CIE iGCSE Co-ordinated Sciences-C2.5 Simple molecules and covalent bonds- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C2.5 Simple molecules and covalent bonds – Study Notes
CIE iGCSE Co-ordinated Sciences-C2.5 Simple molecules and covalent bonds – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Covalent Bond Formation
- A covalent bond is formed when a pair of electrons is shared between two atoms. This sharing allows both atoms to achieve a stable electronic configuration, often similar to that of the nearest noble gas.
- Covalent bonds usually form between non-metal atoms that cannot completely transfer electrons to each other because of similar electronegativities.
- In covalent bonding:
Single covalent bonds involve one shared pair of electrons, double bonds involve two shared pairs, and triple bonds involve three shared pairs.
Example
Explain how a covalent bond forms in a hydrogen molecule and how it achieves a noble gas configuration.
▶️Answer/Explanation
Each hydrogen atom has one electron in its 1s orbital. By sharing their electrons, both hydrogen atoms attain the stable helium configuration (\( 1s^2 \)):
\( \text{H} \cdot + \cdot \text{H} \rightarrow \text{H}:\text{H} \)
The colon (:) represents the shared pair of electrons forming a single covalent bond. This sharing ensures that both atoms achieve a full outer shell, which is the most stable arrangement of electrons.
Example
Describe covalent bonding in a chlorine molecule and how it achieves noble gas configuration.
▶️Answer/Explanation
Each chlorine atom has seven electrons in its outer shell. By sharing one electron each, both chlorine atoms achieve the full octet configuration similar to argon:
\( \text{Cl} \cdot\cdot\cdot\cdot\cdot\cdot \cdot + \cdot \text{Cl} \cdot\cdot\cdot\cdot\cdot\cdot \cdot \rightarrow \text{Cl}:\text{Cl} \)
This shared pair of electrons forms a single covalent bond, giving both atoms a complete outer shell and a stable electronic configuration.
Example
Explain covalent bonding in water and how it achieves stable electronic configurations for all atoms.
▶️Answer/Explanation
Oxygen has six electrons in its outer shell and needs two more to complete an octet. Each hydrogen atom shares one electron with oxygen:
\( \text{H} \cdot + \cdot \text{O} \cdot + \cdot \text{H} \rightarrow \text{H}-\text{O}-\text{H} \)
Through this sharing:
Oxygen completes its octet, achieving a noble gas-like configuration.
Each hydrogen attains the helium configuration (\( 1s^2 \)).
The shared electrons form two single covalent bonds, giving stability to the molecule.
Formation of Covalent Bonds in Simple Molecules
Covalent bonds are formed when atoms share pairs of electrons to achieve a stable noble gas configuration. In simple molecules, the atoms are held together by strong covalent bonds within the molecule, while weak intermolecular forces act between molecules. Dot-and-cross diagrams can be used to show how electrons are shared between atoms in these molecules.
- Hydrogen (\( \text{H}_2 \))
Each hydrogen atom has one electron. By sharing electrons, both atoms achieve a helium-like configuration:
\( \text{H} \cdot + \cdot \text{H} \rightarrow \text{H}:\text{H} \)
- Chlorine (\( \text{Cl}_2 \))
Each chlorine atom has seven outer electrons. Sharing one electron each completes the octet for both:
\( \text{Cl} \cdot\cdot\cdot\cdot\cdot\cdot \cdot + \cdot \text{Cl} \cdot\cdot\cdot\cdot\cdot\cdot \cdot \rightarrow \text{Cl}:\text{Cl} \)
- Water (\( \text{H}_2\text{O} \))
Oxygen has six outer electrons and needs two more. Each hydrogen shares one electron:
\( \text{H} \cdot + \cdot \text{O} \cdot + \cdot \text{H} \rightarrow \text{H}-\text{O}-\text{H} \)
- Methane (\( \text{CH}_4 \))
Carbon has four outer electrons and needs four more. Each hydrogen shares one electron with carbon:
\( \text{H} \cdot + \cdot \text{C} \cdot + \cdot \text{H} \) (repeated for four hydrogens)
- Ammonia (\( \text{NH}_3 \))
Nitrogen has five outer electrons and needs three more. Each hydrogen shares one electron with nitrogen, leaving one lone pair on nitrogen:
\( \text{H} \cdot + \cdot \text{N} \cdot + \cdot \text{H} \) (with one lone pair on N)
- Hydrogen Chloride (\( \text{HCl} \))
Hydrogen shares one electron with chlorine to complete its outer shell:
\( \text{H} \cdot + \cdot \text{Cl} \cdot\cdot\cdot\cdot\cdot\cdot \cdot \rightarrow \text{H}-\text{Cl} \)
- Methanol (\( \text{CH}_3\text{OH} \))
Carbon shares electrons with three hydrogens and one oxygen; oxygen shares one electron with hydrogen:
\( \text{H} \cdot + \cdot \text{C} \cdot + \cdot \text{H} + \text{O} \cdot + \cdot \text{H} \rightarrow \text{CH}_3\text{OH} \)
- Ethene (\( \text{C}_2\text{H}_4 \))
Each carbon has four electrons; two are shared between them forming a double bond, and each carbon also bonds to two hydrogens:
\( \text{H}-\text{C}=\text{C}-\text{H} \) (each carbon with two H atoms)
- Oxygen (\( \text{O}_2 \))
Each oxygen atom shares two electrons to form a double bond:
\( \text{O}=\text{O} \)
- Carbon Dioxide (\( \text{CO}_2 \))
Carbon shares two electrons with each oxygen atom, forming double bonds to complete octets:
\( \text{O}=\text{C}=\text{O} \)
- Nitrogen (\( \text{N}_2 \))
Each nitrogen atom shares three electrons with the other, forming a triple bond:
\( \text{N} \equiv \text{N} \)
Properties of Simple Molecular Compounds
Simple molecular compounds are composed of small molecules held together by strong covalent bonds inside the molecules and weak intermolecular forces between molecules. These structural features explain their characteristic physical properties.
1. Low Melting Points and Boiling Points
- Atoms in a molecule are strongly bonded covalently, but the weak intermolecular forces between molecules require only a small amount of energy to overcome. This results in low melting points and low boiling points.
- The covalent bonds within the molecules remain intact during melting or boiling.
2. Poor Electrical Conductivity
- Simple molecular compounds do not have free electrons or ions.
- Since electrons are held tightly within covalent bonds, these compounds cannot conduct electricity in solid, liquid, or dissolved form.
Example
Why does carbon dioxide (\( \text{CO}_2 \)) exist as a gas at room temperature and not conduct electricity?
▶️Answer/Explanation
Carbon dioxide has weak intermolecular forces, so the molecules separate easily, existing as a gas at room temperature (sublimes at -78°C). It does not conduct electricity because it contains no free electrons or ions.
Example
Explain why methane (\( \text{CH}_4 \)) has a very low melting and boiling point and does not conduct electricity.
▶️Answer/Explanation
Methane has very weak forces between molecules, so only a small amount of energy is needed to separate them. Its melting point is -182°C and boiling point is -161°C. Methane does not conduct electricity because it has no free-moving charged particles; all electrons are held in covalent bonds.