Bonding Flashcards
What is ionic bonding?
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions formed by electron transfer.
Formation of cation
A positive ion (cation) is formed when an atom loses an electron
Formation of anion
A negative ion (anion) is formed when an atom gains an electron
Migration of ions as evidence for existence of charged particles (ions):
Electrolysis of copper (III) chromate (VI) on a wet filter paper shows evidence for charged particles
When a drop of green copper (III) chromate (VI) is placed on wet filter paper and electricity passed through it, the ions start to separate
The positive Cu2- ions move towards the negative cathode (opposite charges attract). You can see the blue solution move
The negative CrO42- ions move towards the positive anode (opposite charges attract). You can see the yellow solution move
Physical properties of ionic compounds:
Ionic compounds, like sodium chloride, have a giant ionic structure
They conduct electricity when molten or dissolved in solution as the ions are free to move around
Most ionic compounds dissolve in water as water molecules are polar and so they can attract the positive and negative ions and break up the structure
Have high melting points as there are many strong electrostatic forces between oppositely charged ions.
Lots of energy is needed to overcome these forces
They are brittle. When struck with a hammer the layers slide and you get positive ions aligned with positives and negative ions aligned with negatives. They repel, and the structure breaks apart
What are covalent bonds?
Covalent bond is the strong electrostatic attraction between two nuclei and the shared pair of electrons between them:
Forms between elements that have high electronegativity values (non metals)
There are single, double and triple covalent bonds. More electrons are being shared
How can covalent bonds be represented?
Covalent bonds can be represented by lines too
An arrow represents a coordinate bond
The direction of the arrow goes from the atom that is providing the lone pair to the atom that is deficient
Reasons for the shapes offend bond angles in, simple molecules
Electron pairs repel and try to get as far apart as possible (or to a position of minimum repulsion.)
If there are no lone pairs the electron pairs repel equally
If there are lone pairs of electrons, then state that lone pairs repel more than bonding pairs.
Lone pairs repel more than bonding pairs and so reduce bond angles (by about 2.5o per lone pair
2 bonding pairs
0 lone pairs
Linear shape
Bond angle - 180
3 bonding pairs
0 lone pairs
Trigonal planar shape
Bond angle - 120
4 bonding pairs
0 lone pairs
Tetrahedral shape
Bond angle - 109.5
5 bonding pairs
0 lone pairs
Trigonal bipyramidal shape
Bond angle - 120 and 90
6 bonding pairs
0 lone pairs
Octahedral shape
Bond angle - 90
3 bonding pairs
1 lone pairs
Trigonal pyramidal shape
Bond angle - 107
2 bonding pairs
2 lone pairs
Brent shape
Bond angle 104.5
Factors affecting electronegativity:
Electronegativity increases across a period as the number of protons increases and the atomic radius decreases because the electrons in the same shell are pulled in more.
It decreases down a group because the distance between the nucleus and the outer electrons increases and the shielding of inner shell electrons increases
Ionic and covalent bonding
Ionic and covalent bonding are the extremes of a continuum of bonding type.
Differences in electronegativity between elements can determine where a compound lies on this scale
A compound containing elements of similar electronegativity and hence a small electronegativity difference will be purely covalent
A compound containing elements of very different electronegativity and hence a very large electronegativity difference(> 1.7) will be ionic
Why may molecules with polar bonds may not be polar molecules?
If the polar bonds in a molecule are arranged symmetrically, the partial charges cancel out and the molecule is non-polar.
Due to directly opposing pulls by the electronegative element.
London forces
London forces occur between all molecular substances and noble gases. They do not occur in ionic substances
In any molecule the electrons are moving constantly and randomly.
As this happens the electron density can fluctuate, and parts of the molecule become more or less negative. This leads to instantaneous dipoles
Instantaneous dipole induces a dipole in nearby molecules. Induced dipoles attract one another
Main factor affecting size of London Forces
The more electrons there are in the molecule the higher the chance that temporary dipoles will form.
This makes the London forces stronger between the molecules and more energy is needed to break them so boiling points will be greater.
Permanent dipole-dipole forces
- Permanent dipole-dipole forces occurs between polar molecules
- It is stronger than London forces and so the compounds have higher boiling points
- Polar molecules have a permanent dipole. (commonly compounds with C-Cl, C-F, C-Br H-Cl, C=O bonds)
- Polar molecules are asymmetrical and have a bond where there is a significant difference in electronegativity between the atoms.
Hydrogen bonding:
It occurs in compounds that have a hydrogen atom attached to one of the three most electronegative atoms of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons. e.g. a–O-H -N-H F-H bond.
There is a large electronegativity difference between the H and the O,N,F
Understand the interactions in molecules, such as H2O, liquid NH3 and liquid HF, which give rise to hydrogen bonding:
Hydrogen bonding is stronger than the other two types of intermolecular bonding.
The anomalously high boiling points of H2O, NH3 and HF are caused by the hydrogen bonding between these molecules in addition to their London forces.
The additional forces require more energy to break and so have higher boiling points
How does the shape of the molecule affect the size of London forces?
The shape of the molecule can also have an effect on the size of the London forces.
Long straight chain alkanes have a larger surface area of contact between molecules for London forces to form than compared to spherical shaped branched alkanes and so have stronger London forces