Molecular Geometry and Bond Angles

Elements that are naturally unstable combine with other elements to gain stability. Inert gases or noble gases are stable, but the rest of the elements are not stable. Molecules can be diatomic, triatomic or polyatomic and can be made of the same type of atoms or different types of atoms. Molecules that are made of two atoms are binary compounds and those made of three atoms are tertiary compounds. Compounds have a fixed composition irrespective of the source from which they are prepared, and this follows the law of constant proportion. Molecules have atoms bonded around them that occupy space and can have different bonds like covalent and ionic.

Ionic and covalent molecules are very different from one another. The ionic bond is formed by the transfer of electrons, forms ions and is crystalline and non-directional. The covalent bond is usually formed by sharing electrons, without ions and are directional.

Molecules forming a covalent bond with the same atoms, like homoatomic molecules, are usually arranged linearly; for example, H2, I2, Br2, Cl2, F2 etc. are binary molecules. Molecules forming a covalent bond with the same atoms with more than two atoms are polyatomic molecules; for example, O3, P4, S8. Molecules forming a covalent bond with different atoms usually are arranged in various forms. According to different theories, the arrangement of bonds around the central atom can be known through which bond angles are explained.

To explain molecular geometry and bond angles, the most important theory we need to discuss is the VSEPR theory.

Valence Shell Electron Pair Repulsion Theory (VSEPR):

This theory is mainly proposed to explain the geometries, shapes and bond angles of the molecules based on valence electrons in the central atom.

Postulates:

1. The element can combine with other elements to attain stability by sharing electrons.

2. In an element, the electrons present in the outermost shell are only responsible for bond formation. They are called valence electrons.

3. Valence electrons can be mainly of two types – bonded electrons and lone pair electrons.

4. Bonded pairs of electrons are those which are shared between two atoms in a molecule, whereas lone pairs of electrons are present only on a single atom that is the central atom.

5. Since all the electrons are the same when they are closely placed, they undergo repulsion. Repulsions can be of different types, like bond pair-bond pair repulsions, lone pair-bond pair repulsions and lone pair-lone pair repulsions.

6. The order of repulsive forces is bond pair- bond pair repulsions

Lone pair repulsions are stronger than the other repulsive forces because lone pairs are present on the central atom and occupy greater space. Bond pair electrons are those shared between two atoms where the electron density is distributed between two atoms, and the effect decreases. This results in a greater effect of lone pair repulsions.

7. Due to the repulsion of electrons in the molecule, atoms move far apart and occupy a position in the space that provides minimum repulsion for the molecule. This provides the shapes and geometries to the various molecules. The geometries of molecules can be calculated by the number of lone pairs and bond pairs in the molecule.

I) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 2, then the molecule is said to be sp hybridised.

II) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 3, then the molecule is said to be sp2 hybridised.

III) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 4, then the molecule is said to be sp3 hybridised.

IV) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 5, then the molecule is said to be sp3d hybridised.

V) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 6, then the molecule is said to be sp3d2 hybridised.

VII) If the number of lone pairs on the central atom and the total bond pairs in a molecule is equal to 7, then the molecule is said to be sp3d3 hybridised.

Examples:

Conclusion:

The VSEPR theory is used to predict the electron pairs’ arrangement around the central atom of molecules. An atom, which is bonded to two or more other atoms, is known as the central atom, and the atom bonded to only one other atom is known as the terminal atom. In VSEPR theory, the electrons pair repel each other whether they are in a bond pair or a lone pair or not. To minimise the repulsion, electron pairs spread themselves as far as from each other. Valence shell electron pair repulsion can predict the shape of all compounds having the central atom, as long as the central atom is not metal.