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Flashcards in Structure and Bonding Deck (20)
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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.


Describe the properties (melting and boiling points, conductivity) of ionic  compounds.

All ionic compounds have a high melting point and boiling point. They conduct when molten or in solution as the ions are free to move.


Describe the properties (melting and boiling points, conductivity) of covalent network compounds.

All covalent network structures have very high melting points and boiling points. They are all hard and do not conduct electricity.


Describe the properties (melting and boiling points, conductivity) of covalent molecular compounds.

They have low melting points and boiling points. They do not conduct electricity. Some covalent molecular compounds have higher melting points than expected.


Sometimes two molecules that seem similar can have very different properties because of the intermolecular bonds present. Consider ethanol and ether.

Which molecule would have the higher melting and boiling points and why?

Both ethanol and ether have the same molecular mass, however, their melting points and boiling points are different due to the ethanol containing 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.


Describe the solubility of ionic compounds, polar molecules and non-polar molecules.

Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as water and insoluble in non-polar solvents. Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents.


As a general rule "like dissolves like" so, polar molecules are soluble in polar solvents (such as water) and vice-versa.