National Chemistry

 

 KEY FACTS  4 & 7 Bonding

 

 

4.1  Atoms can be held together by bonds

                        same atoms bonded together = element   (H2)

                        different atoms bonded together = compound  (HCl)

 

4.2 

 

substance                                      Bonding                   melting pt.         Structure

metal element                                 metallic                       HIGH             NETWORK

Non-metal element/compound       covalent                      HIGH              NETWORK

Non-metal element/compound       covalent                      LOW              MOLECULE

Non-metal+metal compound           ionic                           HIGH               NETWORK (ionic lattice)

 

4.3   Low melting point substances exist as molecules - molecules must have covalent 

  bonding brought about by a sharing of pair(s) of electrons between non-metal atoms.

 

4.4    A diatomic molecule is made from two atoms and can be an element (H2, C2, O2) or   compound (CO, HCl)

Naming simple covalent molecules

 

4.5  Atoms can achieve a stable  electron arrangement of a full outer energy level

   (usually 8) by either

(i) losing electrons   2)8)1 loses 1 electron  becoming  2)8   -positive metal ion

 

(ii) gaining electrons  2)7 gains one electron becoming   2)8   -negative non-metal ion

 

(iii) sharing a pair of electrons to form a covalent bond share a pair of electrons

               = covalent bond (Cl-F)

 

4.6 Many molecules have a tetrahedral shape

eg   CCl4        SiH4           CH4                   

 

   TiCl4  - help! a metal bonded to a non-metal with a covalent molecular structure -

TRUE -there are some com[pounds like this. It has a low melting point and does not conduct electricity so it must be a covalent molecule!

 

 

7.1 Ionic bonding

 Metals form positive ions by losing electrons click

Non-metals form ions by gaining electrons (check in data book page 7)

Both form a full outer orbital of electrons - usually 8

See how to write ionic formulae for  sodium chloride, calcium oxide, magnesium chloride and calcium phosphide,

See how to write formula using the complex (polyatomic)  ions found on page 4

 

7.2

 

substance                                     conducts when             Bonding             melting pt.         Structure

metal element                                solid or molten            METALLIC             HIGH             NETWORK

                                                                                                                       HIGH             NETWORK

Non-metal element/compound                  NO                   COVALENT  

                                                                                                                        LOW           MOLECULE

 

metal-Non-metal compound           solution or molten      IONIC                     HIGH            NETWORK

Noble gas                                                  NO                  'None'                    LOW               atom

 

 

Low melting point = Covalent (bonded) molecule or noble gas

 

Conduction:

Metals conduct when solid or molten by movement of electrons (electricity in metal wires).

 

Ionic compounds conduct when molten or in solution when ions can move

 

In covalent substances the charged particles are never free to move

 

carbon in the form of graphite is the only non-metal that conducts - used as electrodes.

 

7.3 Ionic compounds exist as networks (or lattices) of positive ions and negative ions.

Ionic compiunds do not conduct when solid because the ions are held tightly and cannot move.

Ionic compounds can conduct electricity when molten or when in solution because the charged particles, the ions, are free to move.

When the ionic compound dissolves in water the lattice breaks up completely.

Covalent and metallic substances cannot be electrolysed as they do not have positive and negative charges able to move

 

7.4 A molten ionic compound  or ionic solution (aq) which conducts electricity is called an electrolyte.

The passing of electricity through a molten ionic compound or an ionic solution (using diret current) causes the decomposition of the compund into its elements - this is called ELECTROLYSIS.

 

(Some rervision notes call the electrodes anode and cathode - tghis information is not required by the Scottish syllabus - as far as you are concerned cathode is negative and anode positive

 

Electrolysis of molten lead bromide

2Br-                ----->  Br2 + 2e-

Pb2+      +2e   -----> Pb

 

Positive metal ions are attracted to the negative electrode to gain electrons and produce the metal element<BR><BR>

eg. Pb2+      + 2e ---->  Pb

Negative non- metal ions are attracted to the positive electrode to lose electrons and produce the non-metal element

eg. 2Br- -------->  Br2 + 2e

 

Electrolysis of copper chloride

 

 

7.5 Ion colour migration

Cu2+, the copper(II) ion, is blue. If a copper compound is being electroysed the blue colour would move to the negative electrode.

Eventually brown copper would appear on the electrode as the copper(II) ion gained electrons.<BR>

Cu2+ + 2e -------->  Cu

 

7.6 Covalent substances have covalent bonds produced by a sharing of a pair of elctrons - the electrons in a caovalent substance are held too tightly to move between atoms so electricity cannoy be conducted.

Covalent molecules are usually gases or liquids and always have LOW melting/boiling points.

Generallly covalent substances do not dissolve in water (eg petrol, hydrocarbons) but some can (eg glucose).

 

7.7 High melting point - network solids - exist because of strong bonds between the atoms linking all of the atoms together.

Low melting point covalent molecules have weak (easily broken) forces between the molecules. (Covalent bonds are just as strong as metallic and ionic bonds.

 

Ionic and covalent bonding animation

National 4 and 5

Bonding and Properties

 

These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.

 

The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.

 

 The notes are arranged under the following topics

 

National 3          Unit 1

                          Unit 2

                          Unit 3

 

National 4 and 5  Unit 1            Rates of Reaction

                                                  Atomic Structure

                                                  Bonding and Properties

                                                  Acids and bases

 

National 4 and 5  Unit 2       Fuels and Homologous Series

                                                Consumer Products

                                                Carbohydrates

 

National 4 and 5  Unit 3         Metals

                                                   Plastics

                                                   Fertilisers

                                                   Nuclear Chemistry

                                                  Chemical analysis

 

 

 

 

 

 

 

 

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