Covalent Bonding and Giant Covalent Structures

Comprehensive practice flashcards covering covalent bonding definitions, dot-and-cross diagrams, simple molecular properties, and the characteristics of giant covalent structures like diamond, graphite, and graphene.

What is the definition of a covalent bond?

A covalent bond is formed when two non-metal atoms share a pair of electrons to achieve a full outer electron shell or stable noble gas configuration.

In a dot-and-cross diagram, what do the dots and crosses represent?

Dots (•) represent electrons from one atom, and crosses (×) represent electrons from the other atom. Only outer shell electrons are shown.

How many electrons are shared in a double covalent bond, such as in an Oxygen ($O_2$) molecule?

In a double bond, 2 pairs of electrons are shared, totaling 4 shared electrons.

What type of bond is found in a Nitrogen ($N_2$) molecule, and how many electrons are shared?

A triple bond is found in nitrogen, where 3 shared pairs result in 6 shared electrons.

What is the total number of shared pairs and shared electrons in Carbon Dioxide ($O_2=C=O_2$)?

There are 4 shared pairs total, resulting in 8 shared electrons.

Why do simple molecular substances have low melting and boiling points?

While the covalent bonds within molecules are strong, the forces between molecules (intermolecular forces) are weak and easily overcome.

Why do simple molecular substances generally not conduct electricity?

They have no free electrons or ions; the electrons are shared and localised in bonds, meaning they cannot carry a charge.

What is the shape and bond angle of a Methane ($CH_4$) molecule?

The shape is tetrahedral with a bond angle of approximately $109.5^{\circ}$.

Why does Ammonia ($NH_3$) have a trigonal pyramidal shape instead of being flat?

The presence of one lone pair on the nitrogen atom causes the pyramidal shape.

What is the fundamental structural difference between simple molecules and giant covalent structures regarding melting points?

Simple molecules have low melting points due to weak intermolecular forces, whereas giant covalent structures have very high melting points because many strong covalent bonds throughout the lattice must be broken.

Describe the structure and bonding of Diamond.

Each carbon atom is bonded to 4 other carbon atoms in a tetrahedral arrangement, forming a 3D giant lattice.

Why does Graphite conduct electricity while Diamond does not?

In graphite, each carbon atom is bonded to only 3 others, leaving one delocalised electron per carbon atom that is free to move. In diamond, all electrons are fixed in bonds.

Why is Graphite soft and slippery?

Carbon atoms are arranged in flat hexagonal layers held together by weak van der Waals forces, allowing the layers to slide over each other.

What is the structure of Silicon Dioxide ($SiO_2$)?

It is a 3D giant lattice similar to diamond where each silicon atom is bonded to 4 oxygen atoms and each oxygen atom is bonded to 2 silicon atoms.

How does Silicon Dioxide ($SiO_2$) differ from Carbon Dioxide ($CO_2$) in terms of structure?

Carbon dioxide is a simple molecule (a gas), whereas silicon dioxide is a giant covalent structure (a solid like sand or quartz).

What is Graphene?

Graphene is a single layer of graphite that is one atom thick, arranged in a honeycomb lattice.

What are three key properties of Graphene?

It is the strongest material known (200$\times$ stronger than steel by weight), an excellent electrical conductor (electrons move in 2D), and is incredibly light and transparent.

What is the essential IGCSE exam tip for explaining the melting point of giant covalent structures?

Always state that 'many strong covalent bonds must be broken' to explain the high melting point.

According to IGCSE tips, how should lone pairs be represented in dot-and-cross diagrams?

Lone pairs must be shown as two dots outside of the shared area; examiners specifically look for them.