''Elements in Periodic Table show periodicity of properties.'' List any four such properties.

Mention any two trends exhibited by elements when we go from left to right across the period of the periodic table.

An element $X$ has a mass number of $35$ and the number of neutrons is $18$. Determine the atomic number and electronic configuration of $X$. Also, identify the group number, period number, and valency of $X$.

$K$ and $Ca$ have atomic numbers $19$ and $20$ respectively,and they belong to the same period.
$(a)$ Which among them would have a smaller atomic size?
$(b)$ Which one would be more electropositive?
$(c)$ To which group would each of them belong?

List any two properties of the elements belonging to the first group of the modern periodic table.

Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?