Question: Using the periodic table to locate each element, write the electron configuration of (a) Ru; (b) Cu; (c) Ni.

The distribution of electrons in atomic or molecular orbitals of an atom, molecule, or other physical structure is known as electron configuration.

(a) Based on the periodic table, Ru has the atomic number of $44$, therefore it has $44$electrons with electron configuration:

$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^{2}3{\mathrm{d}}^{10}4{\mathrm{p}}^{6}5{\mathrm{s}}^{1}4{\mathrm{d}}^7$or$\left[\mathrm{Kr}\right]5{\mathrm{s}}^{1}4{\mathrm{d}}^7$

Instead of having a full 5 s orbital, the atom is more stable when there is more unpaired electrons.

(b) Based on the periodic table, Cu has the atomic number of $29$, therefore it has $29$electrons with this electron configuration:

$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^{1}3{\mathrm{d}}^{10}$or $\left[\mathrm{Ar}\right]4{\mathrm{s}}^{1}3{\mathrm{d}}^{10}$

The atom is more stable when the d orbital is fully filled, and the 4 s orbital is only half-filled.

(c) Based on the periodic table, Ni has the atomic number of $28$, therefore it has $28$electrons with this electron configuration:

$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^{2}3{\mathrm{d}}^8$or $\left[\mathrm{Ar}\right]4{\mathrm{s}}^{2}3{\mathrm{d}}^8$