Transition Metals

Transition metals are d-block elements that have their d-orbitals partially filled. Zinc and Scandium are not considered transition metals, even though they're d-block elements. They do not have partially filled d-orbitals. Zn has the electronic configuration [Ar]4s²3d¹⁰. Sc has the electronic configuration [Ar]4s²3d¹.

Properties of transition metals:
1. They have high melting and boiling temperatures.
In their metallic structure, 3d as well as 4s electrons are available for delocalisation. Since there are more electrons within the sea of electrons, there is a greater electrostatic attraction - hence the higher melting temperatures.
2. They have variable oxidation states - except Scandium (Sc) and Zinc (Zn)
3. They form coloured compounds and ions - except Sc and Zn form white compounds and colorless ions
4. They show catalytic activity

Sn can only form Sn ³⁺ ion which has the same electronic configuration as [Ar].
Zn loses both of its 4s electrons to form only Zn²⁺ ions (with the electronic configuration of
[Ar]3d¹⁰).

Electronic Configurations:

In d block, electrons are added to an inner d-oribital and this shields the outer 4s electrons from the increased nuclear charge (i.e. they're easier to lose). Therefore, the atomic radius only decreases slightly and electronegativity and ionisation energies increase only slightly.

Examples of some electronic configurations:
Sc ----> [Ar]4s²3d¹
Zn ----> [Ar]4s²3d¹⁰
Cu ----> [Ar]4s¹3d¹⁰
Cr ----> [Ar]4s¹3d⁵

In Cr and Cu, the 3d orbitals fill first before the 4s orbitals. This is due to the increased stability offered by full and half-filled shells.
Moreover,
Fe²⁺ (3d⁶) is readily oxidised to Fe³⁺ (3d⁵)
Mn²⁺ (3d⁵) is not readily oxidised to Mn³⁺ (3d⁴)

When transition metals form ions, they lose electrons from the 4s sub-shell first before 3d sub-shell. Fe²⁺ has the eletronic structure [Ar]3d⁶  rather than [Ar]4s²3d⁴.
This occurs because of the repulsion forces on the 4s electrons, repelling them further away from the nucleus, by the 3d electrons. Therefore, 4s electrons are pushed to a higher energy level, higher than 3d, thus electrons in 4s sub-shell are lost before 3d sub-shell.