Simple Molecules & Covalent Bonds (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards
Exam code: 0653
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What is a covalent bond?
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What is a covalent bond?
A covalent bond is formed when a pair of electrons is shared between two non-metal atoms. Each atom contributes one electron to the shared pair, and both atoms achieve a full outer shell with a noble gas electronic configuration.
True or False?
Covalent bonding can occur between a metal atom and a non-metal atom.
False.
Covalent bonding only occurs between non-metal atoms. When a metal reacts with a non-metal, electrons are transferred rather than shared, forming an ionic bond instead.
In a covalent bond, electrons from one atom are shown as .......... and electrons from the other atom are shown as .......... in a .......... diagram.
In a covalent bond, electrons from one atom are shown as dots and electrons from the other atom are shown as crosses in a dot-and-cross diagram.
Why do non-metal atoms form covalent bonds rather than transferring electrons?
Non-metal atoms both need to gain electrons to achieve a full outer shell. Instead of one transferring electrons to the other (as in ionic bonding), they share electrons so that both atoms benefit from the shared pair and both achieve a stable full outer shell.
How many covalent bonds does a water molecule (H2O) contain and how are they formed?
Water (H2O) contains two single covalent bonds. The oxygen atom has six outer electrons and needs two more to fill its shell. Each hydrogen atom shares one pair of electrons with the oxygen, forming two O-H bonds and giving each atom a full outer shell.
True or False?
In a dot-and-cross diagram for a covalent compound, the shared electrons appear in the overlapping region of the two electron shells.
True.
In a dot-and-cross diagram for a covalent compound, the electron shells of the two bonded atoms overlap. The shared pair of electrons is shown in the overlap region, with one dot and one cross to indicate one electron from each atom.
In H2, two hydrogen atoms each contribute .......... electron to form a .......... covalent bond. Both atoms then have a full first shell containing .......... electrons.
In H2, two hydrogen atoms each contribute one electron to form a single covalent bond. Both atoms then have a full first shell containing 2 electrons.
What do covalent bonding and ionic bonding have in common in terms of the outcome for the atoms involved?
In both covalent and ionic bonding, the atoms involved end up with a full outer shell of electrons, achieving the stable noble gas electronic configuration. The difference is that covalent bonding achieves this by sharing electrons, while ionic bonding achieves it by transferring electrons.
What is a double bond in covalent chemistry?
A double bond is formed when two adjacent atoms share two pairs of electrons. Double bonds are found in molecules such as O2 (oxygen) and CO2 (carbon dioxide), where atoms need to share more than one pair to achieve a full outer shell.
True or False?
In nitrogen gas (N2), the two nitrogen atoms are joined by a triple bond. (Extended Tier Only)
True.
Each nitrogen atom has five outer electrons and needs three more to fill its shell. The two nitrogen atoms share three pairs of electrons, forming a triple bond (N≡N) and giving each atom a full outer shell of eight electrons.
In ethene (C2H4), the two carbon atoms share .......... pairs of electrons, forming a .......... bond between them. (Extended Tier Only)
In ethene (C2H4), the two carbon atoms share two pairs of electrons, forming a double bond between them.
Why do oxygen atoms form a double bond in O2 rather than two separate single bonds? (Extended Tier Only)
Each oxygen atom has six outer electrons and needs two more to fill its shell. Sharing one pair would give each atom only seven outer electrons, which is not a full shell. By sharing two pairs (double bond), both atoms achieve a full outer shell of eight electrons.
In a dot-and-cross diagram, how do you identify that a compound is covalent rather than ionic? (Extended Tier Only)
In a covalent compound, the electron shells of the atoms overlap and shared electron pairs are shown in the overlap region. There are no square brackets or ionic charges. A covalent compound also contains only non-metal atoms.
Carbon dioxide (CO2) contains two .......... bonds, with the carbon atom forming a double bond with .......... oxygen atom. (Extended Tier Only)
Carbon dioxide (CO2) contains two double bonds, with the carbon atom forming a double bond with each oxygen atom.
True or False?
A triple bond involves the sharing of three pairs of electrons between two atoms. (Extended Tier Only)
True.
A triple bond is formed when two atoms share three pairs (six electrons) between them. This is seen in N2, where each nitrogen needs three extra electrons to complete its outer shell.
How many bonds does the carbon atom form in methanol (CH3OH) and what type are they? (Extended Tier Only)
In methanol (CH3OH), carbon forms four single covalent bonds: three C-H bonds and one C-O bond. Carbon has four outer electrons and needs four more to fill its shell, so it forms four bonds in total.
What is the typical physical state of a simple molecular compound at room temperature, and why?
Simple molecular compounds are typically liquids or gases at room temperature because they have low melting and boiling points. The forces between the small molecules are weak and require little energy to overcome.
True or False?
Simple molecular compounds are good conductors of electricity.
False.
Simple molecular compounds have poor electrical conductivity. They contain no free electrons or ions, which are the charged particles needed to carry an electric current.
Simple molecular compounds have .......... melting and boiling points. As molecular size .........., the melting and boiling points generally .......... .
Simple molecular compounds have low melting and boiling points. As molecular size increases, the melting and boiling points generally increase.
Why do simple molecular compounds have low melting and boiling points?
Simple molecular compounds consist of small molecules with weak forces between them. Only a small amount of energy is needed to overcome these intermolecular forces and change state, resulting in low melting and boiling points. Note: the covalent bonds within the molecules are strong, but these are not broken on melting or boiling.
True or False?
Larger simple molecules tend to have higher melting and boiling points than smaller simple molecules.
True.
As molecular size increases, the intermolecular forces between molecules become stronger, requiring more energy to overcome. This leads to generally higher melting and boiling points for larger simple molecular compounds.
Why can't simple molecular compounds carry an electric current?
Simple molecular compounds have no free electrons or mobile ions — the only charged particles that can carry an electric current. The atoms within each molecule are held together by covalent bonds with no overall charge, so there are no charge carriers available.
Simple molecular compounds are made of small molecules with atoms .......... bonded together. They have .......... electrical conductivity because they contain no free .......... or .......... .
Simple molecular compounds are made of small molecules with atoms covalently bonded together. They have poor electrical conductivity because they contain no free electrons or ions.
Compare the melting points of simple molecular compounds with those of ionic compounds, and explain the difference.
Simple molecular compounds have much lower melting points than ionic compounds. Ionic compounds require a large amount of energy to break the many strong electrostatic forces throughout the giant lattice. Simple molecular compounds only need to overcome weak intermolecular forces between small molecules, which requires far less energy.
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