# The Atomic Number

The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The number of protons determines how many electrons surround the nucleus, and it is the arrangement of these electrons that. Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons.

1. Atomic number The number of protons in the nucleus of an atom. In electrically neutral atoms, this number is also equal to the number of electrons orbiting about the atom's nucleus.
2. Atomic number, the number of a chemical element in the periodic system, whereby the elements are arranged in order of increasing number of protons in the nucleus. Accordingly, the number of protons, which is always equal to the number of electrons in the neutral atom, is also the atomic number.

#### Learning Objective • Determine the relationship between the mass number of an atom, its atomic number, its atomic mass, and its number of subatomic particles

#### Key Points

• Neutral 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

• atomic massThe average mass of an atom, taking into account all its naturally occurring isotopes.
• mass numberThe sum of the number of protons and the number of neutrons in an atom.
• atomic numberThe number of protons in an atom.

## Atomic Number

Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons. The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons. The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms). For instance, iron, Fe, can exist in its neutral state, or in the +2 and +3 ionic states.

## The Atomic Number 5

An element’s mass number (A) is the sum of the number of protons and the number of neutrons. 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. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.

Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine (Cl) is 35.45 amu because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 amu (17 protons and 18 neutrons) and some with an atomic mass of 37 amu (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 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4). Show Sources

Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:

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“A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Atoms.”

http://en.wikibooks.org/wiki/A-level_Chemistry/OCR/Atoms,_Bonds_and_Groups/Atoms_and_Reactions/Atoms
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### Learning Objectives

• Define the atomic number.
• Relate the number of electrons in an element to the atomic number for that element.

### What is unique about each one of us?

For the vast majority of people, it is not their name, because it is quite possible for others in the world to have the same name (check it out by doing an internet search for your name and see how many other of “you” there are). It is not your physical description. Eye-witnesses to crime scenes often pick the wrong person when trying to identify the criminal.

There may be some unique identifiers for us. If you have a cell phone in your name, nobody else in the world has that number. Email addresses are different for each of us, which is a good thing since we can email almost anywhere in the world. Our DNA is unique, but getting a DNA analysis is expensive and time-consuming, so we really don’t want to have to explore that.

## Organizing the Elements

One of the goals of science is to discover the order in the universe and to organize information that reflects that order. As information about the different elements was made known, efforts were made to see if there were patterns in all of the data. An early attempt to organize data was made by Mendeleev, who developed the first periodic table. His data set was based on atomic weights and was instrumental in providing clues as to the possible identity of new elements. Once we learned the details of the atomic nucleus, the table was based on the number of protons in the nucleus, called the atomic number of the element.

### Atomic Number

Figure 1. How can you determine the atomic number of an element?

The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. This means that the number of protons is the characteristic which makes each element unique compared to all other elements. Elements are different because of their atomic number. The periodic table displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element’s atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Following on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, and so forth.

Since atoms are neutral, the number of electrons is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Manganese (atomic number 25) would have twenty-five protons and twenty-five electrons.

Figure 2. The periodic table classifies elements by atomic number.

The classification of elements by atomic number allows us to understand many properties of the atom and makes it possible to predict behaviors instead of just having to memorize everything.

### Summary

• The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element
• The number of electrons is equal to the number of protons in an element.

### Practice 1. What letter is used by convention to designate the atomic number?
2. What determines the chemical properties of an element?
3. What are the atomic numbers of the elements that appear in nature?
4. How many elements were known in John Dalton’s day?

### Review

1. Name two unique identifiers of people.
2. Who developed the first periodic table?
4. What is the current periodic table based upon?
5. What does the atomic number represent?
6. How many protons are in the following elements:?
1. Ne
2. Ca
3. Pt
7. Write the symbol for the element with the following atomic number:
1. 18
2. 41
3. 82
4. 12

## The Atomic Number Is Equal To ## Glossary

• periodic table: This table displays all of the known elements and is arranged in order of increasing atomic number.
• atomic weight: Each chemical element has an atom with a given mass.
• atomic number : The number of protons in the nucleus of each atom of that element.
Show References

## References

1. Laura Guerin, based on image by User:Materialscientist/Wikimedia Commons. CK-12 Foundation (original image: http://commons.wikimedia.org/wiki/File:Atomic_number_depiction.jpg).
2. User:Cepheus/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Periodic_table.svg.