- Define and explain Atomic Number and Mass Number in your own words
- Use key terms such as isotope accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Atomic Number and Mass Number: use proton and nucleon numbers to describe any atom.
Use proton and nucleon numbers to describe any atom.
Key ideas
Mass number counts the heavy particles
The mass (nucleon) number is the total of protons plus neutrons, since electrons weigh almost nothing. So number of neutrons = mass number − atomic number: for example, an atom with atomic number 6 and mass number 14 has 14 − 6 = 8 neutrons. This simple subtraction unlocks the full structure of any atom.
The nucleus decides the element
Almost all the mass of an atom sits in its tiny central nucleus, made of protons and neutrons. The number of protons — the atomic (proton) number — decides which element the atom is: every carbon atom has 6 protons, every oxygen atom has 8. Change the proton number and you change the element itself.
Key term — isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.
An atom has atomic number 12 and mass number 24. State the numbers of protons, neutrons and electrons, and give its electron arrangement.
Protons = atomic number = 12. Electrons = protons in a neutral atom = 12. Neutrons = mass number − atomic number = 24 − 12 = 12. Arrange 12 electrons in shells: first shell 2, second shell 8, third shell 2, giving 2,8,2.
Answer: 12 protons, 12 neutrons, 12 electrons; electron arrangement 2,8,2 (magnesium).
- Mixing up atomic number and mass number The atomic number counts protons only; the mass number counts protons plus neutrons. Remember: neutrons = mass number − atomic number, not the other way round.
- Thinking isotopes are different elements Isotopes have the same number of protons, so they are the same element with the same chemistry. They differ only in neutron number, and therefore in mass — like carbon-12 and carbon-14.
Practice
Oxygen; mass number = 8 + 8 = 16.
A neutral atom has equal numbers of protons (each +1) and electrons (each −1), so the charges cancel out.
2,8,7.
An electron's relative mass is about 1/1836 of a proton's, which is negligible next to the whole-number masses of protons and neutrons, so it makes no difference to the total.
Quick check
Which of these best defines "isotope"?
An ion has 11 protons and 10 electrons. What is its charge?
Carbon-12 and carbon-14 are isotopes. State one similarity and one difference between their atoms.
- Atomic Number and Mass Number: use proton and nucleon numbers to describe any atom.
- Mass number counts the heavy particles: The mass (nucleon) number is the total of protons plus neutrons, since electrons weigh almost nothing.
- proton: A positively charged particle in the nucleus with a relative mass of 1.
- Watch out for: mixing up atomic number and mass number