Why does the basic character decrease along a period? Give reasons.

Compare the radii of two species $X$ and $Y$. Give reasons for your answer.
$(a)$ $X$ has $12$ protons and $12$ electrons.
$(b)$ $Y$ has $12$ protons and $10$ electrons.

Which of the following are the characteristics of isotopes of an element?
$(i)$ Isotopes of an element have same atomic masses
$(ii)$ Isotopes of an element have same atomic number
$(iii)$ Isotopes of an element show same physical properties
$(iv)$ Isotopes of an element show same chemical properties

Write true or false for the statement: In Mendeleev's periodic table,elements were arranged in the order of their increasing atomic masses and it was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.

Name the following:
$(a)$ $A$ metal that can be cut with a knife.
$(b)$ $A$ metal that is liquid at room temperature.
$(c)$ $A$ metal that has $3$ valence electrons.

When Mendeleev started his work,the number of known elements were: