Bonding & structure flashcards
13 cards on Bonding & structure: the key points, worked examples and the slips to watch. Swipe through them, or print them.
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Bonding & structure
Ionic bonding
What happens to the electrons when a metal reacts with a non-metal, and what holds the ions together?
Answer it in your head, then turn the card over.
Ionic bonding
Key points
- Electrons transfer: the metal atom loses its outer electrons and the non-metal atom gains them.
- The metal forms a positive ion and the non-metal a negative ion, each with a full outer shell.
- An ionic bond is the strong electrostatic attraction between oppositely charged ions.
- Dot-and-cross diagrams show the transfer: the ions in square brackets, the charge outside.
Sodium and chlorine
Na (2, 8, 1) gives its outer electron to Cl (2, 8, 7).
Na⁺ (2, 8) and Cl⁻ (2, 8, 8) form, and attract each other.
Watch out
Ions form by moving electrons only: sodium becomes Na⁺ by losing an electron, never by gaining a proton.
Bonding & structure
Working out an ionic formula
How do you work out the formula of an ionic compound from the charges on its ions?
Answer it in your head, then turn the card over.
Working out an ionic formula
Key points
- Group 1 metals form 1+ ions, Group 2 form 2+, Group 6 non-metals form 2− and Group 7 form 1−.
- Use enough of each ion for the charges to cancel out to zero.
- Write the metal first, with the number of each ion as a small number after it.
Magnesium chloride
Mg²⁺ has a charge of +2 and Cl⁻ a charge of −1.
One Mg²⁺ needs two Cl⁻: +2 − 2 = 0, so the formula is MgCl₂
Watch out
The charges are not written in the formula: it is MgCl₂, not Mg²⁺Cl₂⁻.
Bonding & structure
Giant ionic lattices
What is the structure of an ionic compound such as sodium chloride, and why does it melt at a high temperature?
Answer it in your head, then turn the card over.
Giant ionic lattices
Key points
- It is a giant lattice: a regular, repeating arrangement of positive and negative ions.
- Strong electrostatic forces act between the ions in all directions.
- Melting means overcoming a great many of these forces, which takes a lot of energy, so the melting point is high.
- There are no molecules in sodium chloride, only ions.
Watch out
Write about ions and the forces between them: "sodium chloride molecules" do not exist.
Bonding & structure
Covalent bonds and simple molecules
What is a covalent bond, and why do simple molecules such as water melt and boil at low temperatures?
Answer it in your head, then turn the card over.
Covalent bonds and simple molecules
Key points
- A covalent bond is a shared pair of electrons between two non-metal atoms.
- Examples of simple molecules: H₂, Cl₂, O₂, HCl, H₂O, NH₃ and CH₄.
- The bonds inside each molecule are strong, but the intermolecular forces between molecules are weak.
- Melting and boiling only overcome those weak forces, so it takes little energy. Bigger molecules have stronger forces.
- They do not conduct: the molecules have no overall charge.
Watch out
When water boils, its covalent bonds do not break: whole H₂O molecules just move apart.
Bonding & structure
Diamond and graphite
How are the carbon atoms bonded in diamond and in graphite, and how does that explain their properties?
Answer it in your head, then turn the card over.
Diamond and graphite
Key points
- Both are giant covalent structures of carbon: allotropes, with very high melting points.
- Diamond: each carbon forms four strong covalent bonds, so it is very hard.
- Graphite: each carbon forms three bonds, in layers of hexagons with no covalent bonds between the layers.
- The layers slide over each other, so graphite is soft and slippery.
- Silicon dioxide (sand) is another giant covalent structure.
Watch out
Graphite's layers are not joined by weak covalent bonds: there are no covalent bonds between them at all.
Bonding & structure
Graphene and fullerenes
What are graphene and fullerenes, and what are they used for?
Answer it in your head, then turn the card over.
Graphene and fullerenes
Key points
- Graphene is a single layer of graphite, one atom thick. It is very strong and conducts electricity.
- It is useful in electronics and in composite materials.
- Fullerenes are carbon molecules with hollow shapes, built from rings of carbon atoms. The first found was C₆₀.
- Carbon nanotubes are cylindrical fullerenes: very long and thin, strong, and good conductors.
