Electron arrangementIB MYP Chemistry: Revision notes
Section 1
Electron shells
Electrons move around the nucleus in shells (also called energy levels). Each shell can hold only a certain number of electrons, and the shells fill from the inside outwards.
- First shell: up to 2 electrons
- Second shell: up to 8 electrons
- Third shell: up to 8 electrons (for the first 20 elements)
The fourth shell starts to fill once the third has 8, so potassium and calcium have electrons in four shells.
Do not start a new shell until the one before it is full. Sodium is 2,8,1, not 2,7,2.
Section 2
Electron arrangements of the first 20 elements
Write the arrangement as numbers separated by commas, filling shells in order.
- H 1, He 2
- Li 2,1; Be 2,2; B 2,3; C 2,4; N 2,5; O 2,6; F 2,7; Ne 2,8
- Na 2,8,1; Mg 2,8,2; Al 2,8,3; Si 2,8,4; P 2,8,5; S 2,8,6; Cl 2,8,7; Ar 2,8,8
- K 2,8,8,1; Ca 2,8,8,2
Worked example: sulfur has 16 electrons. 2 go in the first shell, 8 in the second, and the remaining 6 in the third, giving 2,8,6.
Check that your numbers add up to the atomic number of the element.
Section 3
Electron arrangement and the periodic table
The arrangement of electrons decides where an element sits in the periodic table.
- The number of outer-shell electrons is the group number (Group 1 to Group 7, with Group 0 for a full shell)
- The number of occupied shells is the period number
For example, sulfur (2,8,6) has 6 outer electrons so it is in Group 6, and 3 shells so it is in period 3. Elements in the same group have the same number of outer electrons, so they react in similar ways.
Do not mix up the two: the group comes from the outer electrons, the period from the number of shells.
Section 4
Valence electrons
The electrons in the outer shell are called valence electrons. They decide how an atom reacts, because they are the electrons that are lost, gained or shared.
- Group 1 metals have 1 valence electron and are very reactive
- Group 7 elements have 7 valence electrons
- Group 0 elements (noble gases) have a full outer shell, 2 for helium and 8 for the others, so they are very unreactive
Must know
- Shells hold 2, 8, 8 electrons (first 20 elements) and fill from the inside
- Write arrangements such as 2,8,1 and check that they add up to the atomic number
- Outer electrons = group number; number of shells = period number
- Valence electrons are the outer-shell electrons
- Elements in the same group have the same number of valence electrons, so similar reactions
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electron arrangement
- A teacher tells her class that a neutral sulfur atom has 16 electrons. She asks the students to work out how those electrons are arranged in shells, and what the arrangement shows about where sulfur sits in the periodic table.Explain how the electron arrangement of sulfur shows its group and period in the periodic table.2 marks
- A chemist compares three elements with different reactivity. Argon has atomic number 18 and is very unreactive. Sodium has atomic number 11 and potassium has atomic number 19, and both are very reactive metals that must be stored under oil.Explain why sodium and potassium have similar chemical properties.2 marks
- Calcium has atomic number 20 and is a metal in the same group of the periodic table as magnesium, which has atomic number 12. A student is using electron arrangements to compare the two metals.State the electron arrangement of calcium and use it to give the group and period of calcium.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).