An introduction to chemical bonds

 

Group 0 - The Noble gases:

Facts:

Electron configuration:

 

 

Bonding:

Chemical bonding:

                    Metals        +            Non - metals

Type of Bonding Which types of atoms combine Electrons
Ionic Metal               +     Non-metal Transferred:  Metal Non-metal
Covalent Non-metal     +     Non-metal Shared:  Between the atoms
Metallic Metal     +     Metal Shared:  Between all atoms

 

Questions:  p49, 1 / p73, 12 /

 

Ionic Bonding

 

Ionic bonding results from the electrostatic attraction between oppositely charged ions.

 

        Metals become positive ions (cations) electrons are removed.

        Non-metals become negative ions (anions) electrons are gained. 

        Electrons are transferred from metal atoms to non-metal atoms.  Both will have full outer shells.

        In an ionic compound the ions are held in place by strong forces of attraction between oppositely charged ions (and repulsion between like charged ions).

        Attraction outweighs repulsion and the forces are balanced.

 

Lithium Fluoride:       Click  for DEMO

 

        Metals form positive ions which we call cations.

        Non - metals form negative ions which we call anions.

        Dot and cross diagrams is a means of electron counting.

 

 

Becomes

However we only show the ions and only the outer shells:-

 

 

Sodium oxide:

 

2Na 2Na+ + 2e-
1s22s22p63s1 1s22s22p6    
O + 2e- O2-
1s22s22p4       1s22s22p6

 

Giant ionic lattices

 

                                                           

 

Further examples of ionic bonding:

 

CaO AlF3
Ca Ca2+ + 2e-
Al Al3+ + 3e-
O + 2e- O2-
F + e- F- 3 times

 

Questions:  p51, 1-2 /  p73, 3

 

Ions and the Periodic Table

 

Predicting charges:

Group 1 2 3 4 5 6 7 0
No e's in outer shell 1 2 3 4 5 6 7 Full
No electrons lost gained Lose 1 Lose 2 Lose 3 x Gain 3 Gain 2 Gain 1 x
Charge on ion 1+ 2+ 3+ x 3- 2- 1- x

Transitions metals:

Element Charge on ion / oxidation number
Mn   2+ 3+ 4+ 5+ 6+ 7+
Roman numeral   II III IV V VI VII

            Iron (II)    Fe2+

            Copper (II)    Cu2+

Molecular ions (common ions)

1+ 1- 2- 3-
Ammonium NH4+ Hydroxide OH- Carbonate CO32- phosphate PO43-
Nitrate NO3- Sulphate SO42-
Nitrite NO2- Sulphite SO32-
Hydrogen carbonate HCO3- Dichromate Cr2O72-

 

Predicting ionic formula:

                Sodium Chloride -            Na+               Cl-                Write the ions with charges

                                                            1+         :       1-                 Write the ratio of the charges

                                                                                                      Multiply up if necessary to =0

                                                                     NaCl                        Bring together omitting the charges

 

                Copper (II) Chloride          Cu2+              Cl-              Write the ions with charges

                                                              2+       :         1-  (x2)      Write the ratio of the charges

                                                            Cu2+     :        Cl-2            Multiply up if necessary to =0

                                                                     CuCl2                      Bring together omitting the charges

 

                Calcium Hydroxide           Ca2+              OH-            Write the ions with charges

                                                             2+          :        1-  (x2)     Write the ratio of the charges

                                                           Ca2+       :      (OH-)2        Multiply up if necessary to =0

                                                                     Ca(OH)2                 Bring together omitting the charges

 

Questions  p53, 1-3 / p73, 4

 

Covalent Bonding

Octet rule (more accurately - full outer shell rule)

Single covalent bonds

Chlorine:

  • Each chlorine has 7e in its outer shell.

  • Each Cl has 1 unpaired electron in its outer shell.

  • Each Cl contributes an electron each = 2e forming 1 single covalent bond:

Other examples:

Group 4 5 6 7
Element C N O F
Electrons in outer shell 4 5 6 7
Lone pairs 0 1 2 3
Unpaired electrons in outer shell 4 3 2 1
No of covalent bonds 4 3 2 1

  • Lone pairs of electrons are when the paired electrons are not used in a covalent bond.

