If an atom has the electron configuration 1s22s22p 1 , how many electrons must it gain to achieve an octet?

According  to  octet  rule  element  has  eight  electrons  in  its valence shell.This  give it  the  same  electronic  configuration  as  a  group  18  elements.Atom  with  electronic  configuration  1s2 2s2 2p1  require  five  electrons  to  achieve  octet. This  is  because the  valence  electrons  are  3 in 2s2  and 2p1  and  hence require  5  electrons  to  be  filled.

They did not study the continental drift

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.