Structure and electronic configuration

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The modern structure of the atom

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

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

The 3S orbital is lower energy then 3P orbitals which again are of lower energy than 3d orbitals. However, orbitals belonging to a particular type ( P or d or f ) will be of equal energy (degenerate) state of an atom or an ion.

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

The orbital energy level of an atom

Admittedly it is often difficult for the readers to remember the orbital energy level of an atom. A trivial but distinctly more convenient way for electronic configuration is
Orbital energy level of an atom
Orbital energy level of an atom
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.

An element which contains 36 electrons the electronic configuration of this element according to the above diagram is,
1S² 2S² 2P⁶ 3S² 3P⁶ 4S² 3d¹⁰ 4P⁶

Aufbau or Building Up Principle

The question that arises now how many electrons can be accommodated per orbital. The answer to this follows from Pauli's exclusion principle.

Pauli's exclusion principle

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

This principle tells us that in each orbital maximum of two electrons can be allowed. The two electrons have the same three quantum numbers namely the same n, same l, and the same ml.

Any conflict with the Pauli Principle can now be avoided if one of the electrons has the spin quantum numbers is (+1/2) and (-1/2).

An alternative statement of Pauli's exclusion principle is,

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 are given the maximum accommodation of electrons.

Capacities of electronic levels

The total number of electrons for a particular n is given by 2n².

Capacity of electronic level
The capacity of electronic level
To determine 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 (increasing n ) until all the electrons have been accommodated.
  2. Electrons will tend to maintain maximum spin. So long orbitals of similar energy are available for occupation electrons will prefer to remain unpaired.
  3. In other words, electrons tend to avoid the same orbital, that is, hate to share space. This rule is known as Hund's Rule of maximum spin Multiplicity.
  4. Spin pairing can occur only when vacant orbitals of similar energy are not available for occupation, and when the next available vacant orbital is of higher energy.

Electronic configuration of elements

Hydrogen to helium and lithium to neon

Hydrogen has its only one electron in the 1S orbital and this electronic configuration represented below. the bar on the pictorial representation indicates the orbital and the arrow ↑ a single spinning electron in the orbital.

Helium, the second electron occupies the 1S orbital since the next 2S orbital is much higher energy.
Obeying Pauli principle the configuration 1S² represented below.

From the above rule, we can represent the electronic configuration of hydrogen to neon.
Hydrogen (H) 1S¹
1S
Helium (He) 1S²
↑↓
1S
Lithium (Li) 1S² 2S¹
↑↓
1S 2S
Beryllium (Be) 1S² 2S²
↑↓ ↑↓
1S 2S
Boron (B) 1S² 2S² 2P¹
↑↓ ↑↓

1S 2S 2Px 2Py 2Pz
Carbon (C) 1S² 2S² 2P²
↑↓ ↑↓
1S 2S 2Px 2Py 2Pz
Nitrogen (N) 1S² 2S² 2P3
↑↓ ↑↓
1S 2S 2Px 2Py 2Pz
Oxygen (O) 1S² 2S² 2P⁴
↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz
Fluorine(F) 1S² 2S² 2P⁵
↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz
Neon (Ne) 1S² 2S² 2P⁶
↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz

Electron distribution from sodium to argon

After neon, the next available orbital is 3S being followed by 3P. The orbitals are then progressively filled by electrons.

Thus the electronic configuration of sodium to argon given below.

Sodium (Na) 1S² 2S² 2P⁶ 3S¹
↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S
Magnesium (Mg) 1S² 2S² 2P⁶ 3S²
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S
Aluminum (Al) 1S² 2S² 2P⁶ 3S² 3P¹
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓

1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz
Silicon(Si) 1S² 2S² 2P⁶ 3S² 3P²
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz
Phosphorus (P) 1S² 2S² 2P⁶ 3S² 3P³
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz
Sulphur (S) 1S² 2S² 2P⁶ 3S² 3P⁴
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz
Chlorine (Cl) 1S² 2S² 2P⁶ 3S² 3P⁵
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz
Argon (Ar) 1S² 2S² 2P⁶ 3S² 3P⁶
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1S 2S 2Px 2Py 2Pz 3S 3Px 3Py 3Pz

Elements of potassium and calcium

4S orbital, being of lower energy then the 3d, is filled. the elements involve potassium and calcium are represented as,

Potassium (K) [Ar] 4S¹

[Ar] 4S
Calcium (Ca) [Ar] 4S²

↑↓
[Ar] 4S

Chemistry articles for school-college courses

3d-block elements scandium to zinc

In Scandium(21) the twenty-first electron goes to the 3d orbital, the next available orbital of the higher energy. There are five 3d orbitals with the capacity of ten electrons. From scandium to Zinc these 3d orbitals are filled up.

The presence of partially filled d orbitals generates some special properties of the elements. Elements with partially filled d or f block elements are called transition elements.

Scandium (Sc) [Ar] 4S²3d¹

↑↓



[Ar] 4S 3d 3d 3d 3d 3d
Titanium (Ti) [Ar] 4S² 3d²

↑↓


[Ar] 4S 3d 3d 3d 3d 3d
Vanadium (V) [Ar] 4S² 3d³

↑↓

[Ar] 4S 3d 3d 3d 3d 3d
Chromium (Cr) [Ar] 4S² 3d⁴

↑↓
[Ar] 4S 3d 3d 3d 3d 3d

In reality, experimental studies on chromium revel their electronic configuration and better represented as

Chromium (Cr) [Ar] 4S¹ 3d⁵

[Ar] 4S 3d 3d 3d 3d 3d

This reordering of electrons is due to extra stability associated with a half-filled sub-shell.

Manganese (Mn) [Ar] 4S² 3d⁵

↑↓
[Ar] 4S 3d 3d 3d 3d 3d
Iron (Fe) [Ar] 4S² 3d⁶

↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d
Cobalt (Co) [Ar] 4S² 3d⁷

↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d
Nickel (Ni) [Ar] 4S² 3d⁸

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

↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d

Studies on copper also reveal that their electronic configuration and are better represented as

Copper (Cu) [Ar] 4S¹ 3d¹⁰

↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d

This reordering of electrons is due to extra stability associated with a filled sub-shell.

Zinc (Zn) [Ar] 4S² 3d¹⁰

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d

These 3d orbital are represented as dxy, dxz, dyz, dz², dx²-y².

Distribution from gallium to krypton

In Gallium(31) the thirty-first electron goes to the 4P orbital, the next available orbital of the higher energy. There are three 4P orbitals with the capacity of six electrons. From Gallium to Krypton these 4P orbitals are filled up.

Gallium (Ga) [Ar] 4S² 3d¹⁰ 4P¹

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓

[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P
Germanium (Ge) [Ar] 4S² 3d¹⁰ 4P²

↑↓↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P
Arsenic (As) [Ar] 4S² 3d¹⁰ 4P³

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P
Selenium (Se) [Ar] 4S² 3d¹⁰ 4P⁴

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P
Bromine (Br) [Ar] 4S² 3d¹⁰ 4P⁵

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P
Krypton (Kr) [Ar] 4S² 3d¹⁰ 4P⁶

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
[Ar] 4S 3d 3d 3d 3d 3d 4P 4P 4P

Electronic configuration of elements, capacities of electronic levels, electronic configuration from hydrogen to neon, 3d-block, and 4p-block elements

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