- Define and explain Electron Shells in your own words
- Use key terms such as electron accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Electron Shells: arrange electrons in shells and link the arrangement to the periodic table.
Arrange electrons in shells and link the arrangement to the periodic table.
Key ideas
Electrons are arranged in shells
Electrons occupy shells around the nucleus, filling from the innermost shell outwards; the first shell holds up to 2 electrons and the second and third hold up to 8. An atom's electron arrangement decides how it reacts — atoms with full outer shells, like the noble gases, are unreactive. Sodium's arrangement 2,8,1 explains why it loses one electron so readily.
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.
Key term — electron: A negatively charged particle found in shells around the nucleus, with negligible mass (about 1/1836 of a proton).
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).
- Putting too many electrons in the first shell The first shell holds a maximum of 2 electrons — it can never hold 8. The pattern is 2 in the first shell, then up to 8 in the second and third.
- 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.
Practice
2,8,7.
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.
Similarity: both have 6 protons (and 6 electrons), so both are carbon. Difference: carbon-14 has 2 more neutrons, so it is heavier.
Quick check
Which of these best defines "electron"?
Explain why the mass of an electron is ignored when calculating mass number.
An ion has 11 protons and 10 electrons. What is its charge?
- Electron Shells: arrange electrons in shells and link the arrangement to the periodic table.
- Electrons are arranged in shells: Electrons occupy shells around the nucleus, filling from the innermost shell outwards; the first shell holds up to 2 electrons and the second and third hold up to 8.
- proton: A positively charged particle in the nucleus with a relative mass of 1.
- Watch out for: putting too many electrons in the first shell