Watch out
Fullerenes are molecules with a fixed number of atoms, such as C₆₀, unlike the giant structures of diamond and graphite.
Bonding & structure
Polymers
What is a polymer, and why are most polymers solid at room temperature?
Answer it in your head, then turn the card over.
Polymers
Key points
- A polymer is a very large molecule made of many small repeating units, called monomers.
- The atoms are joined by strong covalent bonds.
- The molecules are so long that the forces between them are fairly strong.
- So most polymers are solids at room temperature, unlike small simple molecules.
Watch out
Polymers are solid because the forces between their long molecules are fairly strong: the covalent bonds are inside each molecule.
Bonding & structure
Metallic bonding
How are the atoms in a metal held together, and why can a pure metal be bent?
Answer it in your head, then turn the card over.
Metallic bonding
Key points
- A metal is a giant structure of positive ions in a regular arrangement.
- Their outer electrons are delocalised: a sea of electrons free to move through the whole structure.
- Metallic bonding is the strong attraction between the positive ions and the delocalised electrons, so most metals have high melting points.
- The layers of ions can slide over each other, so pure metals can be bent and shaped.
Watch out
The sea is of electrons, not of positive ions: it is the ions that sit in a fixed arrangement.
Bonding & structure
Alloys
What is an alloy, and why is it harder than the pure metal?
Answer it in your head, then turn the card over.
Alloys
Key points
- An alloy is a mixture of a metal with other elements, such as steel (iron with carbon).
- Pure metals are often too soft to use.
- Atoms of a different size distort the layers in the structure.
- So the layers cannot slide over each other as easily, and the alloy is harder.
Watch out
Alloys are harder because the layers cannot slide, not because their atoms are bonded more strongly.
Bonding & structure
Solids, liquids and gases
What does particle theory say about the arrangement and movement of particles in a solid, a liquid and a gas?
Answer it in your head, then turn the card over.
Solids, liquids and gases
Key points
- Solid: close together in a regular pattern, vibrating about fixed positions.
- Liquid: close together but random, moving past each other.
- Gas: far apart and random, moving quickly in all directions.
- State symbols: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water.
Watch out
Particles in a liquid are almost as close as in a solid, not spread out: that is why liquids are hard to squash.
Bonding & structure
Changes of state
What happens to the particles when a substance melts or boils, and how do you tell its state at a given temperature?
Answer it in your head, then turn the card over.
Changes of state
Key points
- Heating gives the particles energy to overcome the forces between them: melting, then boiling.
- Cooling reverses it: condensing, then freezing.
- The stronger the forces between the particles, the higher the melting and boiling points.
- Below the melting point it is solid, between the two it is liquid, above the boiling point it is gas.
Melts at −7 °C, boils at 59 °C
At 20 °C: above −7 °C, so it has melted.
Below 59 °C, so it has not boiled: it is a liquid.
Watch out
The particles themselves never melt or get bigger: only their energy and arrangement change.
Bonding & structure
Which substances conduct, and why
Which kinds of substance conduct electricity, and what carries the charge in each?
Answer it in your head, then turn the card over.
Which substances conduct, and why
Key points
- A current needs charged particles that are free to move.
- Metals conduct as solids and liquids: delocalised electrons carry the charge. They conduct heat this way too.
- Graphite conducts: each carbon has one delocalised electron. Diamond does not: all four are used in bonds.
- Ionic compounds conduct only when molten or dissolved, when the ions are free to move.
- Simple molecules do not conduct: they have no charged particles.
Watch out
In molten sodium chloride the ions carry the charge, not electrons, and solid sodium chloride does not conduct.
Bonding & structure
Identifying a structure from its properties
How can melting point and conductivity tell you what kind of structure a substance has?
Answer it in your head, then turn the card over.
Identifying a structure from its properties
Key points
- Low melting point, no conduction: simple molecules.
- High melting point, conducts only when molten or dissolved: ionic.
- Very high melting point, does not conduct: giant covalent, such as diamond or silicon dioxide.
- High melting point, conducts as a solid, can be bent: metallic.
Watch out
Graphite is the exception: giant covalent, yet it conducts as a solid.
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Put it into practice
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