CHAPTER 1. ATOMICCOMBINATIONS 1.
+ +(1)
(2)
(3)Figure 1.1: Forces acting on two approaching atoms: (1) repulsion between electrons, (2) attraction
between protons and electrons and (3) repulsionbetween protons.
- repulsive force between the electrons of the atoms, since like charges repel
- attractive force between the nucleus ofone atom and the electrons of another
- repulsive force between the two positively-charged nuclei
Now look at Figure 1.2to understand the energy changes that take place when the two atoms move
towards each other.
Energy+0Distance between atomic nuclei
AQPXFigure 1.2: Graph showing the change in energythat takes place as atoms move closer togetherIn the example of the two hydrogen atoms, where the resultant force between them is attraction, the
energy of the system iszero when the atoms are far apart (point A), because there is no interaction
between the atoms. When the atoms move closer together, attractive forces dominate and the atoms
are pulled towards eachother. As this happens,the potential energy of the system decreasesbecause
energy would now needto be supplied to the system in order to move the atoms apart. However, as
the atoms continue to move closer together (i.e. left along the horizontal axis of the graph), repulsive
forces start to dominateand this causes the potential energy of the systemto rise again. At some point,
the attractive and repulsive effects are balanced,and the energy of the system is at its minimum (point
X). It is at this point, when the energy is at a minimum, that bonding takes place.
The distance marked ’P’ is the bond length, i.e. the distance between the nuclei of the atoms when
they bond. ’Q’ represents the bond energy i.e. the amount of energy that must be added tothe system
to break the bonds thathave formed. Bond strength means how strongly one atom attracts and is held
to another. The strengthof a bond is related to the bond length, the sizeof the bonded atoms and the
number of bonds between the atoms. In general, the shorter the bondlength, the stronger thebond
between the atoms, andthe smaller the atoms involved, the stronger thebond. Also, the more bonds
that exist between the same atoms, the stronger the bond.
See simulation: VPgxl at http://www.everythingscience.co.za))