Electron configuration of elements

Order of filling orbital and electron configuration

Orbitals of a multi-electron atom not likely quite the same as the hydrogen atom orbitals. But for practical purposes, however, the number of orbitals and there shapes in multi-electron cases may be taken the same as for the hydrogen atom.

In multi-electron atoms, experimental studies of spectra show that orbitals with the same value of n but different l values have different energies.

Thus 3S orbital lower energy than 3P orbitals which again lower energy than 3d orbitals. But the orbitals belonging to a particular type ( P or d or f ) will be of equal energy (degenerate) state of an atom or an ion.

So three P orbitals or the five d orbitals originating from the same n will be degenerate.

But the separation of orbitals of a major energy level into sub-levels is primarily due to the interaction among the many electrons.

This interaction leads to the following relative order of the energies of each type of orbitals.

1S〈2S〈2P〈3S〈3P〈4S〈3d 〈4P〈5S〈4d〈5P〈6S〈4f〈5d 〈6P〈7S〈5f〈6d……

Orbital energy levels in many-electron atoms

Admittedly it is often difficult for the readers to remember the orbital energy level in many-electrons atoms. Thus a trivial but distinctly more convenient way for electron configuration

Orbital energy levels in many-electron atoms
Orbital energy level

The different orbitals originating from the same principal quantum number n are written in the horizontal lines.

Now inclined parallel lines are drawn through the orbitals according to the above picture. Filling up the different orbitals by electrons will follow these lines.

If an element which contains 36 electrons the electronic configuration of this element according to the above diagram

1S2 2S2 2P6 3S2 3P6 4S2 3d10 4P6

Aufbau building up principle

The question that arises now, how many electrons accommodated per orbital?. The answer to this question given by Pauli’s exclusion principle.

Pauli exclusion principle electron configuration

No two electrons of the atom can have the same four quantum numbers.

So this principle tells us that, each orbital maximum of two electrons is allowed. And these two electrons have the same three quantum numbers namely the same n, same l, and the same ml but a spin quantum number different.

Any conflict with the Pauli Principle can now be avoided if one of the electrons has the spin quantum numbers = (+½) and other = (-½).

An alternative statement of Pauli’s exclusion principle

No more than two electrons can be placed in one and the same orbital.

When an orbital contains two electrons, the electrons are paired. These two electrons per orbital given the maximum accommodation of electrons.

Maximum number of electrons in a subshell

The maximum number of electrons for a particular quantum number n given by 2n2.

How to find the number of electrons in an element?
Number of electrons

For the electronic configuration of elements, electrons are filling in different orbitals obeying certain rules.

Rules Aufbau or Building up the principle

  1. Electrons are fed into orbitals in order of increasing energy until all the electrons have been accommodated.
  2. Electrons will tend to maintain maximum spin. Thus electrons with similar spin occupied first will prefer to remain unpaired.
  3. According to Hund’s rule, electrons are filling in the orbital with maximum spin multiplicity.
  4. Spin pairing occurs only when vacant orbitals of similar energy are not available for occupation.

Electronic configuration hydrogen to neon

Hydrogen has its only one electron in the 1S orbital and this electron occupied in 1S orbital.

But helium, the second electron occupies the 1S orbital since the next 2S orbital much higher energy.

Thus the electron configuration from hydrogen to neon with the pictorial representation given as

Hydrogen (H)1S1
Helium (He)1S2
↑↓
Lithium (Li)1S22S1
↑↓
Beryllium (Be)1S22S2
↑↓↑↓
Boron (B)1S2 2S2 2P1
↑↓↑↓
1S2S2Px2Py2Pz
Carbon (C)1S² 2S² 2P²
↑↓↑↓
1S2S2Px2Py2Pz
Nitrogen (N)1S2 2S2 2P3
↑↓↑↓
1S2S2Px2Py2Pz
Oxygen (O)1S2 2S2 2P4
↑↓↑↓↑↓
1S2S2Px2Py2Pz
Fluorine (F)1S2 2S2 2P5
↑↓↑↓↑↓↑↓
1S2S2Px2Py2Pz
Neon (Ne)1S2 2S2 2P6
↑↓↑↓↑↓↑↓↑↓
1S2S2Px2Py2Pz

The electron configuration of the second period

After neon, the next available orbital is 3S being followed by 3P. Thus these orbitals filling progressively by electrons to given the electronic configuration of sodium to argon.

