The chemical bond

Energetic considerations

The simplest case for a closer examination of the atomic bond is the hydrogen molecule ${\text{H}}_2$ to get voted. Experience has shown that there is a strong bond in the hydrogen molecule, because at normal temperatures hydrogen consists of ${\text{H}}_2$-Molecules that are largely split into atoms only at several thousand degrees.

If two separate hydrogen atoms are brought closer to one another, six opposing electrostatic forces must be taken into account (Fig. 1),

• the attraction between the nuclei and the electrons A-1, A-2, B-1, B-2 (green),
• the repulsion between nuclei A and B and between electrons 1 and 2 (black).

In the case of larger distances between the nuclei, the attraction between nuclei and electrons predominates; in the case of smaller distances, the repulsion of the nuclei predominates. In a potential-distance diagram (potential energy, ${\mathit{E.}}_{\text{p}}$, against core distance,$\mathit{r}$) this is expressed as follows:

hydrogen

The equilibrium distance (r_e) is 74 for hydrogen $\text{pm}$, the educational energy (${\mathit{H}}_{\text{B.}}$ = Δ${\mathit{E.}}_{\text{p}}$) from ${\text{H}}_2$-Molecules of Atoms -432 $\text{kJ}{\text{mol}}^{-1}$. The energy that is necessary to 1 $\text{mol}$${\text{H}}_2$ split into atoms, the dissociation energy, ${\mathit{E.}}_{\text{d}}$, has the same amount, but with a positive sign.

Interpretation of the atomic bond

The previous discussion of the formation of the ${\text{H}}_2$Molecule gives a rough interpretation of the atomic bond: When the two atoms approach, the two 1s orbitals overlap. In this area of ​​overlap the charge cloud becomes denser, which means that the probability of encountering an electron is greater here than in the vicinity of just one nucleus. Due to the increased density of negative charge, on the one hand the repulsion of the two nuclei is reduced, and on the other hand the electrons are mutually attracted by both nuclei, so that the total distance between the nuclei becomes smaller. The two electrons are no longer assigned to one nucleus, but belong to both nuclei together. The electron 1 can thus also be hit in the vicinity of the nucleus and vice versa (compare (Fig. 1)).

The fact that the experimentally determined nuclear distance with 74 $\text{pm}$ is much smaller than twice the Bohr radius of the H atom, which roughly corresponds to that of the 1s orbital calculated by wave mechanics: 2 x 53 pm> 74 $\text{pm}$.

The atomic bond, like the other types of bond, is to be seen initially as the effect of electrostatic forces.

Video: Van der Waals Kräfte. Bindungen (May 2022).