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 of the following orders is not correct about the property shown against it?

Which of the following sequences represents the atomic numbers of only representative elements?

In what properties do the first elements of a group differ from the subsequent elements of the same group?

The electronic configurations of four elements are $:-$
$(I) \ [Xe] 6s^1$
$(II) \ [Xe] 4f^{14} 5d^1 6s^2$
$(III) \ [Ar] 4s^2 4p^5$
$(IV) \ [Ar] 3d^7 4s^2$
Select the incorrect statement about these elements $:-$

State whether the following statements are true or false:
$(i)$ Higher the ionization enthalpy,lower the screening effect.
$(ii)$ The ionization enthalpy of $Be$ is higher than that of $B$.
$(iii)$ The shielding effect increases as we move from left to right in a period.
$(iv)$ The increasing order of the first ionization energy is $B < Be < O < N$.