What Is the Atomic Number?

The Significance of the Atomic Number in Chemistry

Each element has its own unique atomic number, which is the number of protons in its atom.
Each element has its own unique atomic number, which is the number of protons in its atom. Steven Hunt, Getty Images

Each element on the periodic table has its own atomic number. In fact, this number is how you can distinguish one element from another. The atomic number is simply the number of protons in an atom. For this reason, it's sometimes called the proton number. In calculations, it is denoted by the capital letter Z. The symbol Z comes from the German word zahl, which means number of numeral, or atomzahl, a more modern word which means atomic number.

Because protons are units of matter, atomic numbers are always whole numbers. At present, they range from 1 (the atomic number of hydrogen) to 118 (the number of the heaviest known element). As more elements are discovered, the maximum number will go higher. Theoretically, there is no maximum number, but elements become unstable with more and more protons and neutrons, making them susceptible to radioactive decay. Decay may result in products with a smaller atomic number, while the process of nuclear fusion may produce atoms with a larger number.

In an electrically neutral atom, the atomic number (number of protons) is equal to the number of electrons.

Why the Atomic Number Is Important

The main reason the atomic number is important is because it's how you identify the element of an atom. Another big reason it matters is because the modern periodic table is organized according to increasing atomic number.

Finally, the atomic number is a key factor in determining the properties of an element. Note, however, the number of valence electrons determines chemical bonding behavior.

Atomic Number Examples

No matter how many neutrons or electrons it has, an atom with one proton is always atomic number 1 and always hydrogen.

An atom the contains 6 protons is by definition an atom of carbon. An atom with 55 protons is always cesium.

How to Find the Atomic Number

How you find the atomic number depends on the information that you are given.

  • If you have an element name or symbol, use a periodic table to find the atomic number. There may be many numbers on a periodic table, so how do you know which one to pick? The atomic numbers go in order on the table. While other numbers may be decimal values, the atomic number is always a simple positive whole number. For example, if you are told the element name is aluminum, you can find the name or symbol Al to determine the atomic number is 13.
  • You can find the atomic number from an isotope symbol. There is more than one way to write an isotope symbol, but the element symbol will always be included. You can use the symbol to look up the number. For example, if the symbol is 14C, you know the element symbol is C or that the element is carbon. The atomic number of carbon is 6.
  • More commonly, the isotope symbol already tells you the atomic number. For example, if the symbol is written as 146C, the number "6" is listed. The atomic number is the smaller of the two numbers in the symbol. It is typically located as a subscript to the left of the element symbol.

    Terms Related To Atomic Number

    If the number of electrons in an atom varies, the element remains the same, but new ions are produced. If the number of neutrons changes, new isotopes result.

    Protons are found together with neutrons in the atomic nucleus. The total number of protons and neutrons in an atom is its atomic mass number (denoted by the letter A). The average sum of the number of protons and neutrons in a sample of an element is its atomic mass or atomic weight.

    The Quest for New Elements

    When scientists talk about synthesizing or discovering new elements, they are referring to elements with higher atomic numbers than 118. How will these elements be formed? Elements with new atomic numbers are made by bombarding target atoms with ions. The nuclei of the target and the ion fuse together to form a heavier element.

    It's difficult to characterize these new elements because the super-heavy nuclei are unstable, readily decaying into lighter elements. Sometimes the new element itself isn't observed, but the decay scheme indicates the higher atomic number must have been formed.

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    Helmenstine, Anne Marie, Ph.D. "What Is the Atomic Number?" ThoughtCo, Jan. 12, 2018, thoughtco.com/what-is-the-atomic-number-4031221. Helmenstine, Anne Marie, Ph.D. (2018, January 12). What Is the Atomic Number? Retrieved from https://www.thoughtco.com/what-is-the-atomic-number-4031221 Helmenstine, Anne Marie, Ph.D. "What Is the Atomic Number?" ThoughtCo. https://www.thoughtco.com/what-is-the-atomic-number-4031221 (accessed May 23, 2018).