Structure & Bonding (AQA GCSE Combined Science: Synergy: Physical Sciences): Flashcards

In a dot and cross diagram for an ionic compound, each ion is shown inside square brackets, with the charge written in superscript outside at the top right.

A cation is a positively charged ion formed when an atom loses electrons. Metals typically form cations. For example, Na⁺ is formed when sodium loses one electron.

True.

Group 7 non-metals gain one electron to achieve a full outer shell, forming ions with a 1− charge (e.g. Cl^-).

Oppositely charged ions are held together by strong electrostatic forces of attraction, which act in all directions throughout the ionic lattice.

True.

When Group 1 metals lose one electron and Group 7 non-metals gain one electron, the resulting ions both achieve a full outer shell that matches the electronic structure of a noble gas (Group 0).

The formula is MgCl₂.

Magnesium (Group 2) forms a Mg²⁺ ion; chlorine (Group 7) forms a Cl^- ion. Two chloride ions are needed to balance the 2+ charge.

A giant ionic lattice is the regular structure of an ionic compound, consisting of large numbers of oppositely charged ions held together by strong electrostatic forces of attraction acting in all directions.

Ionic compounds have high melting and boiling points because they have a giant ionic lattice structure with strong electrostatic forces of attraction between oppositely charged ions in all directions. A large amount of energy is needed to break these forces.

False.

Ionic compounds are poor conductors in the solid state because the ions are in fixed positions in the lattice and cannot move to carry a charge.

Ionic compounds conduct electricity when molten or dissolved in water because the ions are then free to move and carry a charge.

The greater the charge on the ions, the stronger the electrostatic forces between them, and therefore the higher the melting point. For example, MgO (Mg²⁺ and O^2-) has a higher melting point than NaCl (Na⁺ and Cl^-).

False.

It is the ions (not electrons) that move and carry the charge when an ionic compound is molten or dissolved in water.

The electrostatic force of attraction is the force that holds oppositely charged particles together. In ionic compounds, it acts between positive metal ions and negative non-metal ions in all directions throughout the lattice.

The statement is incorrect. Ionic compounds conduct electricity because free ions (not electrons) are able to move and carry a charge. This only happens when the compound is molten or dissolved in water.

A covalent bond is formed when two non-metal atoms share a pair of electrons. The bond arises from the electrostatic attraction between the shared electrons and the positive nuclei of both atoms.

A water molecule contains two covalent bonds. Each hydrogen atom shares one pair of electrons with the central oxygen atom, forming two single covalent bonds.

In a dot and cross diagram, the bonding pair of electrons must be drawn inside the overlapping region between the two atoms. Only outer shell electrons should be shown.

The empirical formula shows the simplest whole number ratio of atoms of each element in a compound. It can be deduced from a dot and cross diagram by counting atoms and simplifying the ratio.

True.

Giant covalent structures such as diamond, graphite, and silicon dioxide are always solids at room temperature because all atoms are linked by strong covalent bonds that require a large amount of energy to break.

A dot and cross diagram fails to show the 3D arrangement of atoms and electron shells, and does not indicate the relative sizes of the atoms.

True.

A ball and stick model is useful for illustrating the arrangement of atoms in 3D space and visualising the shape of a molecule. However, it does not show the movement of electrons.

The empirical formula of ethene is CH₂.

The ratio C:H = 2:4, which simplifies to 1:2.

Intermolecular forces are weak forces of attraction that act between molecules in a covalently bonded substance. They are much weaker than the covalent bonds within the molecules.

Simple covalent molecules have low melting and boiling points because only weak intermolecular forces need to be overcome to separate the molecules. The strong covalent bonds within the molecules are not broken.

False.

When a simple covalent substance melts, only the intermolecular forces between molecules are overcome. The covalent bonds within the molecules remain intact.

A giant covalent structure is a substance in which all atoms are linked to other atoms by strong covalent bonds throughout a large network. Examples include diamond, graphite, and silicon dioxide. These substances have very high melting points.

Simple covalent molecules do not conduct electricity because the molecules have no overall electric charge and there are no free ions or electrons to carry a current.

Larger molecules have stronger intermolecular forces between them. More energy is needed to overcome these forces, so they have higher melting and boiling points.

True.

Polymers have very large molecules, so the intermolecular forces between chains are much stronger than in small molecules. More energy is needed to overcome these forces, which is why polymers are solids at room temperature.

Three examples of giant covalent structures are:

1. Diamond

2. Graphite

3. Silicon dioxide

A metallic bond is the strong electrostatic attraction between positively charged metal ions and the surrounding 'sea' of delocalised electrons. This bonding occurs throughout the metallic lattice.

Delocalised electrons are the outer-shell electrons of metal atoms that are no longer attached to any specific atom. They are free to move throughout the entire metallic lattice structure.

False.

In metallic bonding, the outer-shell electrons are delocalised and shared freely throughout the structure. They are not transferred as in ionic bonding.

Metals consist of giant structures of atoms arranged in a regular pattern, with a 'sea' of delocalised electrons between the positive metal ions.

A metallic lattice is the regular arrangement of positive metal ions in a metal structure. The ions are held in place by the attraction of the surrounding delocalised electrons that move freely between them.

The outer-shell electrons become delocalised — they leave their original atoms (which become positive metal ions) and are free to move throughout the entire metallic lattice structure.

False.

Metallic bonding occurs in both metallic elements and alloys (mixtures of metals, or a metal with a non-metal such as carbon).

Malleable describes the property of metals that allows them to be hammered or bent into shape. This is possible because the layers of positive ions can slide over each other when a force is applied.

Most metals have high melting and boiling points because they have a giant structure with strong metallic bonds (strong electrostatic forces between the positive ions and delocalised electrons). A large amount of energy is needed to break these bonds.

True.

In alloys, atoms of different sizes distort the regular layers of the metallic lattice. This prevents the layers from sliding easily over each other, making alloys harder than pure metals.

An alloy is a mixture of two or more metals, or a metal with a non-metal (e.g. steel is iron and carbon). Alloys are often harder, stronger, and more resistant to corrosion than the pure metals they contain.

Metals conduct electricity because the delocalised electrons are free to move through the structure and carry electrical charge.

In a pure metal, the atoms are arranged in layers, which can slide over each other. This is why pure metals can be bent and shaped.

False.

Pure metals are often too soft for many structural uses. Alloys are preferred because the different-sized atoms prevent layers sliding, making alloys much harder than the pure metal.

1. The electrons are delocalised (free to move)

2. They carry electrical charge

3. They move through the metal structure