What does the electron configuration of sodium, 2,8,1, tell you?

2 electrons in the first shell, 8 in the second and 1 in the third; 11 electrons altogether.

Which electrons decide an element's group number?

The outer electrons: the number of electrons in the outer shell equals the group number.

What does the number of occupied shells tell you?

The period number of the element.

Why is argon unreactive?

It has a full outer shell of 8 electrons, so it has no tendency to gain, lose or share electrons.

Why is helium unreactive when it has only 2 electrons?

Its outer shell is the first shell, which holds a maximum of 2, so it is full.

Where does the 19th electron of potassium go?

Into a fourth shell, because the third shell is treated as full with 8 electrons.

How do you draw an atom's electron arrangement?

A nucleus in the centre, a ring for each shell, and dots or crosses on the rings for the electrons.

Why do elements in the same group react in similar ways?

They have the same number of outer electrons.

Electron configurationIB MYP Sciences: Flashcards

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Question

How many electrons can the first, second and third shells hold (first 20 elements)?

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How many electrons can the first, second and third shells hold (first 20 elements)?: 2, 8 and 8.
What does the electron configuration of sodium, 2,8,1, tell you?: 2 electrons in the first shell, 8 in the second and 1 in the third; 11 electrons altogether.
What is the electron configuration of oxygen (8 electrons)?: 2,6.
What is the electron configuration of chlorine (17 electrons)?: 2,8,7.
What is the electron configuration of calcium (20 electrons)?: 2,8,8,2.
Which electrons decide an element's group number?: The outer electrons: the number of electrons in the outer shell equals the group number.
What does the number of occupied shells tell you?: The period number of the element.
Which group and period is an atom with configuration 2,8,4 in?: Group 4, period 3 (silicon).
Why is argon unreactive?: It has a full outer shell of 8 electrons, so it has no tendency to gain, lose or share electrons.
Why is helium unreactive when it has only 2 electrons?: Its outer shell is the first shell, which holds a maximum of 2, so it is full.
Where does the 19th electron of potassium go?: Into a fourth shell, because the third shell is treated as full with 8 electrons.
How do you draw an atom's electron arrangement?: A nucleus in the centre, a ring for each shell, and dots or crosses on the rings for the electrons.
Why do elements in the same group react in similar ways?: They have the same number of outer electrons.

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

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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).