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Question
If an atom ‘A’ has atomic number 19 and mass number 39, state –
- Its electronic configuration.
- The number of valence electrons it possesses.
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Solution
Atom ‘A’ has mass number A = 39 and atomic number Z = 19 = p
∴ A = Z + n
A = p + n
39 = 19 + n
n = 39 – 19 = 20
But e = p = 19
- A (K, L, M, N)
19 = 2, 8, 8, 1
There will be 2 electrons in K-shell or 1st shell
8 electrons in 2nd shell or L-shell
8 electrons in 3rd shell or M-shell
1 electron in 4th shell or nth-shell - The number of valence electrons i.e. in outer most shell = 1 electron.
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