Cobalt (Co) Revision Guide

Atomic Model Details of Cobalt (Co)

Cobalt (Co) is a transition metal located in Group 9 and Period 4 of the periodic table. Its atomic structure is characterized by the following:

Atomic Number (Z): 27
- This indicates 27 protons in the nucleus of each cobalt atom.
- For a neutral cobalt atom, there are also 27 electrons.
Mass Number (A): The most abundant isotope of cobalt is Cobalt-59 ($^{59}\text{Co}$), which accounts for 100% of naturally occurring cobalt.
- Number of Neutrons: For $^{59}\text{Co}$, the number of neutrons is $\text{A} - \text{Z} = 59 - 27 = 32$.
Atomic Size:
- Covalent Radius: Approximately 126 picometers (pm). This value reflects the size of the atom when bonded covalently.
- Atomic size generally decreases across a period for transition elements due to increasing nuclear charge and relatively poor shielding by d-electrons, leading to a stronger pull on the valence electrons.

The electronic configuration of an atom describes the distribution of its electrons in atomic orbitals.

Ground State Electronic Configuration:
- Full configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^7 4s^2$
- Noble gas configuration: $[Ar] 3d^7 4s^2$
- The outermost shell is the 4th shell, containing 2 electrons in the $4s$ orbital. The incomplete $3d$ subshell with 7 electrons is characteristic of a transition metal.
Shell Diagram Representation (Electron Distribution per Shell):
- K-shell (n=1): 2 electrons ($1s^2$)
- L-shell (n=2): 8 electrons ($2s^2 2p^6$)
- M-shell (n=3): 15 electrons ($3s^2 3p^6 3d^7$)
- N-shell (n=4): 2 electrons ($4s^2$)
This distribution illustrates that the valence electrons (those involved in chemical bonding) are primarily the $4s$ electrons and the partially filled $3d$ electrons.

The position of Cobalt in the periodic table influences its chemical properties through several periodic trends.

First Ionization Enthalpy (IE1): Approximately 760 kJ/mol.
Trend: For transition metals, ionization enthalpies are generally higher than s-block metals but lower than p-block elements in the same period. Across the 3d series, IE1 generally shows an increasing trend with minor irregularities. Cobalt’s IE1 is relatively high due to its increasing nuclear charge and contraction of atomic radius across the period, requiring more energy to remove the first electron.

Pauling Scale Electronegativity: Approximately 1.88.
Trend: Electronegativity generally increases across a period. Cobalt’s electronegativity is typical for a transition metal, falling between highly electropositive s-block elements and more electronegative p-block elements. It indicates a moderate tendency to attract electrons in a chemical bond.

Electron Gain Enthalpy: Approximately -64 kJ/mol (exothermic, indicating a slight tendency to accept an electron).
Trend: For metals, electron gain enthalpy values are generally positive or slightly negative, indicating that they do not readily accept electrons. Cobalt’s small negative value is characteristic of a metallic element.

Covalent Radius: Approximately 126 pm.
Trend: Atomic radii in the 3d transition series generally decrease from Sc to Cr/Mn, then remain relatively constant or show a slight increase towards the end of the series (Ni, Cu, Zn). This trend is attributed to the increasing nuclear charge (contracting effect) and the increasing number of d-electrons providing shielding (expanding effect), which tend to counterbalance each other for later elements in the series. Cobalt falls in the plateau region.

Cobalt is a lustrous, silvery-blue metal with distinct physical characteristics.

Density: 8.90 g/cm$^3$ at 20 °C.
State at Room Temperature: Solid.
Color: Silvery-blue metallic luster.
Melting Point: 1495 °C.
Boiling Point: 2927 °C.
Magnetic Properties: Cobalt is ferromagnetic, meaning it can be strongly magnetized, similar to iron and nickel. This property arises from the presence of unpaired electrons in its 3d orbitals.
Hardness: It is a relatively hard and brittle metal.