LEWIS diagrams

 Lewis dot diagram and lewis dot-dash diagram

 Lewis dot diagram is used to illustrate the atomic skeleton, nature of the bonding present (single, double and triple bond) and the distribution of the valance shell electronsaround each atom of the given chemical formula. In Lewis dot-dash structure, a bonding electron pair is denoted by a short line drawn between the two atoms.

👉Chemical formula =Cl2 

👉Lewis dot diagram 

👉Lewis dot-dash structure

👊The following factors need to be considered when drawing Lewis dot diagrams:

🤜 Elements H and F are generally not considered as the central atom since these can form only single bonds. Atoms which are capable of forming multiple bonds are  placed as the central atom. 

🤜 The element with the lower electronegativity is generally the central atom.

🎗It is important to consider the following facts for molecules and ions with one central atom. 

(i) Identification of the central atom and surrounding atoms.

(ii) Calculating the total number of electrons for a given chemical formula considering all electrons in the valence shell of each atom.

👉👉e.g.: In H2O, oxygen atom contributes 6 electrons and one electron from each hydrogen atom (two electrons from two hydrogen atoms), which sum up to 8 when considering the total electron count (6 + 2 = 8) of the valence shells. 

👉👉If it is a negatively charged ion, then negative charges should be counted as well. e.g.: In the OH-ion, contribution of electrons from oxygen atom is six and hydrogen atom one together with one electron due to negative charge of the ion adding up to eight as the total number of electrons (6 + 1 + 1 = 8).

👉👉If the ion is positively charged, then an equivalent number is deducted from the total valance electron count.

e.g.: In NH4

+ ion, N atom contributes 5 valance electrons and four hydrogen atoms contribute 4 electrons. However, since it is a cation, then one electron (equivalent to number of positive charges) is deducted resulting 8 electrons in the valance shell of nitrogen atom (5 + 4 – 1 = 8).

(iii) A bond is denoted by a pair of dots between the central atom and a surrounding atom. Each surrounding atom is connected to the central atom with at least one bond.

(iv) Bonding electron pairs are denoted first as a pair of dots (Lewis dot diagram) or a short line (Lewis dot-dash diagram) drawn between central atom and each of surrounding atoms. Next, the remaining electrons are distributed, starting from the most electronegative atom, to complete the octet. Each electron pair is marked by pair of dots (lone pair electrons). If the electronegative atoms are surrounding atom, then lone pairs are marked on these surrounding atoms in order to complete the octet of each atom. CCl4 is an example for this.

In the case of ammonia, surrounding atoms are hydrogen, remaining pair of electrons is marked on the nitrogen atom.

In the Lewis dot diagrams bond electrons between two atoms can be represented as follows.

(v) If electron pairs are remaining after distributing electron pairs on the surrounding atoms (satisfying the octet rule), then left over pairs of electrons are marked on the central atom.

(vi) After distributing all the electron pairs, the number of electrons on each atoms should be compared with the number of electrons in the non-bonded state of the atom (free atom) to assign the formal charge and then check completion of the octet. In the case of a bond, one electron is counted for each atom and if lone pairs are present, both electrons are counted to the particular atom. Priority is given for completion of the octet.

As an example, NH2−ion:

Here the total electron count on the nitrogen atom is 8. Though the nitrogen atom has contributed only 5 electrons, from the Lewis dot diagram, it appears as if the nitrogen has contributed 6 electrons. Since it has one extra electron, (-1) the charge is marked on the nitrogen atom as the formal charge.

(vii) Electron distribution shall be rearranged in order to minimize the formal chargeon atoms and completion of octet by converting lone pair of electrons to bonding pairs of electrons.

If SO32−ion is taken as an example, sulphur atom will contribute 6 electrons and each oxygen will contribute 6 electrons. Hence 18 electrons come from three oxygen atoms. Addition of two more electrons due to (-2) charge add up to 26 electrons (6 + 3(6) + 2 = 26) for the Lewis dot structure.

All atoms of Lewis dot structure (d) have satisfied the octet, but it is not stable, since it has maximum formal charge distribution. Hence the lone pair electrons are rearranged in order to obtain the stable Lewis structure having minimum formal charge distribution. The following sketch shows the way of rearrangement.

Finally, the Lewis dot dash structure for SO32− is given as below.

Here, all oxygen atoms have completed octets. There are total of 10 electrons in the valance shell of sulphur atom which exceeds the octet. However, this is allowed due tothe presence of empty d orbitals in addition to the p orbitals in the valance shell of the sulphur atom.It is important to know the skeleton of atoms of a given chemical formula when multiple central atoms (e.g.: C3H6O) are present. Table 2.1 shows Lewis dot diagrams and Lewis dot-dash structures of selected molecules and ions.

Lewis dot diagrams and Lewis dot dash structures of some selected molecules and ions

You can also see

1.https://generalideasinchemistryforbegginners.blogspot.com/2021/01/just-introduction.html

2.https://generalideasinchemistryforbegginners.blogspot.com/2021/01/the-cathode-ray-experiment.html

3.https://generalideasinchemistryforbegginners.blogspot.com/2021/01/periodic-table-of-elements.html

4.https://generalideasinchemistryforbegginners.blogspot.com/2021/01/chemical-bonds.html

5.https://generalideasinchemistryforbegginners.blogspot.com/2021/01/covalent-bonds-and-dative-covalent-bonds.html