Trends Across a Series: Generally, $E^\circ$ values for $M^{2+}/M$ become less negative (or more positive) across the 3d series, indicating a decreasing tendency to form $M^{2+}$ ions. This correlates with increasing ionisation enthalpies.
Anomalies:
- Mn, Ni, Zn: More negative $E^\circ$ than expected.
  - Mn: Due to the stability of $Mn^{2+}$ ($d^5$ configuration).
  - Zn: Due to the stability of $Zn^{2+}$ ($d^{10}$ configuration).
  - Ni: Due to unusually high negative enthalpy of hydration ($\Delta_{hyd}H^\circ$) of $Ni^{2+}$ ions, which compensates for the high ionisation energy.
- Copper (Cu): Has a positive $E^\circ$ value ($+0.34V$) for $Cu^{2+}/Cu$. This means copper does not liberate hydrogen from acids. Its high energy required to transform $Cu(s)$ to $Cu^{2+}(aq)$ is not compensated by its hydration enthalpy.
$E^\circ$ for $M^{3+}/M^{2+}$:
- Mn: Very high positive $E^\circ$ for $Mn^{3+}/Mn^{2+}$ ($+1.57V$), indicating $Mn^{3+}$ is a strong oxidising agent (prefers to be reduced to $Mn^{2+}$ which has stable $d^5$).
- Fe: Relatively low $E^\circ$ for $Fe^{3+}/Fe^{2+}$ ($+0.77V$), indicating $Fe^{3+}$ is less oxidising than $Mn^{3+}$ (due to stable $d^5$ for $Fe^{3+}$).
- Cr: Negative $E^\circ$ for $Cr^{3+}/Cr^{2+}$ ($-0.41V$), indicating $Cr^{2+}$ is a strong reducing agent (prefers to be oxidised to $Cr^{3+}$ which has stable $d^3$).

Most transition metals are electropositive and react with mineral acids.
Early transition metals like Ti and V are passive to dilute non-oxidizing acids at room temperature due to oxide layer formation.
The reactivity generally decreases across the series.
$Ti^{2+}$, $V^{2+}$ and $Cr^{2+}$ are strong reducing agents. For example, $Cr^{2+}$ (d$^4$) can be easily oxidized to $Cr^{3+}$ (d$^3$, stable half-filled $t_{2g}$ orbitals).
$Mn^{3+}$ and $Co^{3+}$ are strong oxidizing agents. For example, $Mn^{3+}$ (d$^4$) is easily reduced to $Mn^{2+}$ (d$^5$, stable half-filled d-orbitals).

Transition metals exhibit either diamagnetism or paramagnetism.

Diamagnetism: Substances repelled by a magnetic field, caused by the presence of only paired electrons.
Paramagnetism: Substances attracted to a magnetic field, caused by the presence of unpaired electrons. The magnetic moment ($\mu$) is calculated using the "spin-only" formula: $$\mu = \sqrt{n(n+2)}$$ where $n$ is the number of unpaired electrons and $\mu$ is in Bohr magnetons (BM).
Ferromagnetism: An extreme form of paramagnetism where substances are very strongly attracted to magnetic fields (e.g., Fe, Co, Ni).
Orbital Angular Momentum: For 1st series transition metals, the contribution of orbital angular momentum to magnetic moment is often quenched and can be ignored.

Ion	Configuration	Unpaired electrons (n)	$\mu$ (Calculated)	$\mu$ (Observed)
Sc$^{3+}$	$3d^0$	0	0	0
Ti$^{3+}$	$3d^1$	1	1.73	1.75
Ti$^{2+}$	$3d^2$	2	2.84	2.76
V$^{2+}$	$3d^3$	3	3.87	3.86
Cr$^{2+}$	$3d^4$	4