Covalent Bonding

1. Covalent Bonding

2. Covalent Bonding • A chemical bond forms when outer-shell electrons come close enough to each other to interact and rearrange themselves into a more stable arrangement—one with a lower chemical energy. • This chemical energy is the sum of the chemical potential energy of the particles and their kinetic energy.

3. Covalent Bonding • The electrostatic attraction between the positively charged nuclei and the negatively charged electrons is the most significant source of the chemical potential energy. • As the two (or more) atoms approach one another, the positively charged nuclei repel one another, as do the negatively charged electrons.

4. Covalent Bonding • These repulsion forces increase the potential energy of the system. • At the same time, however, the oppositely charged particles are attracting one another, causing the potential energy to decrease. • At some point, the distance separating the atoms or ions will be such that the repulsive forces of the particles with the same charge exactly balance the attractive forces between oppositely charged particles.

5. Covalent Bonding • It is in this stable arrangement that a chemical bond is formed.

6. The Nature of the Covalent Bond • What is the simplest of all atoms? • Hydrogen • What does it consist of? • A single proton in its nucleus and a single electron occupying the space around it. • Hydrogen exists as diatomic molecules—a pair of hydrogen atoms joined together (H2). • Why is this a more stable arrangement than single hydrogen atoms, or perhaps a triatomic molecule?

7. The Nature of the Covalent Bond • A molecule may be formally defined as “a discrete group of non-metal atoms covalently bonded to one another.” • Of particular significance in this definition is the fact that molecules contain specific numbers of atoms in a set ratio. • A molecule of water contains two atoms of hydrogen bonded to one atom of oxygen; the molecule is always the same whether the water is present in the solid, liquid or gaseous phase.

8. The Nature of the Covalent Bond • The average radius of a hydrogen atom is 1.2 Å (1.2 x 10-10 m). • The distance seperating the two nuclei in a hydrogen molecule is just 0.74 Å. • This means that there must be significant overlap of the atomic radii of the individual atoms when the molecule is formed.

9. The Nature of the Covalent Bond • As the two atoms approach one another, electrostatic attractions and repulsions occur between the positively charged nuclei and the negatively charged electrons. • The minimum overall energy of this system occurs at a separation of 0.74 Å. • In the formation of the hydrogen molecule, each hydrogen atoms has contributed its single electron to occupy the space between the nuclei as a pair.

10. The Nature of the Covalent Bond

11. The Nature of the Covalent Bond A D B C

12. The Nature of the Covalent Bond • In 1916 Gilbert Lewis was the first to propose that when a pair of electrons is shared between two atoms, a covalent bond is formed. • The electrons of the bond make up a bonding pair. • As only one pair of electrons is occupying the space between the two nuclei, a single covalent bond has been formed.

13. The Nature of the Covalent Bond • Any other pairs of valence-shell electrons that do not actually take part in the bond are known as non-bonding pairs (lone pairs). • These non-bonding pairs of electrons can be very important in helping to determine the shape of the molecule, which can have significant effect on the properties of the substance.