Question

here are some elements and their electrons configurations:
he: 1s²
be: 1s²2s²
c: 1s²2s²2p²
mg: 1s²2s²2p⁶3s²
si: 1s²2s²2p⁶3s²3p²
ca: 1s²2s²2p⁶3s²3p⁶4s²
the total number of the elements above with 2 valence electrons is
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Explanation:

Step1: Determine valence electrons for each element

Valence electrons are in the outermost shell.

• For \(He:1s^{2}\), the valence shell is \(1s\), number of valence electrons \(= 2\).
• For \(Be:1s^{2}2s^{2}\), the valence shell is \(2s\), number of valence electrons \(= 2\).
• For \(C:1s^{2}2s^{2}2p^{2}\), the valence shell is \(2s^{2}2p^{2}\), number of valence electrons \(=4\).
• For \(Mg:1s^{2}2s^{2}2p^{6}3s^{2}\), the valence shell is \(3s\), number of valence electrons \(= 2\).
• For \(Si:1s^{2}2s^{2}2p^{6}3s^{2}3p^{2}\), the valence shell is \(3s^{2}3p^{2}\), number of valence electrons \(=4\).
• For \(Ca:1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}\), the valence shell is \(4s\), number of valence electrons \(= 2\).

Answer:

\(4\)