Figure 3. Spatial orientation of p-orbitals

3. Principles for distribution of electrons in atoms Distributions of electrons in atoms on energy levels and sub-levels may be presented in the form of electronic formulas.

It may be compelled in the following way:

• Note Arabian figure, which indicates the main quantum figure (energy level);

• Note symbol of orbital, that determined electronic sub-level;

• Quantity of electrons on this sub-level is made by Arabian figure above it in the right upper corner.

Example: To show electronic formula of Nitrogen: ₇N 1s² 2s² 2p³. Atom of Nitrogen has 7 electrons, 2 - on the first level on s-sub-level, and 5 others – on the second level, s and p-sub-levels.

Principles, according to which the electrons are distributed in atoms, are shown in the Table 5.

Table 5. General rules for electronic formulas compilation

Name	Formulation	Using
Principle of energy minimum	Minimum of energy responds to the most stable state of electrons in atom	Electron occupies a atomic orbital with minimum energy
Pauli’s Principle	Atom may not have two electrons with the same values of all four quantum figures	N=2n2 (quantity of on the level) Max quantity of on the sub-level is equal to 2(2l+1) s=2; p=6; d=10; f=14
Rule of Klech-kovsky	Energy sub-levels are filled up according increasing of sum n+l	Indicate an order of sub-levels occupation
Rule of Hound	Summary spin value of electrons on sub-level must be maximum	Indicate an order of occupation of equal atomic orbitals: ↑ ↑ ↑ Correctly ↑ ↑↓ Incorrectly

In addition to electron formulas graphic pictures of electrons distributions in atoms is widely used. Orbitals are represented by rectangle

where may be unpaired electrons or paired electrons with antiparallel spins:

↑↓

↑

For each atom different quantity of electrons states, notable for energy, is possible. The most stable state of electrons in atom corresponds to minimum possible value of its energy. Such state is named as normal (ground) one. All other states are named as excited ones. Atoms may pass on to excited state if they have free orbitals and take energy from the outside. At the same time electrons may be separated.

Example. ₁₇Сl 1s²2s²2p⁶3s²3p⁵

Electrons located on the last, third level, may go into free d-sublevel when atom was excited.

Ground state

Excited states

4. Valency and Oxidation number as function of electrons distribution

Using electron formulas, possible values of valency and oxidation numbers for chemical elements may be predicted.

In general case valency is equal to the quantity of unpaired electrons on the last (for s- and p-elements) and next to last (for d-elements) sub-levels.

According to this idea valency of Chlorine may be equal to I (normal state), III (the first excited state), V (the second excited state) and VII (the last, third excited state).

For estimation of oxidation number the theory of “octet configuration” must be used. It says that all atoms pressing towards to obtain the same configuration of the last electron shell are similar to noble (indifferent) gases. Why? Because the noble gases are the most stable chemical elements and they have 8 (excluding He) electrons on the last electron shell (ns²np⁶). Chemical elements may form configuration like noble gases giving up unpaired electrons to other atoms or joining them from other atoms. If after such operation atom has structure of last or next to last electron shell absolutely like noble gas it will be stable. If its configuration is not “ideal”, we may predict that such one is not stable and of course compounds included these atoms.

According to this theory, we may predict that in the first (normal) state atom of Chlorine may have three figures of oxidation number: -1 (when it joined one unpaired electron from any other atom); 0 (without interchange of electrons with other atoms) and +1 (when it lost one electron). Only the first figure of oxidation number will respond to an octet configuration of the last electron shell (3s²3p⁶) and Cl^-1 is a stable oxidation number for this chemical elements. Truly in nature this element exists only in the form of Chlorides where Chlorine has oxidation number –1.

Let’s consider the first excited state of Chlorine atom. We see, that there are 3 unpaired electrons on the last electron shell. What can be said about possible degree of oxidation? The first possibility – to join 3 electrons from other atom and to have oxidation number –3. This state in not stable, because atom has not octet configuration. If it lost 3 elections, its configuration is more stable, because atom has two pairs of electrons.

Analogously it may be shown that oxidation numbers of Chlorine in the second and third excited states will be equal to +5 and +7. Oxidation number +5 is more stable that +1 and +3, but the most stable compounds respond to oxidation number +7.

Note, that valency and oxidation number are the different notions. Oxidation is the virtual figure, mathematical value. For example, in molecules H₂ and O₂ oxidation numbers are equal to 0, but valency is equal to 1 and 2 correspondingly.