Electron configurationIB MYP Sciences: Revision notes
Section 1
Electron shells
Electrons move around the nucleus in energy levels called electron shells. The shell closest to the nucleus has the least energy and fills first. Each shell can hold only a limited number of electrons:
- first shell: up to 2 electrons
- second shell: up to 8 electrons
- third shell: up to 8 electrons (for the first 20 elements)
The number of electrons in a neutral atom equals its atomic number, so this tells you how many electrons you have to place.
Section 2
Filling order and writing configurations
Electrons fill the shells from the inside outwards. Fill the first shell with 2, then the second with 8, then the third with 8, then start the fourth. The electron configuration is written as the numbers in each shell, separated by commas.
- Lithium (3 electrons): 2,1
- Carbon (6): 2,4
- Sodium (11): 2,8,1
- Chlorine (17): 2,8,7
- Argon (18): 2,8,8
- Calcium (20): 2,8,8,2
After argon, the next electron (potassium, 19) goes into a fourth shell, not into the third.
Do not put more than 2 electrons in the first shell. Check that your numbers add up to the atomic number.
Section 3
Drawing configurations
To draw an atom, show the nucleus as a circle in the centre with the atomic number written inside. Draw each occupied shell as a ring around it, and place the electrons on the rings as dots or crosses.
For sodium (2,8,1): draw three rings, with 2 electrons on the inner ring, 8 on the middle ring and 1 on the outer ring. Draw the electrons in pairs where you can, and make sure the total number matches the atomic number.
Count your dots at the end. If the total is not the atomic number, one shell is wrong.
Section 4
Outer electrons and group number
The electrons in the outermost shell are the outer electrons. For the main groups, the number of outer electrons equals the group number.
- Sodium 2,8,1: Group 1
- Magnesium 2,8,2: Group 2
- Aluminium 2,8,3: Group 3
- Chlorine 2,8,7: Group 7
Elements in the same group have the same number of outer electrons, which is why they react in similar ways. Some books number the groups 13 to 18, so aluminium is Group 13 and chlorine is Group 17. The noble gases are Group 0 (or 18).
Section 5
Shells and period number
The number of occupied shells equals the period number, which is the row of the periodic table.
- Lithium 2,1: two shells, period 2
- Sodium 2,8,1: three shells, period 3
- Potassium 2,8,8,1: four shells, period 4
To place any of the first 20 elements, write its configuration, then read the last number for the group and count the numbers for the period. For example, 2,8,6 is Group 6 and period 3, which is sulfur.
Do not mix up the two rules. Outer electrons give the group; number of shells gives the period.
Section 6
Stable full outer shell
An atom with a full outer shell is stable. It has no tendency to gain, lose or share electrons, so it is unreactive. This is true of the noble gases:
- helium 2 (full first shell)
- neon 2,8
- argon 2,8,8
Other atoms react in order to get a full outer shell. Sodium (2,8,1) has one electron to lose, and chlorine (2,8,7) is one electron short, so both are reactive. Helium is the exception to the rule of 8: its outer shell is full with just 2 electrons.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electron configuration
- A museum in Lagos is building a display of the first twenty elements. Each element card shows its electron configuration, written as the number of electrons in each shell from the inside outwards (for example, lithium is 2,1).An element X has the electron configuration 2,8,6. State the group and the period of X, giving a reason for each.2 marks
- A science teacher in Nairobi shows her class a neon sign and a sodium street lamp. She tells them that a neon atom has the electron configuration 2,8 and a sodium atom has the electron configuration 2,8,1.Explain, in terms of electron arrangement, why neon is unreactive but sodium is reactive.2 marks
- A student in Mumbai writes out electron configurations for ten elements and looks for patterns. Her results: lithium 2,1; beryllium 2,2; boron 2,3; carbon 2,4; nitrogen 2,5; oxygen 2,6; fluorine 2,7; neon 2,8; sodium 2,8,1; magnesium 2,8,2. She proposes the hypothesis: "The number of electrons in the outer shell of an atom is equal to the period number of its element."Describe the pattern in the student's data from lithium to neon, and what happens at sodium.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).