Chemical Bonding

Ionic Bonding

Remembering that elements gain or lose electrons, when forming compounds,so that they achieve a full outer shell - let's now look at the reaction between sodium and chlorine.

Example 1: Reaction between sodium and chlorine

A sodium atom loses one electron to achieve a full outer shell and chlorine gains one electron to complete a full outer shell. So when a sodium atom reacts with a chlorine atom, the sodium atom loses its one electron to chlorine. The two ions formed are a sodium ion, Na+ and a chlorine ion Cl-.

The two ions have opposite charges, they attract one another.

The force of attraction between them is an electrostatic one. This type of attraction is strong. It is called an ionic or electrovalent bond.

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Example 2: Reaction between magnesium and oxygen

Other metals and non-metals react together to form ionic compounds This is because metals tend to lose electrons, whereas non-metals tend to gain electrons.

A magnesium atom has two electrons in its outer shell, whereas oxygen has six electrons. This means that magnesium wants to lose two (to oxygen) and oxygen wants to gain two (from magnesium) so that they can have full outer shells.

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The ions attract each other due to their opposite charges. Magnesium ions and oxide ions are formed. The product is magnesium oxide, MgO.

Example 3: Reaction between magnesium and chlorine

To obtain full outer shells magnesium must lose two electrons and chlorine must gain one electron. So when we react magnesium in chlorine, one magnesium atom reacts with two chlorine atoms to form magnesium chloride, MgCl2.

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Covalent Bonding

When two non-metals react together, they both need to gain electrons to complete full outer shells. The only way this can be achieved is if they share their outer electrons

Hydrogen: Each hydrogen atom has only one electron and needs one more to complete its first shell. When two hydrogen atoms get close together their shells can overlap and then they can share their electrons.

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Since, electrons are being shared, there is a strong force of attraction between them. This force is a covalent bond.

The bonded atoms form molecules. Hydrogen's molecular formula is H2.

Chlorine: A chlorine atom needs a share of one other electron to obtain a full outer shell. If two chlorine atoms are placed together the result is as shown below:

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Oxygen: Each oxygen atom requires a share of two electrons.

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Since each oxygen atom has a share of two pairs of electrons, we call this a double covalent bond.

There is a vast number of compounds that exist as molecules.

Water: In each molecule, H2O, one oxygen atom shares electrons with two hydrogen atoms.

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Ammonia: In each molecule, NH3, one nitrogen atom shares its electrons with three hydrogen atoms, so that they all reach full shells.

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Methane: Its formula is CH4. One carbon atom shares its electrons with four hydrogen atoms.

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Types of Solids

The four types that we shall study in this quick learn are:

1. Metals

2. Ionic

3. Molecular

4. Giant molecular

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In a metal, the atoms are very tightly packed, leaving little space between them. Due to this tight packing, the outer electrons become delocalised from their atoms. This results in a 'sea' of electrons around a lattice of ions or 'pseudo' cations.

Properties of metals

Here are some general properties, but remember there are always exceptions!

  1. They are hard.
  2. They are tough.
  3. They are not easily compressed.
  4. High tensile strength - not easily stretched.
  5. Malleable - can be bent or hammered into a shape.
  6. Ductile - can be drawn into wires.
  7. Good conductors of heat and electricity because of sea of electrons that can move around the lattice carrying heat energy or charge.
  8. Usually high melting points.
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Ionic solids are made up of a lattice composed of oppositely charged ions. One of the most common ionic solids is sodium chloride. Sodium chloride is made up of sodium and chloride ions packed - a lattice. The ions are held by electrostatic charges in an ionic bond.

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Properties of ionic solids

  1. High melting points and boiling points due to strong ionic bonds. Most are solids at room temp.
  2. They are brittle - will shatter with a hammer.
  3. Usually soluble in water. Insoluble in non-polar solvents.
  4. Do not conduct electricity in solid state. They do conduct when molten or dissolved in water since the ions are free to carry the charges as the ionic bonds do not hold them firmly in the liquid state.
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In a molecular solid, the molecules are held together by weak Van Der Waal's force, but packed in a regular pattern. Iodine is an example of a molecular solid. Each iodine molecule is made up of 2 iodine atoms, held together by a strong covalent bond. Each iodine molecule is held to another by weak Van Der Waal's forces.

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Properties of molecular solids

  1. Low melting and boiling point due to weak forces between molecules.
  2. They are brittle.
  3. Insoluble in water but soluble in non-polar solvents such as tetrachloromethane and petrol.
  4. Do not conduct electricity. Molecules do not carry a charge so even when melted, molecular solids cannot conduct.

Diamond and graphite

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Diamond: Is made up of a lattice of carbon atoms. Each carbon atom can make 4 covalent bonds to 4 other carbon atoms. Each outer atom then bonds to 3 more and so on. Eventually millions of carbon atoms are bonded to form a giant lattice.

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Properties of diamond

  1. Very hard - hardest known substance. Each atom held to 4 others by strong covalent bonds - this explains the high melting point.
  2. Does not conduct electricity due to no ions or free electrons to carry charge.

Graphite: Graphite is made up of flat sheets of carbon atoms.

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Each carbon atom makes three covalent bonds to other carbon atoms. T his gives rings of 6 atoms. The flat sheets that lie on top of each other are held by weak forces - Van Der Waal's.

