For an atom with atomic number $Z = 25$,determine the total number of $s$ electrons present.

$(i)$ Write the electronic configurations of the following ions:
$(a)$ $H^{-}$
$(b)$ $Na^{+}$
$(c)$ $O^{2-}$
$(d)$ $F^{-}$
$(ii)$ What are the atomic numbers of elements whose outermost electrons are represented by
$(a)$ $3s^{1}$
$(b)$ $2p^{3}$ and
$(c)$ $3p^{5}$?
$(iii)$ Which atoms are indicated by the following configurations?
$(a)$ $[He] 2s^{1}$
$(b)$ $[Ne] 3s^{2} 3p^{3}$
$(c)$ $[Ar] 4s^{2} 3d^{1}$

Explain the meaning of the $+ve$ and $-ve$ signs in the context of the wave function $\psi$ for an electron.

Explain the arrangement of orbitals in increasing order of energy on the basis of the $(n + l)$ rule.

The correct order of decreasing energy for the electrons whose quantum numbers $n$ and $l$ are given below, is
$A$. $n=5, l=2$
$B$. $n=5, l=0$
$C$. $n=4, l=3$
$D$. $n=4, l=1$

The electronic configuration of $Cu$ is