CHEMISTRY FORM 2
- 1.1 Structure of the atom
- 1.2 Atomic Number and Mass Number
- 1.3 Isotopes
- 1.4 Energy levels and electron arrangement
- 1.5 Development of the Periodic Table
- 1.6 Relative Atomic Mass and Isotopes
- 1.7 Ion Formation
- 1.8 Chemical Formulae
- 1.9 Chemical Equations
- 2.1 Alkali metals (Group I elements)
- 2.2 Alkali Earth Metals (Group II elements)
- 2.3 Halogens (Group VII elements)
- 2.4 Noble gases (Group VIII elements)
- 2.5 Properties and Trends Across the Periodic Table
- 3.1 Bond
- 3.2 Ionic bond
- 3.3 Giant ionic structure
- 3.4 Covalent bond
- 3.5 Co-ordinate bond
- 3.6 Molecular structures
- 3.7 Giant covalent structures
- 3.8 Metallic Bond
- 3.9 Types of bond across a period
- 3.10 Oxides of elements in Period 3
- 3.11 Chlorides of Period 3 elements
- 4.1 What is a salt?
- 4.2 Types of salt
- 4.3 Solubility of salts in water
- 4.4 Methods of preparing salts
- 4.4.1 Reacting a Metal with an Acid
- 4.4.2 Reacting an Acid with a Base (Neutralization)
- 4.4.3 Reacting an Acid with a Carbonate (or hydrogencarbonate of metal)
- 4.4.4 Combining elements Directly (Direct Combination of elements)
- 4.4.5 Precipitation (Double decomposition)
- 4.5 Action of heat on salts
- 4.6 Uses of salts
- 5.1 Electrical conduction
- 5.2 Electrical conductivity of molten substances
- 5.3 Electrical conductivity of substances in aqueous state
- 5.4 Electrolysis
- 5.5 Applications of electrolysis
- 6.1 Allotropes of carbon
- 6.2 Chemical properties of carbon
- 6.3 Carbon (IV) oxide
- 6.4 Carbon (II) oxide (CO)
- 6.5 Large scale production of sodium carbonate and sodium hydrogencarbonate
- 6.6 Effect of carbon (II) oxide and carbon (IV) oxide on the environment
- 6.7 Carbon cycle
Chemical Bonding and Structure: Molecular structures
3.0 Chemical Bonding and Structure
3.6 Molecular structures
Let us revisit some of the substances we have dealt with so far under covalent and coordinate bonds (Figure 3.6).
Figure 3.6: Molecular structures
In each unit in Figure 3.6, energy levels of all the atoms are filled. So they are stable like noble gases and exist independently. They are molecules; their structure is molecular. Unlike ions, they are neutral and do not attract or combine with neighboring particles. Also, they are non-metals and compounds of non-metals only, and occur mainly as gases and liquids.
- Define a molecular structure.
- Which bond type leads to molecular structures?
- Which class of elements forms bonds that lead to molecular structures?
- Distinguish between a bond and structure.
- Comment about the texture (hardness and softness), and melting and boiling points of molecular structures.
- Do substances with molecular structures conduct electricity? Explain your answer.
Answers to Questions 3.6
Both co-ordinate and covalent bonds lead to molecular structures.
Van der Waal's forces of attraction
Because water exists as molecules, we would expect it to be a gas; that is, a state in which molecules are far apart because there is no bond holding them together. But water is a liquid, which can be readily condensed to ice. That means there are forces of attraction holding its molecules together.
The forces that hold molecules together are called Van der Waal's forces of attraction. Van der Waal's forces are not chemical bonds, but weak attractions between molecules. They occur in all molecular structures. Ice, liquid water and other substances with molecular structures have low melting and boiling points because of the weak Van der Waal's forces of attraction in them.
Properties of substances with molecular structures
- They have low melting and boiling points, because molecules exist independently of one another, only linked by weak Van der Waal's forces of attraction.
- They occur as gases, liquids and soft solids.
- They do not conduct electricity in solid, molten state or solution because molecules are electrically neutral. This property is used to identify molecular structures.