Question

Ques 1. why we take only carbon-12 atom as measurement for the atomic mass of any element ?
Ques 2. on what basis elements are numbered in periodic table ?
Ques 3. on what basis elements are given a particular valency by scientists ?
Ques 4.  on what basis elements are given there ions by scientists ? 
please answer all my four questions 

Answer 1

Ans-1 ) because it is the most naturally happening isotope of it and .....by composition it is the greatest of all other carbons....thus is taken as the unviversal mass

Ans-2 ) in increasing atomic number from left to right horizontally ....with 8 elements in each row..and there is much more to it.....if u want to know more than tell me..

Ans-3 ) Valencies or energy level.....they are determined by the number of outer shell electrons.......for transition metals u do some algebra....u should learn tht in grd 9........ if u wanna know tht leave a comment..

Ans-4 ) I dont get the question that much ..but ions of the element are formed if they either loose or gain electrons ....metals loose all the valencies and form positively charged cations....and non metals gain electrons from metal to produce negatively charges anions.......u can remember it onions bring tears right so its negative........cations are positive....u can remember a cat ot something :P hehe........hope I helped