Chemical Bonding and Structure Flashcards
Multiple covalent bonds are caused by what?
Strong electrostatic forces of attraction between the nuclei and shared electrons
Lone pairs of electrons can affect what about the molecule?
It’s shape.
Lone pairs of electrons can form what with other atoms?
Dative covalent bonds (coordinate bonds).
A sigma bond is what?
A sigma bond is your basic head-on covalent bond with the bond in line with the bonding orbitals.
How many sigma bonds can you only ever have between any two atoms?
One.
A pi bond is what?
A pi bond is a covalent bond between orbitals perpendicular to the bond direction. This is usually p-orbitals, never s.
Electron pairs do what to each other?
Repel each other, as they are negatively charged.
Molecules are shaped so that what can be said to be true?
Molecules are shaped so that electron pairs are arranged to be as far away from each other as possible.
2e pairs makes what shape and what bond angle?
Linear, 180 degrees.
3e pairs are arranged in why shape with what bond angle?
Trigonal planar, 120 degrees.
4e pairs makes what shape with what bond angle?
Tetrahedral, 109.5 degrees.
Electron pair repulsion theory is the theory that…
…a lone pair exerts more repulsion than a bonded pair. This is about 2.5 degrees per lone pair and examples of these are water and ammonia bond angles (104.5 and 107).
Double and triple bonds act as what when we consider molecule shapes?
They act as a centre of negative charge causing similar shapes to molecules with singly bonded electron pairs.
If the two atoms covalently bonded are the same what can be said about the electrons?
They are shared equally between the two atoms.
What is a polar covalent bond?
When two atoms are covalently bonded but one atom has a nucleus that the electrons are more attracted too.
What happens to a atoms of they have a polar covalent bond between them?
Dipoles form with a delta negative where the electrons are closer too and a delta positive where the electrons are further from. Overall there is no charge as these charges are equal and cancel.
What is electronegativity?
Electronegativity is the ability of an atom to attract the bonding electrons in a covalent pair.
How can electronegativity be used to predict the extent to which the bonds between different atoms can be polar?
The greater the difference of the electronegativity of the atoms bonding, the greater the polarity of the covalent bond between them.
Electronegativity does what across a period?
Increases.
Why does electronegativity increase across a period?
The nuclear charge increases but the number of shielding electrons remains constant, so the attraction for the shared electrons pairs increases.
What type of diffraction can be used for chemists to work out structure?
X-ray diffraction.
Microwave spectroscopy can give chemists information about what?
Bond lengths between atoms.
Bond length depends on what?
Size of atom. Single bonds are longer than multiple bonds as there are less electrons in the shared electron cloud.
Sigma bonds are how strong relative to pi bonds?
Sigma bonds are far stronger as they are closer to the nucleus and experience stronger attraction, therefore are more difficult to break.
Lone pairs of electrons can be used to form what?
Dative covalent bonds (coordinate bonds).
Example of a dative covalent bond:
An ammonium ion, +1 charge due to a H+ bonding with the lone pair on ammonia.
An hydroxonium ion, +1 charge due to a H+ bonding with a lone pair on water.
Electronegativity is what?
Electronegativity is the pull of atoms on electrons or electron pairs.
What happens to electronegativity across a period?
Electronegativity increases across a period.
What happens to electronegativity as we go up a group?
It increases.
Why does electronegativity increase up a group?
It increases due to decreasing atomic radius.
Pauling did what?
Gave elements values based on their electronegativity on a scale from 0-4.
What are the three most electronegative elements?
Nitrogen, oxygen and fluorine (3.0, 3.5 and 4.0 on Pauling’s system).
If two atoms are covalently bonded together and are exactly the same is the molecule polar and if so where are the poles?
No, because both atoms have the same electronegativity and neither attracts the electrons slightly more than the other, therefore no partial charges are present.
If two atoms covalently bonded are not the same, for example HCl, is the molecule polar and if so where are the poles?
Yes, because the chlorine is more electronegative than hydrogen meaning there is a δ- charge towards the chlorine and a δ+ charge towards the hydrogen.
Polar bonds in a molecule don’t necessarily mean that the molecule is polar. Why not?
If the molecule is symmetrical the dipoles can cancel.
Why is there no difference in charge between carbon and hydrogen?
They have a similar electronegativity (difference of 0.4) so no dipoles form.
What type of force is a London force?
A weak intermolecular force.
What happens to cause London forces?
Electrons are never in a fixed position so temporary dipoles within the molecule can form which induce temporary dipoles in neighbouring molecules. Opposites attract and intermolecular forces are experienced.
Do London forces only occur when electrons are symmetrical or asymmetrical?
Asymmetrical.
Dipole-dipole forces occur in addition to what forces?
London forces.
Dipole-dipole interactions take place when?
When molecules have permanent dipoles.
How does dipole-dipole bonding work?
The positive dipole on one molecule attracts the negative of another and vice-versa.
Molecules with permanent dipoles have a higher what than those without?
Boiling and melting point(s).
Hydrogen bonding can be described as an extreme type of what?
Dipole-dipole bonding.
How many times weaker than a covalent bond is a hydrogen bond?
At least 10 times.
Hydrogen bonding acts in addition to…
…London forces and dipole-dipole forces.
Hydrogen bonding effects:
Molecules with a H bonded to a NOF.
Molecules with a lone pair on a NOF atom.
How does hydrogen bonding work?
The NOF atom attracts electrons strongly away from the H leaving it δ+. The re is then a strong intermolecular force between the δ+ H and the lone pair on another atom on a nearby molecule.
What is the bond angle of hydrogen bonding?
180 degrees.
For one substance to dissolve another what must happen?
The bonds in the substance have to break.
The bonds in the solvent have to break.
New bonds must form between the substance and the solvent.
Ionic substances dissolve in polar solvents because:
The ions are attracted to the oppositely charged poles of the solvent molecules.
The ions are pulled away from the ionic lattice by the solvent molecules.
Ions being pulled away from the lattice by water molecules and surrounding the ions is known as what?
Hydration.
Alcohols dissolve in polar solvents because:
The polar OH bond in an alcohol is attracted to the poles in a polar solvent.
The carbon chain of the alcohol isn’t attracted to the solvent, so the more carbon atoms in the chain the less soluble the alcohol in a polar solvent.
Alcohols dissolve in water because:
The polar OH bond in an alcohol is attracted to the polar OH bonds in water. Hydrogen bonds form between the lone pairs on the oxygen and the delta + hydrogen.
Which molecules have polar bonds but don’t dissolve in water?
Halogenoalkanes.
Why can halogeboalkanes dissolve in water?
The dipoles in halogenoalkanes aren’t strong enough to form hydrogen bonds with water. The hydrogen bond in water is stronger than the bonds between halogenoalkanes.
Non polar substances dissolve best in what?
Non polar substances.
Why do non polar substances dissolve best in other non polar substances?
They form similar bonds (London forces) between molecules.
Why don’t polar substances substances dissolve very well in non polar solvents and vice-versa?
The molecules of the polar substance are attracted together more strongly than non polar molecules.
What phrase can we use for solubility?
“Like dissolves like”
What does miscible mean?
When two liquids mix to form a homogeneous mixture.
What is the opposite to miscible?
Immiscible.
