(1) The difference between O levels and A levels chemistry is
something like your PSLE and Sec 4 Maths, except that without your
O levels Chemistry, your A levels Chemistry is as good as
dead.
(2) Cannot. There's no such thing as (2H)^2 = 4H. The 'power' is
not applicable in the arithmetic sense in chemistry. Superscripts
are used to denote the charge of the species and subscripts are
used to denote the number of that particular species.
(3) To understand this, you need to read up on how electron
subshells are filled up.
Basically, during your O levels, you do remember that Potassium and
Calcium have a 'weird' electronic configuration of 2,8,8,1 and
2,8,8,2 instead of 2,8,9 and 2,8,10. This is because of certain
subshells are filled up first (more specifically, your s subshells
are filled up before your d subshell of the same principal quantum
subshell).
At A levels chemistry, you will learn about the electron subshells
and how they are filled up. Your periodic table actually gives you
a pretty good guide of which shells are filled up in which order as
each particular section of elements are due to filling up of
certain subshells (eg first 2 groups are s subshells, transition
elements are due to d subshells, groups 3 to 8 (or 13 to 18 if u
include transition metal groups) are due to p subshells, lactinides
and actinides are due to f subshells etc.
I hope that pretty much answers your questions.
