1) Answer is: False.
A period in the periodic table is a horizontal row. For example, sodium and chlorine are in third period and they have very different properties. Sodium is solid metal and chlorine is gaseous nonmetal. Elements in the same row have the same number of electron shells, but different proporties.
2) Answer is: True.
Group is a column of elements in the periodic table. For example, lithium and sodium are in 1 group of the periodic table and they both have one valence electron.
3) Answer is: valence electrons.
For example, electron configuration of oxygen atom: ₈O 1s² 2s² 2p⁴.
Oxygen atom has six valence electrons, so it is in 16 group of Periodic table of elements.
