Bonding, Structure & Properties (WJEC GCSE Science (Double Award): Chemistry): Flashcards

In a metal, the outer electrons become delocalised and are free to move between the positive metal ions, forming what is described as a 'sea of electrons'.

Metals are good conductors of electricity because the delocalised electrons are free to move through the structure and carry a charge.

True.

Metals are malleable because the atoms are arranged in layers that can slide over each other when a force is applied, allowing the metal to be bent or hammered into shape.

Most metals have high melting points because there are strong electrostatic forces of attraction between the positive metal ions and the delocalised electrons, and these need lots of energy to be broken.

Going across a period in the Periodic Table, the melting point of metals increases because the number of delocalised electrons increases, strengthening the forces of attraction.

False.

In metallic bonding, electrons are not transferred to another element. Instead, the outer electrons become delocalised and move freely between the positive metal ions within the metal lattice.

Delocalised electrons are electrons that no longer belong to any specific metal atom. They are free to move through the whole metal structure, which allows metals to conduct electricity and heat.

The melting point increases from sodium to magnesium to aluminium because the number of delocalised electrons increases across the period. This means there are greater forces of attraction between the positive ions and delocalised electrons, so more energy is required to overcome them.

An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons. Ions form so that atoms can achieve a full outer shell of electrons.

Negative ions are called anions and form when atoms gain electrons. Positive ions are called cations and form when atoms lose electrons.

True.

Metals in Groups 1 and 2 lose electrons to other atoms to become positively charged cations, giving them a full outer shell of electrons like a noble gas.

A sodium atom loses its one outer shell electron to form a Na⁺ ion. This gives sodium the same electronic configuration as neon and a full outer shell.

In a dot and cross diagram for an ionic compound, square brackets are drawn around each ion and the charge is written at the top right-hand corner.

Oppositely charged ions are held together by strong electrostatic forces of attraction between the positive and negative ions. This is the basis of ionic bonding.

False.

A chlorine atom gains one electron to form a Cl^– ion with a charge of 1–. This gives chlorine a full outer shell of eight electrons.

Group 1 ions lose one electron and Group 7 ions gain one electron so that both achieve a full outer shell of electrons, giving them the same electronic structure as the nearest noble gas.

If a magnesium atom loses 2 electrons, it forms an ion with a charge of 2+ and the symbol Mg²⁺.

A giant ionic lattice is a regular, three-dimensional arrangement of alternating positive and negative ions held together by strong electrostatic forces of attraction acting in all directions.

True.

Ionic compounds have high melting points because the strong electrostatic forces of attraction act in all directions throughout the giant lattice, and lots of energy is needed to overcome them.

In the solid state, ionic compounds cannot conduct electricity because the ions are in fixed positions in the lattice and are unable to move and carry a charge.

When an ionic compound is melted or dissolved in water, the ions are free to move and carry a charge through the liquid, allowing electricity to flow.

False.

In ionic compounds, it is the ions that move and carry a charge, not electrons. This can only happen when the compound is molten or dissolved in water.

In an ionic lattice, positive and negative ions alternate in a regular arrangement and are tightly packed together. Strong electrostatic forces of attraction hold the lattice together in all directions.

Magnesium oxide has a higher melting point than sodium chloride because the Mg²⁺ and O^2– ions have a greater charge than Na⁺ and Cl^–, creating stronger electrostatic forces. (Higher Tier Only)

The greater the charge on the ions, the stronger the electrostatic forces of attraction between them, so more energy is needed to overcome these forces and the melting point is higher.

Electrostatic forces are the strong forces of attraction between oppositely charged ions in an ionic lattice. They act in all directions and are responsible for the high melting points and rigid structure of ionic compounds.

A covalent bond is a shared pair of electrons between two non-metal atoms. Covalent bonds form so that each atom achieves a full outer shell of electrons.

In a dot and cross diagram, the electron shells of each atom overlap and the shared electrons are shown in the area of overlap.

False.

Covalent bonds only form between non-metal atoms. When a metal atom bonds with a non-metal atom, an ionic bond is formed instead.

A double bond forms when two adjacent atoms share two pairs of electrons, creating two covalent bonds between them. Carbon dioxide (CO₂) contains double bonds between the carbon and oxygen atoms.

A water molecule (H₂O) contains two covalent bonds. Each hydrogen atom shares one pair of electrons with the oxygen atom.

Atoms form covalent bonds to achieve a full outer shell of electrons, giving them the same stable electronic configuration as a noble gas.

