Atomic number — Science definition

The number of protons in an atom; it identifies which element the atom is.

Where it is taught

In use

That count is the atomic number, and it never changes for a given element: every atom with 6 protons is carbon, and anything with 8 protons is oxygen, no exceptions.

Atomic Structure

Questions using atomic number

Problem 1

A sodium (Na) atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer and explanation

12

Neutrons = mass number − atomic number = 23 − 11 = 12.

Problem 2

An atom has 12 protons, 13 neutrons and 10 electrons. State atomic number, mass number and net charge, and explain whether losing electrons changed its element.
Answer and explanation

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Read proton count for atomic number.

Add 12+13 for mass number; compare 12 positive with 10 negative charges.

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 3

Independent chapter check: An atom has 12 protons, 13 neutrons and 10 electrons. State atomic number, mass number and net charge, and explain whether losing electrons changed its element.
Answer and explanation

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Read proton count for atomic number.

Add 12+13 for mass number; compare 12 positive with 10 negative charges.

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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