Journey Inside the Atom · Lesson 9 of 14
How Are Electrons Distributed in Different Energy Levels?
“Bohr and Bury rules build the electronic configurations of the first eighteen elements”
• Use the shell-capacity relation. • State the outermost-shell limit used in the chapter. • Explain shell filling order. • Define electronic configuration. • Build configurations of the first eighteen elements. • Determine outermost-shell electron counts.
The total number of electrons does not by itself explain how atoms combine. We also need to know how those electrons are distributed among energy levels.
How Are Electrons Distributed in Different Energy Levels?
Bohr and Bury Rules
Problem
Apply the shell-capacity relation to K, L and M.
- 1.K has n = 1, so capacity = 2 × 1² = 2.
- 2.L has n = 2, so capacity = 2 × 2² = 8.
- 3.M has n = 3, so capacity = 2 × 3² = 18.
- 4.These are maximum shell capacities, not a requirement that each atom fully fills every shell.
The outermost shell can contain a maximum of eight electrons in the treatment used here, except the first shell, which can hold only two. Electrons fill shells step by step from the shell nearest the nucleus outward. L begins filling after K is complete, and so on.
Building up Atoms
The distribution of an atom's electrons among its shells or energy levels.
Hydrogen has one electron in K. Helium has two and fills K. Lithium has three, so two occupy K and the third goes to L, giving 2, 1. Continuing this process builds the first eighteen elements.
| Element | Symbol | Atomic number | Electronic configuration |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Helium | He | 2 | 2 |
| Lithium | Li | 3 | 2, 1 |
| Beryllium | Be | 4 | 2, 2 |
| Boron | B | 5 | 2, 3 |
| Carbon | C | 6 | 2, 4 |
| Nitrogen | N | 7 | 2, 5 |
| Oxygen | O | 8 | 2, 6 |
| Fluorine | F | 9 | 2, 7 |
| Neon | Ne | 10 | 2, 8 |
| Sodium | Na | 11 | 2, 8, 1 |
| Magnesium | Mg | 12 | 2, 8, 2 |
| Aluminium | Al | 13 | 2, 8, 3 |
| Silicon | Si | 14 | 2, 8, 4 |
| Phosphorus | P | 15 | 2, 8, 5 |
| Sulfur | S | 16 | 2, 8, 6 |
| Chlorine | Cl | 17 | 2, 8, 7 |
| Argon | Ar | 18 | 2, 8, 8 |
Finding Electronic Configuration
Problem
Write the configuration for atomic number 12.
- 1.A neutral atom has 12 electrons.
- 2.Place 2 in K. Remaining = 10.
- 3.Place 8 in L. Remaining = 2.
- 4.Place 2 in M.
- 5.Configuration = 2, 8, 2.
Problem
Write the configuration for atomic number 16.
- 1.A neutral atom has 16 electrons.
- 2.K receives 2. Remaining = 14.
- 3.L receives 8. Remaining = 6.
- 4.M receives 6.
- 5.Configuration = 2, 8, 6.
Problem
Write the configuration for atomic number 18.
- 1.A neutral atom has 18 electrons.
- 2.K receives 2.
- 3.L receives 8.
- 4.M receives the remaining 8.
- 5.Configuration = 2, 8, 8.
Outermost-Shell Electrons
The final number in the configuration is the number of electrons in the outermost occupied shell. Carbon 2, 4 has four outermost electrons, fluorine 2, 7 has seven and silicon 2, 8, 4 has four.
Keep the two limits separate. The expression 2n² gives maximum shell capacity, while the outermost shell is limited to eight electrons in the treatment used here.
Quiz
Which description best matches Electronic Configuration?
Which term matches this description: The distribution of an atom's electrons among its shells or energy levels.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Write the configurations for atomic numbers 12, 16 and 18.
- Find the outermost-shell electron counts of carbon, fluorine and silicon.
- An atom has mass number 23 and 11 protons. Find its electron configuration and neutron count.
- Why does the model fill inner shells before outer shells?
Key Takeaways
• Electrons are arranged in energy levels or shells around the nucleus. • The maximum number of electrons a shell can hold is related to the 2n² rule. • For the first eighteen elements, the outermost shell contains no more than eight electrons. • Electronic configuration describes how an atom's electrons are distributed among its shells.