F>P>Si>Be
On moving left to right in a period valence electron increases by one. The atomic numbers are F(9),P(15),Si(14),Be(4) .
By the electronic configuration
F−2,7,P−2,8,5,Si−2,8,4, Be−2,2.
Hence F has 7 electrons, P has 5 , Si has 4 and Be has 2 electrons in their outermost shell.