Hydrogen occupies a unique position in the modern periodic table because of the following reasons:
(i) Both hydrogen and halogens have similar outer electric configuration. Therefore some of the properties of hydrogen are similar to those of alkali metals and hence it can be placed in group 1 along with alkali metals.
(ii) Both hydrogen and halogens have similar outer electric configuration. Therefore some of the properties of hydrogen are similar to those of halogens and hence it can be placed in group 17 along with halogens.
(iii) In some properties, it differs from both hydrogen and halogens. For examples, the oxide of hydrogen (H ₂O) is neutral but the oxides of alkali metals (Na ₂O, K₂O) are basic while those of halogen (Cl₂O₇, Br₂O₅) are acidic.

In Mendeleev's periodic table, cobalt (Co) with a higher atomic mass of 58.93 u is placed before nickel (Ni) due to the following reasons:
(a) The properties of cobalt are similar to those of rhodium (Rh) and iridium (Ir) and
(b) The properties of nickel are similar to those of palladium (Pd) and platinum (Pt).

(a) Na, Si and Cl have different properties, therefore they do not form a Dobereiner's triad  even though the atomic mass of the middle element Si is approximately the average of the atomic masses of Na and Cl. That is (23+35) / 2 = 29
(b) Be, Mg and Ca have many similar properties and also the atomic mass of the middle element Mg is approximately the average of the atomic masses of Be and Ca that is (9+40)/2 = 24.5. Therefore, they form a Dobereiner's triad.

(a) F and Cl are first and eighth element in the given sequence, therefore, they have similar properties. Although Na and K have similar properties but they are not related as first and eighth element in the given sequence.
(b) The given sequence represents Newland's Law of Octaves.

The arrangements of elements are called a Dobereiner's triad. For example Li, Na, K and Ca, Sr, Ba.