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How have our ideas about atoms changed over time? - OCR 21st CenturyAtomic number and mass number

The atomic model consists of a nucleus containing protons and neutrons, surrounded by electrons in shells. The numbers of particles in an atom can be calculated from its atomic number and mass number.

Part of Combined ScienceChemical patterns

Atomic number and mass number

Atomic number

The of an is the number of in its . Remember that:

  • all the atoms of a given have the same atomic number
  • the atomic number of each element is unique - no two elements have the same atomic number

An atom contains an equal number of protons and . Since protons and electrons have equal and opposite , this means that atoms are overall.

For example, the atomic number of lithium is 3. Every lithium atom has 3 protons and 3 electrons. It has 3 positive charges and 3 negative charges.

Mass number

Atoms of different elements usually have different , but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.

Calculating numbers of subatomic particles

The symbol for an atom can be written to show its mass number and its atomic number.

Fluorine symbol.

To calculate the numbers of in an atom use its atomic number and mass number:

  • number of protons = atomic number
  • number of electrons = atomic number
  • number of = mass number - atomic number

Example

The atomic number of a fluorine atom is 9 and its mass number is 19. Calculate the number of protons, neutrons and electrons it contains.

number of protons = 9

number of electrons = 9

number of neutrons = mass number - atomic number

= 19 - 9 = 10