Unit 1-Structure And Bonding Flashcards
What name is given to bonding:
a) inside molecules
b) between molecules
a) inside molecules (intramolecular bonds)
b) between molecules (intermolecular bonds).
Describe how a covalent bond is held together.
In a covalent bond, atoms share pairs of electrons. The covalent bond is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons.
What are the two types of covalent bonding? (Hint-Electronegativity)
Non-polar (pure) covalent and polar covalent.
What causes a polar covalent bond to be formed?
Polar covalent bonds are formed when the attraction of the atoms for the pair of bonding electrons is different (electronegativity).
How is a polar covalent bond shown in a diagram?
Delta positive (δ+) and delta negative (δ-) notation can be used to indicate the partial charges on atoms, which give rise to a dipole (eg Hδ+ ̶ Clδ-).
What causes a non-polar (pure) covalent bond to be formed?
Non-polar (pure) covalent bonds are formed when the attraction of the atoms for the pair of bonding electrons are similar (electronegativity).
What causes an ionic bond to form?
When there is a large difference in electronegativity, an ionic bond is formed.
Electrons move from the element of lower electronegativity to the element of higher electronegativity which results in the formation of ions. Compounds formed between metals and non-metals are often, but not always ionic.
What is meant by the bonding continuum?
Pure covalent bonding and ionic bonding can be considered as being at opposite ends of a continuous range with polar covalent bonding lying between these two extremes.
What are the weakest intermolecular forces and how do they arise?
London’s dispersion forces which exist between all atoms and molecules.
They are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.
What affects the strength of London dispersion forces?
The strength of London dispersion forces depends on the size of the molecule or atom.
Larger atoms and molecules have more electrons. This leads to larger dipoles being established.
Explain why water is polar but carbon tetrachloride is non-polar despite both containing polar bonds.
A substance that contains polar covalent bonds may not be overall polar due to symmetry.
Water is non-symmetrical (bent), the molecule is polar.
Carbon tetrachloride is a tetrahedral molecule, so the charges are symmetrical and the molecule is non-polar.
Describe what is meant by permanent dipole interactions.
Molecules with a permanent dipole are polar.
Polar molecules display attractions between the oppositely charged ends of the molecules.
This type of intermolecular bond is stronger than London dispersion forces (which are called temporary dipoles).
What name is given to the strongest type of intermolecular bond?
Hydrogen (bonding).
What is needed for hydrogen bonding to occur?
Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.
As with permanent dipole to permanent dipole attractions, the oppositely charged ends of molecules attract.
How do the melting and boiling points of a polar substance compare to a non-polar substance with similar numbers of electrons?
The melting and boiling points of polar substances are higher than the melting and boiling points of non-polar substances with similar numbers of electrons.
Which of the following molecules would have the higher melting and boiling points and why?
Ethanol would have higher MP and BP.
Both ethanol and ether have the same molecular mass, but ethanol contains hydrogen bonding.
Ether is a symmetrical molecule, so even though it contains polar bonds, the molecule itself is non-polar and the only intermolecular force present will be weak London dispersion forces.
