Answer:
a. O: Period 2. VIA family. 6 valence electrons.
b. B: Period 2. IIIA family. 3 valence electrons.
c. Na: Period 3. IA family. 1 valence electron.
d. Mg: Period 3. IIA family. 2 valence electrons.
Step-by-step explanation:
There is a property of the periodic table which is: for neutral atoms, the number of valence electrons is equal to the atom's main group number. The group numbers are found in columns in the periodic table and periods are found in rows.
a. If you see the periodic table, you can see that O belongs to the second period and VIA family, VI is 6, so its valence electrons are 6.
b. You can see that B is in the second period too but is in the IIIA family, III is 3, so the valence electrons of B are 3.
c. Na is in the third period and IA family, I is 1, so Na only contains 1 valence electron.
d. Mg is in the third period too, but is in the IIA family, II is 2, so Mg has 2 valence electrons.