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(a) Lithium, sodium, potassium are all m...

(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

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(a) The atoms of all these elements have one electron each in their valence shell. That is why, these elements are placed in the group 1 known as alkali metal group. The electronic configurations of these elements are given :

All the three elements evolve hydrogen gas on reacting with water
`2Li+2H_(2)Oto2LiOH+H_(2)`
`2Na+2H_(2)Oto2NaOH+H_(2)`
`2K+2H_(2)Oto2KOH+H_(2)`
Apart from this, all the elements happen to be the first elements of their respective periods. For example,
Second period starts from lithium (Li)
Third period starts from sodium (Na)
Fourth period starts from potassium (K).
(b) Both these elements have completely filled shells. Helium (Z = 2) has two electrons in the only shell (K shell). The other element neon (Z = 10) has both K and L shells fully filled (2, 8). Because of the filled shells, the atoms of these elements do not have any desire to take part in chemical combination and they have been placed together in the same group known as group 18 or zero group.
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