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

Fill in the blanks:
$(i)$ Atoms and ions having the same number of electrons are known as ........... .
$(ii)$ The quantitative measure of the tendency of an element to lose an electron is called ........... .
$(iii)$ The screening effect is also known by the name ........... .
$(iv)$ The enthalpy change when a gaseous neutral atom forms a negative ion is called ........... .

The number of the following pairs of elements which exhibit a diagonal relationship is:
$1. B \text{ and } Si$
$2. Be \text{ and } Al$
$3. Li \text{ and } Mg$
$4. Al \text{ and } S$

Which elements in groups $1$ to $17$ differ in many respects from the other elements of their respective groups?

Which of the following orders is incorrect?

Which of the following pairs of atomic numbers belongs to the same group in the periodic table?