Identify the incorrect order of properties among the following:
$(a)$ Basic nature: $Na_2O > MgO > Al_2O_3 > SiO_2$
$(b)$ $2^{nd}$ Ionization Potential $(I.P.)$: $Na > S > P > Si$
$(c)$ Electron affinity: $O > S > Se$
$(d)$ Size: $I^- < I < I^+$

How many of the following statements are correct?
$(a)$ 'He' is the second most abundant element in the universe.
$(b)$ The symbol for the element with atomic number $110$ is Ds.
$(c)$ Osmium has the highest density among all elements.
$(d)$ Francium is the most electropositive element in the periodic table.

Which property decreases from left to right across the periodic table and increases from top to bottom?
$(i)$ Atomic radius
$(ii)$ Electronegativity
$(iii)$ Ionisation energy
$(iv)$ Metallic character

To which block do the elements with the following outer electronic configurations belong?
$(i)$ $6s^2 4f^3$
$(ii)$ $3s^2 3p^4$
$(iii)$ $3s^1$
$(iv)$ $3d^2 4s^2$

$A, B$,and $C$ are hydroxy-compounds of the elements $X, Y$,and $Z$ respectively. $X, Y$,and $Z$ are in the same period of the periodic table. $A$ gives an aqueous solution of $pH < 7$. $B$ reacts with both strong acids and strong alkalis. $C$ gives an aqueous solution which is strongly alkaline.
Which of the following statements is / are true?
$I$: The three elements are metals.
$II$: The electronegativities decrease from $X$ to $Y$ to $Z$.
$III$: The atomic radius decreases in the order $X, Y$,and $Z$.
$IV$: $X, Y$,and $Z$ could be phosphorus,aluminium,and sodium respectively.

In general,the properties that decrease and increase down a group in the periodic table respectively are