Recall the number of elements in each of the first 3 periods
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1st period ( Period 1 ) : 2 elements

2nd period ( Period 2 ) : 8 elements

3rd period ( Period 3 ) : 8 elements

Explanation:

Accordingly to the periodic table, we have the following elements in period 1, 2 and 3 respectively:

Period 1: Hydrogen, H and Helium, He

Period 2: Lithium (Li), Berrylium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Neon (Ne)

Period 3: Sodium (Na), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulphur (S), Chlorine (Cl), Argon (Ar)

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."