Simple Molecules & Covalent Bonds (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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- FrontCovalent Bonds
What is a covalent bond?
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What is a covalent bond?
A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms. Both atoms contribute one electron to the shared pair, giving each atom a full outer shell.
True or False?
Covalent bonds form between metal and non-metal atoms.
False.
Covalent bonds form between non-metal atoms only. When a metal reacts with a non-metal, ionic bonds form instead.
A covalent bond forms when a ______ of electrons is ______ between two atoms. The bond arises from the electrostatic attraction between these electrons and the ______ of both atoms.
A covalent bond forms when a pair of electrons is shared between two atoms. The bond arises from the electrostatic attraction between these electrons and the nuclei of both atoms.
Why do two chlorine atoms form a covalent bond with each other?
Each chlorine atom has 7 electrons in its outer shell and needs one more to achieve a full outer shell of 8. By sharing one pair of electrons, both chlorine atoms gain a full outer shell, forming a stable Cl2 molecule.
True or False?
In a dot-and-cross diagram of a covalent molecule, the shared electrons are shown in the overlapping region between the two atoms.
True.
The overlapping region represents the shared electron pair. Electrons from one atom are shown as dots and from the other as crosses, so the origin of each electron is clear.
Water (H2O) contains two ______ bonds, with each hydrogen atom sharing a pair of electrons with the ______ atom to give each atom a full outer ______.
Water (H2O) contains two covalent bonds, with each hydrogen atom sharing a pair of electrons with the oxygen atom to give each atom a full outer shell.
Give two differences between ionic and covalent bonding.
In ionic bonding, electrons are transferred from a metal to a non-metal to form oppositely charged ions.
In covalent bonding, electrons are shared between non-metal atoms, forming molecules with no overall charge.
What is a double covalent bond?
A double covalent bond forms when two atoms share two pairs of electrons. This occurs when each atom needs to share more than one pair of electrons to achieve a full outer shell.
True or False?
Nitrogen (N2) contains a triple bond because each nitrogen atom needs three more electrons to fill its outer shell.
True.
Each nitrogen atom has 5 electrons in its outer shell and needs 3 more to fill it. By sharing 3 pairs of electrons, both nitrogen atoms achieve a full outer shell, forming a triple bond.
In a double bond, ______ pairs of electrons are shared, while in a triple bond, ______ pairs are shared.
In a double bond, two pairs of electrons are shared, while in a triple bond, three pairs are shared.
Give one example of a molecule that contains a double bond and one that contains a triple bond.
Carbon dioxide (CO2) and oxygen (O2) contain double bonds. Nitrogen (N2) contains a triple bond.
True or False?
In ethene (C2H4), the two carbon atoms are joined by a double bond.
True.
In ethene, each carbon atom shares two pairs of electrons with the other carbon, forming a double bond. This allows both carbon atoms to achieve a full outer shell.
Why must oxygen (O2) form a double bond rather than a single bond?
Each oxygen atom has 6 electrons in its outer shell and needs 2 more to achieve a full outer shell of 8. A single bond would only provide 1 extra electron per atom. By sharing 2 pairs of electrons (a double bond), both oxygen atoms gain a full outer shell.
What are the melting and boiling points of simple molecular compounds, and why?
Simple molecular compounds have low melting and boiling points because only weak intermolecular forces need to be overcome to separate the molecules. The covalent bonds within each molecule remain intact during melting or boiling.
True or False?
Simple molecular compounds have low melting points because their covalent bonds are weak.
False.
The covalent bonds within molecules are strong. Simple molecular compounds have low melting points because the intermolecular forces between molecules are weak and require little energy to overcome.
Simple molecular compounds are usually ______ or ______ at room temperature because the ______ forces between molecules require little energy to overcome.
Simple molecular compounds are usually liquids or gases at room temperature because the intermolecular forces between molecules require little energy to overcome.
Why do simple molecular compounds not conduct electricity?
Simple molecular compounds have no free electrons or ions. All electrons are held in covalent bonds within the molecules and cannot move freely to carry a charge, making these substances electrical insulators.
True or False?
Larger simple molecules tend to have higher melting points than smaller ones.
True.
As molecule size increases, there are more electrons available to create stronger intermolecular forces between molecules. More energy is needed to overcome these forces, so larger molecules have higher melting and boiling points.
Why is methane (CH4) a gas at room temperature while larger alkanes such as decane are liquids?
Methane is a small molecule with weak intermolecular forces between molecules, so little energy is needed to separate them and it exists as a gas at room temperature.
Decane is a much larger molecule with stronger intermolecular forces, requiring more energy to overcome, so it is a liquid.
As molecule size increases, the ______ forces between molecules become ______, so more energy is needed to ______ the molecules.
As molecule size increases, the intermolecular forces between molecules become stronger, so more energy is needed to separate the molecules.
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