Chapter 4 - Covalent Molecular Substnaces Flashcards
How is covalent binding different to ionic bonding?
The formation of the process of the metal atoms giving up their valence electrons to form positive ions and the non-metals gaining these electrons to become negative ions. It is the transfer of electrons between metals and non-metals to form ions.
In the reaction between non-metals, it is not energetically favourable for any of the atoms to lose electrons. Nonetheless, the non-metal atoms do react, but rather than forming ions, they often form a molecule.
What is a molecule?
A molecule is a small group of atoms strongly bonded together. Water, carbon dioxide, methane, chlorine, sulfur and ethanol are examples of substances composed of molecules.
What is a covalent bond?
It was suggested that the atoms in a molecule share electrons. These atoms were proposed to be held together, or bonded together by electrostatic attractions between the shared electrons and their positive nuclei. The form of bonding between atoms in a molecule is called covalent bonding.
Covalent bonds and valence electrons
The number of covalent bonds formed by an atom of an element depends on the number of valence electrons it has.
Properties of covalent molecular substances
- they have low melting and boiling points; many are liquids and gases at room temp.
- they are non conductors of electricity in both solid and liquid state
- they form solids that are generally quite soft, are easily scratched and have a waxy appearance.
- many solids are malleable when bent and do not shatter when hit.
- many are insoluble in water, but are soluble in non-polar solvents such as petrol and kerosine
- many have an odour
Structure of covalent molecular substances
For a covalent molecular substance in the solid state, the neutral molecules are organised in an orderly lattice structure. In hone liquid and gas states, the molecules are arranged randomly. In the gas state, the molecules are much further apart than in the liquid state.
Explain the melting points of covalent molecular substances
Generally, the attractive forces between the molecules of covalent molecular substances are weak, as indicated by the low melting points. For example, ice has the low melting point of 0 degrees because the attractive forces between the water molecules, the intermolecular forces, are relatively weak. However, a large amount of energy is required to break up water molecules into individual hydrogen and oxygen atoms.
Explain the boiling points of covalent molecular substances
As with melting points, when a covalent molecular substance is boiled, it is the weak intermolecular forces between the molecules that are disrupted, not the covalent bonds between the atoms in the molecules.
Explain intermolecular bonding*
*in all covalent molecular compounds! the intermolecular bonding, that is! the bonding between the molecules, is much weaker than the covalent binding between the atoms within the molecules.
Explain softness in covalent molecular substances
Very little force is needed to push the neutral molecules in the solid lattice past one another, due to the weak intermolecular attractive forces.
Explain the non conductivity of covalent molecular substances
For a substance to conduct electricity, mobile charged particles capable of conducting electricity must be present. These charged particles include electrons and positive and negative ions. In covalent molecular substances, there are no ions because the molecules are neutral. Also, in the molecules the valence electrons are localised between or around atoms, as either shared or unshared pairs, and so are not free to move about the solid lattice or the liquid. As a result, the covalent molecular substances are non-conductors of electricity in the solid or liquid state.
Explain the solubility of covalent molecular substances
The solubility properties of covalent molecular compounds vary considerably. Some are very soluble in water, others slightly soluble, but many are insoluble. Numerous covalent molecular substances are soluble in non-polar solvents such as oil or kerosine.
Naming covalent molecular substances
Ide is added to the stem of the name of the second element.
Where a molecule contains more than one atom of one type, the number of atoms is indicated by adding the prefixes: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca.