True.

A nitrogen molecule (N₂) contains a triple bond because the two nitrogen atoms share three pairs of electrons, giving each atom a full outer shell of eight electrons.

A dot and cross diagram shows the electronic configuration of a covalent molecule. Dots represent electrons from one atom and crosses represent electrons from the other, making it clear which atom each electron came from and showing which electrons are shared.

In a covalent bond, if two atoms share two pairs of electrons a double bond is formed, and if they share three pairs a triple bond is formed. (Higher Tier Only)

Intermolecular forces are weak forces of attraction that act between molecules in a simple molecular covalent substance. They are much weaker than the covalent bonds within each molecule.

False.

Simple molecular substances have low melting points because the intermolecular forces between molecules are weak, not because the covalent bonds are weak. The covalent bonds themselves are actually very strong.

In a simple molecular substance, covalent bonds are found between atoms within a molecule and intermolecular forces are found between the molecules.

Simple molecular covalent substances do not conduct electricity because there are no free ions or electrons to move and carry a charge. Most covalent compounds act as insulators.

As the size of molecules in a simple molecular substance increases, the melting and boiling points generally increase because the intermolecular forces become stronger and more energy is needed to break them.

Two examples of simple molecular covalent substances are:

1. Carbon dioxide (CO₂)

2. Methane (CH₄)

False.

When a simple molecular substance melts, it is the intermolecular forces between molecules that are overcome, not the covalent bonds. The covalent bonds within the molecules remain intact.

Plastic coatings are good insulators because they are made of covalent molecules that have no free ions or electrons. This means they cannot carry a charge and do not allow electricity to flow through them.

A giant covalent structure is a structure containing billions of non-metal atoms, each joined to adjacent atoms by strong covalent bonds. Diamond and graphite are both examples.

In diamond, each carbon atom is bonded to four other carbon atoms, forming a tetrahedral shape.

False.

Diamond cannot conduct electricity because all the outer shell electrons of each carbon atom are held in the four covalent bonds, leaving no free electrons to carry a charge.

Diamond is extremely hard because each carbon atom is bonded to four other carbon atoms by strong covalent bonds, creating a rigid three-dimensional network that is very difficult to break.

In graphite, each carbon atom is bonded to three other carbon atoms, leaving one free electron per atom that becomes delocalised.

Graphite is slippery because it is arranged in layers held together by weak intermolecular forces. These layers can slide over each other easily, which is why graphite is used in pencils and as a lubricant.

True.

Graphite can conduct electricity because each carbon atom is bonded to only three others, leaving one delocalised electron per atom that is free to move through the structure and carry a charge.

Both diamond and graphite have very high melting points because they have giant covalent structures with strong covalent bonds between the carbon atoms. These bonds need a lot of energy to break.

In graphite, the layers are held together by weak intermolecular forces, but the carbon atoms within each layer are joined by strong covalent bonds.

Fullerenes are carbon-based structures that form hollow, cage-like shapes. They consist of carbon atoms bonded in arrangements of hexagons and pentagons.

The formula of buckminsterfullerene is C₆₀ and it consists of 60 carbon atoms arranged in 20 hexagons and 12 pentagons.

True.

Fullerenes can trap other molecules inside their hollow cage-like structure, making them useful for targeted drug delivery systems.

Fullerenes are useful as lubricants because they have weak intermolecular forces between each fullerene molecule, allowing them to slide past each other easily.

Fullerenes have a huge surface area which makes them useful for trapping catalyst molecules on their surfaces to speed up chemical reactions.

Buckminsterfullerene (C₆₀) has a spherical shape, similar to a soccer ball. It contains 60 carbon atoms arranged in 20 hexagons and 12 pentagons.

False.

Fullerenes are used in drug delivery because of their hollow shape, which allows them to trap and carry other molecules. It is their large surface area that makes them useful for catalysis.

Graphene is a single layer of graphite consisting of carbon atoms covalently bonded in a continuous hexagonal arrangement. It is essentially a two-dimensional molecule as it is only one atom thick.

True.

Graphene can conduct both electricity and heat because it has delocalised electrons that are free to move through the structure and carry a charge.

Carbon nanotubes are produced by rolling sheets of graphene into a cylindrical shape.

Carbon nanotubes have a high melting point because there are strong covalent bonds between the carbon atoms that require a lot of energy to overcome.

Carbon nanotubes can conduct electricity because each carbon atom is bonded to three other carbon atoms, leaving one delocalised electron per atom that is free to move and carry a charge.

