(A-Level Chem) From s-block to p-block to transition metals

The electronic configurations of the first three Periods of the Periodic Table are easy to follow.

• Period 1 has two elements H and He. It involves the filling up of main energy level n = 1 which consists of only a 1s orbital.
  
• Period 2 has 8 elements Li-Ne. It involves the filling up of main energy level n = 2 which consists of two sub-shells 2s and 2p.
  
• Period 3 has 8 elements Na-Ar. It involves the filling up of sub-shells 3s and 3p. 

Wait a minute. The main energy level n = 3 has three sub-shells 3s, 3p and 3d, not just two. Why is the 3d sub-shell not filled up in Period 3 so that we get 18 elements instead of only 8? This is because the 4s orbital is lower in energy than the 3d orbitals. After the 3p sub-shell is filled in Ar, the next electron goes to 4s. This filling up of 4s leads us to the s-block elements K and Ca in Period 4.

Period 1 to Period 3: Filling up of 1s (Period 1) --> 2s2p (Period 2) --> 3s3p (Period 3)

When we come to Period 4, we realize there are 18 elements K-Kr. 10 of these elements Sc-Zn are in d-block. We were told in O Level that these are transition metals. Well, it's not really so as we'll see later.

In O Level, we learned very little about transition metals. All we know are that, unlike Groups I and II metals, they have variable oxidation states (e.g. Fe²⁺ and Fe³⁺), form coloured compounds (e.g. CuSO₄.5H₂O(s) is blue, KMnO₄(aq) is purple), are dense (e.g. Density of iron = 7.86 g/cm³) and are catalysts of reactions (e.g. Fe used in the Haber Process).

These 10 elements in Period 4 correspond to the filling up of the 3d orbitals. From Sc to Zn, the electronic configuration changes from [Ar]4s²3d¹ to [Ar]4s²3d¹⁰.

Period 4: Filling up of 4s (in s-block: K, Ca) --> 3d (in d-block: Sc-Zn) --> 4p (in p-block: Ga-Kr)

The pattern, however, is not neat. It's broken at the 4^th element Cr and 9^th element Cu. Instead of [Ar]4s²3d⁴, Cr has [Ar]4s¹3d⁵ and instead of [Ar]4s²3d⁹, Cu has [Ar]4s¹3d¹⁰. There's no short explanations for why these happen. Just take them as they are. 

 

Now, what're transitions elements? Most syllabi define a transition metal as one which forms at least one stable ion with partially filled d orbitals. Manganese, iron and copper are examples of transition metals because they form Mn²⁺ (3d⁵), Fe³⁺ (3d⁵) and Cu²⁺ (3d⁹) with incomplete d sub-shell. Scandium and zinc, however, aren't considered transition metals because they form Sc³⁺ (3d⁰) and Zn²⁺ (3d¹⁰) which don't have partially filled d sub-shell.

Not all d-block elements are transition metals...

Try these questions before you check with the answers below.

1. Complete the table that shows the electronic configurations of elements in the first row of the d-block. Scandium has been done for you. 

			4s				3d
Sc	[Ar]		↑↓		↑
Ti	[Ar]		↑↓		↑	↑
V	[Ar]		↑↓		↑	↑	↑
Cr	[Ar]		↑		↑	↑	↑	↑	↑
Mn	[Ar]		↑↓		↑	↑	↑	↑	↑
Fe	[Ar]		↑↓		↑↓	↑	↑	↑	↑
Co	[Ar]		↑↓		↑↓	↑↓	↑	↑	↑
Ni	[Ar]		↑↓		↑↓	↑↓	↑↓	↑	↑
Cu	[Ar]		↑		↑↓	↑↓	↑↓	↑↓	↑↓
Zn	[Ar]		↑↓		↑↓	↑↓	↑↓	↑↓	↑↓

2. Write down the oxidation state of manganese in Mn²⁺, Mn₂O₃, MnO₂, MnO₄^2- and MnO₄^-.
   
3. Write down some of the formulas of the ions formed by the first row of the d-block elements and the number of electrons in their 3d shell. For example, Sc³⁺ (d⁰). 

2. Which elements in the first row of d-block are transition metals? Why? 

Answers: 1.

			4s				3d
Sc	[Ar]		↑↓		↑
Ti	[Ar]		↑↓		↑	↑
V	[Ar]		↑↓		↑	↑	↑
Cr	[Ar]		↑		↑	↑	↑	↑	↑
Mn	[Ar]		↑↓		↑	↑	↑	↑	↑
Fe	[Ar]		↑↓		↑↓	↑	↑	↑	↑
Co	[Ar]		↑↓		↑↓	↑↓	↑	↑	↑
Ni	[Ar]		↑↓		↑↓	↑↓	↑↓	↑	↑
Cu	[Ar]		↑		↑↓	↑↓	↑↓	↑↓	↑↓
Zn	[Ar]		↑↓		↑↓	↑↓	↑↓	↑↓	↑↓

2. Oxidation states: Mn²⁺ (+2), Mn₂O₃ (+3), MnO₂ (+4), MnO₄^2- (+6) and MnO₄^- (+7). 3. Ti: +3 (d¹), +4 (d⁰). V: +2 (d³), +3 (d²), +4 (d¹), +5 (d⁰). Cr: +3 (d³), +6 (d⁰). Mn: +2 (d⁵), +3 (d⁴), +4 (d³), +6 (d¹), +7 (d⁰). Fe: +2 (d⁶), +3 (d⁵). Co: +2 (d⁷), +3 (d⁶). Ni: +2 (d⁸). Cu: +1 (d¹⁰), +2 (d⁹). Zn: +2 only (d¹⁰). 4. Titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper are transition metals because they form ions with incomplete d sub-shell.