Count neutrons from mass number

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or browser document; an ordinary calculator is optional.
• Rule: Neutron count equals mass number minus atomic number. Formula: a-b. Variable key: a = isotope mass number, b = atomic number
• Distinct worked inputs and result: A distinct isotope has mass number 19 and atomic number 9: 19-9=10 neutrons.
• Assessment case 1: Supplied case 1: isotope mass number=14; atomic number=6, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?
• Assessment case 2: Supplied case 2: isotope mass number=23; atomic number=11, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?
• Assessment case 3: Supplied case 3: isotope mass number=24; atomic number=12, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?
• Assessment case 4: Supplied case 4: isotope mass number=31; atomic number=15, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?
• Assessment case 5: Supplied case 5: isotope mass number=35; atomic number=17, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?
• Assessment case 6: Supplied case 6: isotope mass number=40; atomic number=20, using the explicitly named quantities. Neutron count equals mass number minus atomic number. What result follows?

Learn the idea

Mass number counts protons plus neutrons in one isotope. It is not a decimal average atomic mass. Subtract atomic number from the supplied integer mass number.

Neutron count equals mass number minus atomic number. The formula in the supplied model is a-b. Variable key: a = isotope mass number, b = atomic number. Use integer isotope mass number. Atomic number counts protons.

A distinct isotope has mass number 19 and atomic number 9: 19-9=10 neutrons. Do not use an averaged periodic-table mass as mass number. Track the quantity and unit before calculating. Explain the meaning of a positive, negative or fractional result when relevant. A numerical answer supports the stated model calculation; broader claims require the additional measurements and assumptions described here.

1. Name the measured quantities and their units before substituting values.
2. Write the stated rule and use the variable key to match each symbol to its explicitly named quantity.
3. Calculate each supplied case separately, keeping intermediate arithmetic and the result unit.
4. Explain the result in context and identify the assumption that would invalidate direct use of the rule.

Check your reasoning

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Take it further

Changed-input task: use the same stated model with isotope mass number=15; atomic number=6. Expected result: 9 neutrons. Explain what changes and keep this assumption: Do not use an averaged periodic-table mass as mass number.

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