Section 1 of 9Filling the shells
1
Electrons occupy energy levels (shells) starting with the one closest to the nucleus, which has the lowest energy.
2
The first shell holds a maximum of 2 electrons.
3
The second and third shells each hold up to 8 electrons, for the first 20 elements.
4
Fill each shell completely before starting the next.
5
Write the configuration as numbers separated by commas, for example sodium (11 electrons) is 2,8,1.
Section 2 of 9Configurations to know
| Element | Electrons | Configuration |
|---|---|---|
| Carbon | 6 | 2,4 |
| Oxygen | 8 | 2,6 |
| Neon | 10 | 2,8 |
| Magnesium | 12 | 2,8,2 |
| Chlorine | 17 | 2,8,7 |
| Calcium | 20 | 2,8,8,2 |
Section 3 of 9Links to the periodic table
Electron configuration
Group number
- Group number The number of electrons in the outer shell equals the group number.
Period number
- Period number The number of occupied shells equals the period number.
Reactivity
- Reactivity Elements with full outer shells, the noble gases, are unreactive.
Section 4 of 9Energy level changes (HT only)
1Absorbing radiation
- Absorbing radiation An electron takes in electromagnetic radiation of the right energy.
2Moves outward
- Moves outward It jumps to a higher energy level, further from the nucleus.
3Unstable state
- Unstable state The excited electron soon falls back to a lower level.
4Emits radiation
- Emits radiation The energy difference is released as electromagnetic radiation, such as visible light.
Section 5 of 9Worked example: chlorine
1
Chlorine has atomic number 17, so a neutral atom has 17 electrons.
2
Fill the first shell with 2, leaving 15.
3
Fill the second shell with 8, leaving 7.
4
Place the last 7 in the third shell, giving 2,8,7.
5
Seven outer electrons places chlorine in Group 7 and in Period 3.
Section 6 of 9Worked example: potassium
1
Potassium has atomic number 19, so a neutral atom has 19 electrons.
2
Fill the first shell with 2, leaving 17.
3
Fill the second shell with 8, leaving 9.
4
Fill the third shell with 8, leaving 1.
5
Place the last electron in the fourth shell, giving 2,8,8,1; this places potassium in Group 1 and Period 4.
Section 7 of 9Configuration and reactivity
Losing electrons
- Losing electrons Metals such as sodium have 1 to 3 outer electrons and tend to lose them to form positive ions with a full outer shell.
Gaining electrons
- Gaining electrons Non-metals such as chlorine need one or two more electrons to fill their outer shell and tend to gain them to form negative ions.
Noble gases
- Noble gases Helium, neon and argon already have full outer shells, so they have no need to gain, lose or share electrons and are unreactive.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Energy level | A shell at a fixed distance from the nucleus where electrons are found. |
| Electronic configuration | The arrangement of electrons in the shells of an atom. |
| Outer shell | The highest-energy occupied shell, which determines chemical behaviour. |
| Excited electron | An electron that has moved to a higher energy level after absorbing energy. |
| Group | A vertical column of the periodic table; elements in it have the same number of outer electrons. |
| Period | A horizontal row of the periodic table; it matches the number of occupied shells. |
Section 9 of 9Exam technique
1
Putting more than 2 electrons in the first shell.
2
Starting a new shell before the previous is full.
3
Using the period number as the outer-shell electron count; it is the group number that matches.
4
Saying electrons move to a lower level when they absorb energy.