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Electron shells and valence electrons
Age 15 · Chemistry
Electrons fill shells around the nucleus in order: the first shell holds 2, then the next shells fill up one after another. The electrons in the outermost shell, called valence electrons, decide how an atom reacts and which group of the periodic table it belongs to.
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In the picture
- Lithium nucleus: protons and neutrons packed at the centre
- First shell: already full with just 2 electrons
- Lithium’s outer shell: a single valence electron
- Hydrogen’s only electron: also one valence electron, so it sits in the same column as lithium
Examples
- Sodium in table salt and lithium in phone batteries each have one outer electron that they give away easily.
- Neon in glowing signs has a full outer shell of 8 electrons, so it barely reacts with anything.
Key takeaways
- In the first three periods, the first shell holds 2 electrons and the second and third shells hold up to 8.
- Atoms with the same number of valence electrons are in the same group and react in similar ways.
Quiz: Electron shells and valence electrons (Age 15)
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In the atoms of the first three periods, what is the largest number of electrons the first shell can hold?
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Why do hydrogen and lithium sit in the same column of the periodic table?
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What can you say about two atoms that have the same number of valence electrons?
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