False.

Carbon fibres, not carbon nanotubes, are used in bikes and tennis rackets. Carbon nanotubes have a much smaller scale structure than carbon fibres.

Two properties of graphene that make it useful are:

1. It is extremely strong yet very light due to the strong covalent bonds in its unbroken pattern.

2. It conducts electricity due to its delocalised electrons.

Carbon nanotubes have a low density because they are hollow cylindrical structures with a very small diameter. The strong covalent bonds between carbon atoms give them high strength, but the hollow interior means little mass per unit length.

Bulk properties are the physical characteristics of a substance — such as melting point, electrical conductivity, flexibility and hardness — that arise from the type, strength and arrangement of bonds between many atoms. Individual atoms do not possess these properties.

Diamond is extremely hard because each carbon atom is bonded to four other carbon atoms by strong covalent bonds.

True.

Diamond, graphite, fullerenes and graphene all contain only carbon atoms, but their different bonding arrangements give each substance different bulk properties such as hardness, conductivity and melting point.

Graphite is soft and slippery because it is arranged in layers held together by weak intermolecular forces that allow the layers to slide over each other. Diamond is hard because each carbon is bonded to four others by strong covalent bonds in a rigid three-dimensional network.

Bulk silver is a shiny, malleable metal used mainly in jewellery, whereas silver nanoparticles have antibacterial properties due to their high surface area to volume ratio.

Three bulk properties of graphene are:

1. Extremely strong

2. Can conduct electricity and heat

3. Almost transparent

False.

Individual atoms do not possess bulk properties. The hardness of diamond arises from the type and strength of the covalent bonds and the arrangement of many carbon atoms, not from individual atoms.

The bulk properties of a substance are determined by the type and strength of bonds within the substance and how the atoms are arranged. Different arrangements of the same atoms can produce very different bulk properties.

Nanoparticles are particles between 1 and 100 nanometres in size that usually contain only a few hundred atoms. The study of nanoparticles is called nanoscience.

As particles decrease in size, their surface area to volume ratio increases, which is why nanoparticles may have different properties from the same material in bulk.

True.

Silver nanoparticles have antibacterial properties that kill bacteria on the skin. Because body odour is caused by bacteria, silver nanoparticles in sports clothing help to reduce it.

Nanoparticles of titanium dioxide break down dirt on the window surface when UV light from the sun shines on them. They also cause water to spread out in a thin film rather than forming droplets, which helps to rinse the window clean.

Titanium dioxide nanoparticles are used in transparent sunscreens because they can reflect UV light whilst allowing visible light to pass through.

There is concern that nanoparticles are so small they could potentially enter living cells or be absorbed into the bloodstream through the skin. Although no serious short-term side effects have been found, possible long-term effects are not yet fully understood. (Higher Tier Only)

True.

Silver nanoparticles have antibacterial properties. When used as a coating inside fridges, they kill bacteria on surfaces, preventing the odours that bacteria produce.

Nanoparticles have a very high surface area to volume ratio compared to bulk material. This means more atoms are at the surface and can interact with the surroundings, which can give nanoparticles different and more reactive properties.

The use of nanoparticles in science is in its early stages, so there are many unknown factors and potential risks that are not yet fully understood. (Higher Tier Only)

Smart materials are materials that can have their properties changed in a reversible way in response to a change in conditions such as temperature, light or pressure.

Thermochromic pigments change colour as the temperature changes. They are used in colour-changing mugs and strip thermometers.

True.

Photochromic lenses darken when exposed to UV light because they are made from a photochromic pigment that changes colour depending on light intensity. This allows the glasses to act as sunglasses in sunlight.

Polymer gels are smart materials that can absorb up to 1000 times their own volume in water by joining water molecules weakly to the polymer chains. This property makes them useful in nappies and in granules added to soil to help plants retain water.

Shape-memory alloys revert to their original shape when heated. They are used in surgical plates for joining bone fractures because the warmth of the body causes the alloy to apply tension which aids healing.

False.

Shape-memory polymers can be bent out of their original shape but revert back when heated. This reversible behaviour is what makes them smart materials and useful in applications such as car bumpers and gum-shields.

Photochromic pigments change colour in response to changes in light intensity, whereas thermochromic pigments change colour in response to changes in temperature.

Two uses of shape-memory polymers are:

1. Car bumpers — which can return to their original shape after a collision when heated

2. Gum-shields and helmets — which can be moulded to shape and reform when heated