Why could no fixed position be given to hydrogen in Mendeleev’s periodic table?

Question

(i) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table? 

(ii) How and why does the atomic size vary as you go: 

(a) from left to right along a period?

(b) down a group?

Answer 1

(i) The properties of hydrogen resemble with both alkali metals as well as halogens. So, on the basis of its properties it could be placed in group I (with alkali metals) as well as in group VII (with halogens). 

(ii) (a) Atomic size decreases from left to right along a period because of increase in nuclear charge (atomic number) which tends to pull the electrons closer to the nucleus. 

(b) Atomic size increases down the group due to addition of new shells (orbits).