How to assign electrons in Lewis structure?

Lewis electron-Dot symbols:

Atoms interact with each forming chemical bonds in order to achieve a stable electronic configuration. The valence electrons are the only electrons that get involved in forming the chemical bond. Atoms follow octet rule when forming the chemical bond.

The octet rule is the tendency of atoms in a molecule to have eight electrons in their valence shells (two for hydrogen atoms) in order to obtain electronic configuration similar to noble gases because maximum stability occurs when an atom is isoelectronic with a noble gas.

We use Lewis dot symbols to know the number of valence electrons that can get involved in forming a chemical bond. In Lewis dot symbol the atoms or ions are represented by dots placed around the letter symbol of the element. The dots represent the electrons and written as pairs on each side of the element symbol

For example, boron electronic configuration 1s²2s²2p¹, the 2 electrons in 1s orbital are inner electrons, 2s²2p¹ are valence electrons.

Which more closely corresponds to the ground-state configuration [He]2s²2p¹. The first symbol shows boron’s chemistry, in which each single electron (single dot) tends to be involved in bond formation].

Note that in periodic table; (1) elements in the same group have similar outer electronic configurations and hence similar Lewis dot symbols. (2) For group 1 and 2, the number of valence electrons for each element is the same as the group number of elements, means for group 1 and 2 the valence electrons are equal to 1 and 2 respectively. (3) From 13 to 18, the number of valence electrons is equal to the last number. For example, Be is in group 15 so it has 5 valence electrons (five dots). (4) For lanthanides, and actinides, because the inner shells are not completely filled it is hard to anticipate the number of the valence electrons and we cannot write simple Lewis dot symbols for them.

Steps in writing Lewis structures:

Step 1: Calculate the total number of valence electrons for each atom. For anions we add the number of negative changes on this total (For CO₃^2- you add 2 because the -2 charge indicates that there are two more electrons than are provided by the neutral atoms.).

For a polyatomic cation, you subtract the number of positive charges from the total. (For NH⁴⁺  you subtract 1.)

Step 2: Write the chemical symbol of the molecule or ion and draw dots or dash between each bonded atoms.

Step 3: Distribute electrons around the central atom (or atoms) to satisfy the octet rule for these surrounding atoms.

Step 4: Distribute the remaining electrons as pairs. If there are fewer than eight electrons on the central atom, this suggests that a multiple bond is present. (Two electrons fewer than an octet suggests a double bond; four fewer suggests a triple bond or two double bonds.). Atoms that often form multiple bonds are C, N, O, and S.

Example:

Formal charge and Lewis structure:

Assigning the number of electrons an atom in a Lewis structure:

1- All the atom’s nonbonding electrons are assigned to the atom.

2- We break the bond(s) between the atom and other atom(s) and assign half of the bonding electrons to the atom.

Example: assign electrons on ozone molecules (O3) the total charge on ozone molecule is equal to zero (molecules are neutral)

Thus, the formal charges of the atoms in O₃ are:

Rule have to be regarded during assigning Lewis structure:

1. Molecules are electrically neutral so the sum of the charges must add up to zero. Example, the sum of charges on water molecule (H₂O) is equal zero.

2. For cations, the sum of formal charges must be equal to a positive value. For anions, the sum of formal charges must be equal the negative value.