So, the general electronic configuration of these elements are

[Ne] 3S1→2 3P1→6

Sodium (Na)1S2 2S2 2P6 3S1
↑↓↑↓↑↓↑↓↑↓
1S2S2Px2Py2Pz3S
Magnesium (Mg)1S2 2S2 2P6 3S2
↑↓↑↓↑↓↑↓↑↓↑↓
1S2S2Px2Py2Pz3S
Aluminum (Al)1S2 2S2 2P6 3S2 3P1
↑↓
[Ne]3S3Px3Py3Pz
Silicon (Si)1S2 2S2 2P6 3S2 3P2
↑↓
[Ne]3S3Px3Py3Pz
Phosphorus (P)1S2 2S2 2P6 3S2 3P3
↑↓
[Ne]3S3Px3Py3Pz
Sulphur (S)1S2 2S2 2P6 3S2 3P4
↑↓↑↓
[Ne]3S3Px3Py3Pz
Chlorine (Cl)1S2 2S2 2P6 3S2 3P5
↑↓↑↓↑↓
[Ne]3S3Px3Py3Pz
Argon (Ar)1S2 2S2 2P6 3S2 3P6
↑↓↑↓↑↓↑↓
[Ne]3S3Px3Py3Pz

Potassium and calcium electron configuration

4S orbital, being of lower energy than the 3d. Thus for filled electron 4S orbital filling first. So the general electron configuration of potassium and calcium

[Ar] 4S1→2

Potassium (K)[Ar] 4S1
[Ar]4S
Calcium (Ca)[Ar] 4S1
↑↓
[Ar]4S

Electronic configuration of first transition series

The first element in the first transition series starts with Scandium and ending with Zinc.

Thus the twenty-first electron goes to the next available higher energy 3d orbital.

And five degenerate 3d subshell with the capacity of ten electrons.

But due to the presence of partially filled d orbitals, these elements generate some special properties and general electron configuration of these elements are

[Ar] 4S1→2 3d1→10

Although 4S orbital is lower energy than 3d, some exceptional cases 3d electron filled before 4S orbital.

Scandium, titanium, vanadium

Scandium (Sc)[Ar] 4S2 3d1
↑↓
[Ar]4S3d3d3d3d3d
Titanium (Ti)[Ar] 4S2 3d2
↑↓
[Ar]4S3d3d3d3d3d
Vanadium (V)[Ar] 4S2 3d3
↑↓
[Ar]4S3d3d3d3d3d

Electronic configuration of chromium

The general electron configuration of chromium is

Vanadium (V)[Ar] 4S2 3d4
↑↓
[Ar]4S3d3d3d3d3d

But in reality, experimental studies on chromium revel their general electronic configuration and this represented as

Vanadium (V)[Ar] 4S1 3d5
[Ar]4S3d3d3d3d3d

Half filled orbital is more stable than the partially filled orbital. Thus chromium atom reordering of electrons to gain extra stability associated with a half-filled d subshell.

Manganese, iron, cobalt, nickel

Manganese (Mn)[Ar] 4S2 3d5
↑↓
[Ar]4S3d3d3d3d3d
Iron (Fe)[Ar] 4S2 3d6
↑↓↑↓
[Ar]4S3d3d3d3d3d
Cobalt (Co)[Ar] 4S2 3d7
↑↓↑↓↑↓
[Ar]4S3d3d3d3d3d
Nickel (Ni)[Ar] 4S2 3d8
↑↓↑↓↑↓↑↓
[Ar]4S3d3d3d3d3d

Electronic configuration of copper

The general electron configuration of chromium is

Copper (Cu)[Ar] 4S2 3d9
↑↓↑↓↑↓↑↓↑↓
[Ar]4S3d3d3d3d3d

But the actual electron distribution reveals that their electronic configuration and are better represented as

Copper (Cu)[Ar] 4S1 3d10
↑↓↑↓↑↓↑↓↑↓
[Ar]4S3d3d3d3d3d

Filled orbital is more stable than the partially filled orbital due to greater exchange energy. Thus copper atom reordering of electrons to gain extra stability associated with a filled d subshell.

Zinc electron configuration

Zinc is an element with fully filled d orbital and according to the definition of transition elements, zinc is a non-transition element.

But some properties of zinc are similar to the transition elements, thus it is also considered as transition elements.

Zinc (Zn)[Ar] 4S2 3d10
↑↓↑↓↑↓↑↓↑↓↑↓
[Ar]4S3d3d3d3d3d

All the 3d orbital which represented in the above electron configuration designate as

dxy, dxz, dyz, d, dx²-y²

Germanium electron configuration

In Gallium atomic number 31 and thirty-first electron goes to the 4P orbital, the next available orbital of the higher energy level.

Gallium (Ga)[Ar] 4S2 3d10 4P1
[Ar] 4S2 3d104P4P4P
Germanium (Ge)[Ar] 4S2 3d10 4P2
[Ar] 4S2 3d104P4P4P
Arsenic (As)[Ar] 4S2 3d10 4P3
[Ar] 4S2 3d104P4P4P
Selenium (Se)[Ar] 4S2 3d10 4P4
↑↓
[Ar] 4S2 3d104P4P4P
Bromine (Br)[Ar] 4S2 3d10 4P4
↑↓↑↓
[Ar] 4S2 3d104P4P4P
Krypton (Kr)[Ar] 4S2 3d10 4P4
↑↓↑↓↑↓
[Ar] 4S2 3d104P4P4P

Thus for the electron configuration of germanium to krypton, filling three 4P orbitals of next higher energy, with the capacity of six electrons.