Lecture Five Flashcards
Draw a graph which represents the relationship between Internuclear seperation and interaction energy.
You get to a point where the atoms can get any closer due to nuclear repulsion.
Where there is no interaction between atoms, this is the energy required to break the bonds.
Define bond length.
The separation distance at which the molecule has the maximum energetic advantage over the separated atoms is known as the bond length.
Yet experimental studies of molecular motion reveal that nucleu within molecules move continuously, oscilating about their lowest energy seperation distance like two balls attach to opposite ends of a spring.
Define bond energy.
The energy difference between the molecule and the separated atoms at this distance is called the bond energy.
Bnd energy is defined as the energy required to break the bond and therefore is always positive.
It is usual to quote bond energies in the units kJ/mol.
What is a sigma (σ) bond?
I we rotate the molecule about its internuclear axis (an imaginary line joining the two nuclei) we find that the electron distribution between the nuclei looks exactly the same, regardless of the anle through which it was rotated.
In other words, the bond is totally symmetric with respect to rotation about the internulear axis.
Bonds for which this is true are called sigma bonds.
What happens when you have a heteronuclear molecule?
Bonding electrons in the molecule experience unsymmetrical attractive fources.
This is because each nuclei has a different effective nuclear charge (Zeff). (Zeff = atomic nucleus).
E.g. The HF molecule.
This molecule is more stable when its bonding electrons are concentrated closer to the florine atom and away from the hydrogen atom.
This unequal distribution electron density creates a dipole.
The F end has a slightly negitive charge and the H end has a slightly positive charge.
The molecule as a whole remains neutral.
Note: This is still a sigma bond as it is still symmetric about the H-F bond axis.
Define electronegitivity.
Elements have differing abilities to attract electrons, and this is called their electronegitivity and is symbolised by the greek letter chi (χ).
A bond between two elements with different electronegitivities will be polar, and the greater the difference (Δχ), the more polar the bond.
EN gives a numerical value to how strongly an atom attracts the electrons in a chemical bond.
- No units.
- Related to, but distinct from IE and EA.
- INcreases towards the top riht of the periodic table (excluding the noble gases).
- Decreases towards the bottom left of the periodic table.
- Ranges from 0.7 (Fr) to 4.0 (F).
Descrie ionic bonding.
It does not involve the sharing of electrons.
Instead, ionic compounds are held together in 3D arrangements by the attractive fources between oppositly charges ions.
