Question

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how many electrons occur in the valence level of group 3a and 3b elements? why are they different?
elements in group 3a contain valence electrons.
elements in group 3b contain valence electrons and electron in an inner d orbital.
group a elements are representative elements while group b elements are representative elements.
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Explanation:

• For Group 3A elements (e.g., boron - B, aluminum - Al), the general electron configuration for the valence shell is \(ns^{2}np^{1}\). The sum of electrons in the outermost \(n\) (principal quantum number) shell gives the number of valence electrons. Here, \(2 + 1=3\).
• For Group 3B elements (in the context of the older IUPAC numbering where Group 3B corresponds to modern Group 3, e.g., scandium - Sc: \([Ar]3d^{1}4s^{2}\)), the valence electrons are considered as the electrons in the outermost \(s\) - orbital (\(n = 4\) for Sc) which is \(2\) electrons, and there is \(1\) electron in the inner \(d\) - orbital (\(3d\) for Sc). Group A elements are representative elements (they follow the octet rule based on \(s\) and \(p\) - orbital filling in the valence shell), while Group B elements (in the older numbering, now mostly transition metals) are transition elements (where \(d\) - orbitals are being filled in a lower - lying shell while \(s\) - orbitals of the higher shell are filled first in some cases).

Answer:

Elements in Group 3A contain \(3\) valence electrons.
Elements in Group 3B contain \(2\) valence electrons and \(1\) electron in an inner \(d\) orbital.
Group A elements are representative elements while Group B elements are transition elements.