can determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a fraction. For example, the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 (17 protons and 18 neutrons) and some with an atomic mass of 37 (17 protons and 20 neutrons).

Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7) contains three protons (found from Z), three electrons (as the number of protons = the number of electrons), and four neutrons (7 – 3 = 4).

#### Key Term Glossary

atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
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atomic mass
the quantity of matter of a particle, sub-atomic particle, or molecule
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atomic number
The number, equal to the number of protons in an atom that determines its chemical properties. Symbol: Z
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electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
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element
Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means, and are made up of atoms all having the same number of protons.
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fraction
a part of a whole, especially a comparatively small part
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isotope
Any of two or more forms of an element where the atoms have the same number of protons but a different number of neutrons within their nuclei. As a consequence, atoms for the same isotope will have the same atomic number but a different mass number (atomic weight).
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Isotope
Isotopes are variants of a particular chemical element. While all isotopes of a given element share the same number of protons, each isotope differs from the others in its number of neutrons.
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mass
The quantity of matter that a body contains, irrespective of its bulk or volume. It is one of four fundamental properties of matter. It is measured in kilograms in the SI system of measurement.
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mass number
The total number of protons and neutrons in an atomic nucleus.
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neutron
A subatomic particle forming part of the nucleus of an atom and having no charge; it is a combination of an up quark and two down quarks
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nucleus
The massive, positively charged central part of an atom, made up of protons and neutrons.
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proton
A positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down quark