Properties of graphite

  1. It is soft and slippery due to sheets of atoms been able to slide over one another because of weak forces between them.
  2. A good conductor of electricity. This is due to each atom only using 3 out of 4 outer electrons in bonding. The fourth electron of each atom becomes delocalised throughout the lattice, enabling graphite to carry charge.
  3. High melting point due to strong covalent bonds holding atoms of carbon together in the rings.

Exam-style Questions

 

 

 

  1. The diagram below shows the electron arrangements of magnesium and oxygen:

    Install Flash

    a) Draw a diagram showing how a bond is made between magnesium and oxygen.

    (2 marks)

    b) What name is given to this type of chemical bond?

    (1 mark)

    (Marks available: 3)

    Answer

     

    Answer outline and marking scheme for question: 1

    a)

    Install Flash

    (2 electrons moving = 1 mark)

    (complete transfer = 1 mark)

    b)Ionic

    (1 mark)

    (Total = 3 marks)

  2. a) What type of chemical bond would you expect in hydrogen fluoride, HF?

    (1 mark)

    b) Draw a diagram to show how this bond is formed.

    (2 marks)

    (Marks available: 3)

    Answer

     

    Answer outline and marking scheme for question: 2

    a) Covalent

    (1 mark)

    b)

    Install Flash

    (1 mark = 1 shared pair electrons)

    (1 mark = 1 electron from each atom)

    (Total = 3 marks)

  3. Look at the diagram below:

    Install Flash

    a) Which type of structure is shown in the diagram?

    (1 mark)

    b) Will a material with this structure be able to conduct electricity?

    Explain your answer.

    (2 marks)

    (Marks available: 3)

    Answer

     

    Answer outline and marking scheme for question: 3

    a) Metallic.

    (1 mark)

    b) The material will be able to conduct electricity,

    (1 mark)

    because there are free electrons that are able to move through the structure.

    (1 mark)

    (Total = 3 marks)

  4. Look at the table below:

    Substance: Melting point (°C):
    A 445
    B 1907
    C -7

    a) Which substance, A,B or C could have a metallic structure?

    (1 mark)

    b) Which substance, A, B or C could have a simple covalent structure?

    (1 mark)

    (Marks available: 2)

    Answer

     

    Answer outline and marking scheme for question: 4

    a) B (highest melting point)

    (1 mark)

    b) C (lowest melting point)

    (1 mark)

    (Total = 2 marks)

How are Compounds Formed?

Most elements form compounds.

For example: A reaction between sodium and chlorine gives the compound sodium chloride (salt) quite readily.

The noble gases do not usually form compounds. They are different from other elements, since their atoms are described as stable or unreactive. They are stable because their outer electron shell is full. A full outer shell makes an atom more stable.

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Only the noble gases have full outer shells. This is why they are stable.

Other elements react with each other in order to obtain full outer shells, this makes them more stable.

Depending on their electronic configurations, atoms lose or gain electrons in order to achieve a full outer shell.

Losing electrons

The sodium atom has one electron in its outer shell. If it loses this one electron it will achieve a full outer shell. By losing the one electron to another atom, it becomes a sodium ion.

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The sodium ion still has 11 protons but by losing one electron it has only 10 electrons compared to the atom. Hence, its overall charge is +1.

This +1 charge is due to the ion having one more proton than electron.

In naming ions, you take the symbol Na and assign a positive charge. This gives us the sodium ion Na+.

Gaining electrons

A chlorine atom has seven electrons in its outer shell. It can reach a full outer shell by gaining one electron. It will then become the chloride ion, Cl-.

A negative charge is assigned to the ion to signify that the ion contains one more electron than proton.

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Ions

Any atom can become an ion if it gains or loses electrons.

An ion is a charged particle. It is charged due to an unequal number of electrons and protons.

S-Cool Revision Summary

Atoms are most stable if they have a full outer shell.

Atoms bond with one another to achieve full outer shells - this is why most elements form compounds.

Noble gases do not form compounds, since they already have full outer shells.

Metal atoms (and hydrogen), such as sodium, lose electrons to become ions. Ions are charged particles. Since metals lose electrons to achieve full outer shell configurations, the metal ions have positive charges. A positive ion is called a cation.

Non-metal atoms, such as chlorine, gain electrons and become ions. By gaining electrons they can achieve a full outer shell. Since they gain electrons non-metal ions have negative charges. A negative ion is called an anion.

Metal atoms bond with non-metal ions by transferring their electrons. This is called ionic bonding.

For some elements, the energy involved in losing or gaining electrons is too much. In these cases their atoms share electrons. This bonding associated with the sharing of electrons is called covalent bonding.

There are four types of solid, giant ionic, giant covalent, metallic and simple molecular.

Sodium chloride is an example of a giant ionic solid. It consists of oppositely charged ions held together by electrostatic forces. They are soluble in water and conduct electricity when molten or in solution.

Diamond and graphite are examples of giant (covalent) molecular solids. They consist of millions of covalent bonds that cause them to have very high boiling points.

Iodine is an example of a simple molecular solid. It has a low boiling point because molecules of I2 are held together by weak forces. They are insoluble in water and do not conduct electricity.

Metals consist of tightly packed atoms whose outer electrons become part of a ' sea ' of electrons. This delocalisation of electrons explains why metals are good conductors of heat and electricity.

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