The elements K and Na belong to the same group i.e. 1 of the Periodic Table. However they are separately placed in 4th and 3rd periods respectively.
Carbon.
chlorine
chlorine (Cl)
Barium (Ba) Barium (Ba) : is in period 6 group 2. Same group where Magnesium (Mg) is found.
Potassium (K)
Mercury belongs to same group as gold. Both belong to period-6.
Br (Bromine) is a period 4 element in the same group as F.
Carbon.
chlorine
There is no element that has been found yet with those characteristics ( group 5, period 2)
chlorine (Cl)
Barium (Ba) Barium (Ba) : is in period 6 group 2. Same group where Magnesium (Mg) is found.
Potassium (K)
elements in group 2B of the periodic table with its atomic number 30 Zinc 48 Cadmium 80 Mercury 112 Ununbium (the discoverers of this have suggested the name copernicium) These elements are also known as transition elements and they belong to d-block.
Ba ad Ca belongs to same group. The group number is 2.
Element A and element B are the most likely to have similar properties because they belong in the same family, because if you know an element's family you can tell the number of valance electrons and the elements they react with. In periods, the properties are not alike, they slowly change in a pattern.
sodium and lithium are both group 1 elements (like potassium)