  • They do however give a region of concentrated negative charge.

Multiple covalent bonds

  • Some atoms can form more than a single bond, double and triple.

  • This depends on how many single unpaired electrons there are in the outer shell:

 

Ionic / Cov

 

Questions: 1-2 P55

 

Further covalent bonds

Dative covalent bonds:

The oxonium ion:

  • Water has 2 lone pairs.
  • It will react with the hydrogen chloride gas to form the Oxonium ion
HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

 

                                                                              H3O+    or    H+(aq)

The ammonium ion:

                           

Breaking the octet rule

 
  • The problem with this rule is that some atoms do not have enough electrons in their outer shell to form enough covalent bonds to make 8e.

  • Boron is in group 3 of the Periodic table.  This means it has 3e in its outer shell.

  • This means that Boron can only form 3 covalent bonds = 6e in its outer shell:

 

                               

Expanding the octet

 

Period 3, the electron shell is much larger and can accommodate more covalent bonds.

Phosphorous Sulphur Chlorine
If all unpaired electrons are used then there will be 3 covalent bonds = 6e plus the lone pair of 2e = 8e If all unpaired electrons are used then there will be 2 covalent bonds = 4e plus the 2 lone pair of 2e = 8e If all unpaired electrons are used then there will be 1 covalent bond = 2e plus the 3 lone pair of 6e = 8e
If the lone pair unpairs then there are 5 unpaired electrons.  These will form 5 covalent bonds = 10e If one of the lone pair unpairs then there are 4 unpaired electrons.  These will form 4 covalent bonds = 8e plus the remaining lone pair of 2e = 10e If one of the lone pair unpairs then there are 3 unpaired electrons.  These will form 3 covalent bonds = 6e plus the 2 remaining lone pairs of 4e = 10e
  If both of the lone pairs unpair then there are 6 unpaired electrons.  These will form 6 covalent bonds = 12e If 2 of the lone pairs unpair then there are 5 unpaired electrons.  These will form 5 covalent bonds = 10e plus the remaining lone pair of 2e = 12e
    If all of the lone pairs unpair then there are 7 unpaired electrons.  These will form 7 covalent bonds = 14e
Group 5 = 3, 5 bonds Group 6 = 2, 4, 6 bonds Group 6 = 1, 3, 5, 7 bonds
  • All 6 electrons in sulphur have been used to form covalent bonds.
  • This makes 12e in the outer shell of sulphur.
  • Each of the fluorine's have 8e in their outer shell.

A better rule:

  1. All electrons can be used to form covalent bonds from Period 3 onwards
  2. The maximum number of bonds = the number of electrons in the outer shell

Questions:  1-2 P57

 

Shapes of molecules and ions:

 

Electron pair repulsion theory:

                            Bonding pairs & Lone pairs

Bonding pairs of electrons

Activity

 

Trigonal planar

Tetrahedral

Octahedral

Lone pair electrons:

Activity

Tetrahedral
Pyramidal
Non - linear

Explanation:

               Lone Pair Lone Pair       >       Lone Pair Bonding Pair       >       Bonding Pair Bonding Pair

 

Molecules with double bonds:

                                                                                                                            Linear

Shapes of ions:

Summary:

State the number of bonding and lone pairs of electrons

Pairs of electrons repel as far as possible (lone and bonding)

This determines the shape

Lone pairs repel more than bonding pairs as closer to central atom

Each lone pair reducing the bond angle by 2.5o as it is closer to the central atom

3BP (areas)

4BP

3BP / 1LP

2BP / 2LP

2 Areas (4BP)

6BP

 

 

 

 

 

Trigonal planar

Tetrahedral

Pyramidal

Non - linear

Linear

Octahedral

 How does the addition of H+ ion change shape of ammonia (water)

3BP / 1LP

 

4BP

 

The addition of a datively bonded H+ ion uses the lone pair converting it to a bonding pair (similar for water)

Pyramidal

 

Tetrahedral

 

Questions:  1-3 P59 / 5, 9 P73