Unit 2 - Bonding, Structure and Properties of Matter.

Ionic Bonding

When a metal and a non-metal react together, the metal atom loses electrons to form a positively charged ion and the non-metal gains electrons to form a negatively charged ion. These oppositely charged ions are strongly attracted to one another by electrostatic forces. 

Some examples of ionic bonds include:

  • Magnesium Oxide
  • Sodium Chloride 
  • Magnesium Chloride
  • Sodium Oxide

 

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Ionic Compounds

Ionic Compounds have a structure called a giant ionic lattice. The ions form a closely packed regular arrangement and there are very strong electrostatic forces of attraction between oppositely charged ion, in all directions in the lattice.

All Ionic Compounds have similar properties:

  • high melting and boiling points due to strong bonds between the ions
  • can't conduct electricity when they are solid as there are no free ions to carry the charge
  • some ionic compounds also dissolve in water and the ions seperate meaning they are free to move in the solution so they'll carry the charge

 

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Covalent Bonding

When non-metal atoms bond together, they share pairs of electrons to make covalent bonds. The positively charged nuclei of the bonded atoms are attracted to the shared pair of electrons by electrostatic forces, making cavlent bonds very strong.

  • Atoms only share electrons in their outer shells
  • Each single covalent bond provides one extra shared electron for each atom
  • each atom involved generally makes enough covalent bonds to fill up its outer shell, having a full outer shell gives them the electronic structure of a noble gas, which is very stable 
  • covalent bonding happens in compounds of non-metals and in non-metal elements

 

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Polymers and Giant Covalent Structures

Polymers and gaint covalent structures both have covlent bonds.

In a polymer, lots of small units re linked together to form a long chain molecule that has repeating sections. All atoms are joined by covalent bonds. The intermolecular forces between polymer moleculaes are larger than between simple covalent molecules, so more energy is needed to break them. This means that most polymers re solid at room temperature. The intermolecular forces are still weaker than ionic or covalent bonds so they generally have lower boiling points than ionic or giant molecular compounds.

In a giant covalent structures, all the atoms are bonded to each other by strong covlent bonds. They have very high melting and boiling points as a lot of energy is needed to break the covalent bonds between the atoms. They don't contain charged particles, so they don't conduct electricity even when molten. 

Diamond: Each carbon atoms forms four covalent bonds in a very rigid gaint covalent structure.

Graphite: Each carbon atom form three covalent bonds to create layers of hexagons. Each carbon atom has one delocalised electron.

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Simple Molecular Substances

Simple molecular substances are made up of molecules containing a few atoms joined together by covalent bonds. Some exmples include:

  • Hydrogen - 
  • Chlorine - 
  • Oxygen - 
  • Nitrogen - 
  • Methane - 
  • Water - 
  • Hydrogen Chloride - 

Substances containing covalent bonds usually have simple molecular structures. The atoms within the molecules are held together by very strong covalent bonds. However the forces of attration between these molecules are very weak.

  • low melting and boiling points
  • gases or liquids at room temperature
  • doesn't conduct electricity because they aren't charged and don't have any free electrons or ions

 

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Allotropes of Carbon

Diamond has a giant covalent structure, made up of carbon atoms that each form four covalent bonds. This makes diamond really hard. These strong covalent bonds take a lot of energy to break and give a diamond a very high melting point. It doesn't conduct electricity becasue there are no free electrons or ions.

In Graphite, each carbon atom only forms three bonds creating sheets of carbon atoms arranged in hexagonal layers. However the intermolecular force bewteen layers is very wek which means that  they're free to move past each other. This makes graphite soft and slippery, so it's ideal as a lubricating material. Graphite's got a high melting point and conducts electricity and thermal energy. Graphene is one layer of graphite, it's incredibly light so can be added to composite materials to improve their strength without ding too much weight. 

Fullerenes are molecules of carbon, shaped like closed tubes or hollow balls. They're mainly made up of carbon atoms arranged in hexagons and even pentagons or heptagons. Fulerenes can be used to 'cage' other molecules. This could be used to deliever a drug into the body. They have a large surface area so they could help make great industrial catalysts and also make great lubricants. 

Fullerenes can form nanotubes. The ratio between the length and the diameter of nanotube is high. They can conduct both electricity and thermal energy. As well as having high tensile strength.

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Metallic Bonding

Metallic Bonding involves delocalised electrons. The electrons in the outer shell of the metal atom are delocalised. There are strong forces of electrostatic attraction between the positive ions and the shared negative electrons. These forces of attraction hold the atoms together in a regular sructure and are known as metallic bonding. Metallic Bonding is very strong. Substances that are held together by metallic bonding include metallic elements and alloys.

It's the delocalised electrons in the metallic bonds which produce all the properties of metals.

  • most metals are solids at room temperature
  • metals are good conductors of heat and electricity 
  • most metals are malleable 
  • metals are dense

Alloys are harde then pure metals. Pure metals are arranged in layers which can slide as all the atoms are the same size. Whereas alloys are a mixture of different metals with different sized atoms and are arranged irregulary which means the layers can't slide so are much stronger.

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States of Matter

Material come in three different forms - solid, liquid and gas. Which state something is at a certain temperature depends on how strong the forces of attraction are between the particles of the material. How strong the forces are depends on three things: the material, the temperature and the pressure.

In solids, there are strong forces of attraction between particles whcih holds them together in fixed positions to form a very regular lattice arrangement. The particles don't move from their positions, so all solids keep a definite shape and volume, and don't flow like liquids. The paticles vibrate about their positions.

In liquids, there's a weak force of attraction between the particles. They're randomly arranged and free to move past each other but they tend to stick closely together. Liquids have a definite volume but don't keep a definite shape and will flow to the botttom of the container. The particles are constantly moving with random motion.

In gases, the force of attraction between the particles is very weak so they're free to move and are far apart. The particles in gases travel in straight lines. Gases don't keep a definite shape or volume and will fill any container. The particles constantly moving with random motion.

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Changing States

When a solid is heated, its particles gain more energy. This makes the particles vibrate more which weakens the forces that hold the solid together. At a certain temperature, called the melting point, the paticles have enough energy to break free from their positions. This is called melting and the solid turns into a liquid.

When a liquid is heated again the partciles get more energy enough to make the paticles to move faster which weakens and breaks the bonds holding the liquid together.  At a certain temperature , called the boiling point, the particles have enough energy to break their bonds. This is boiling and the liquid becomes a gas.

As a gas cools the particles no longer have enough energy to overcome the forces of attraction between them. Bonds form between the particles. At the boiling point, so many bonds have been formed between the gas particles that the gas becomes a liquid. This is called condensing.

When a liquid cools, the particles have less energy so move around less. There's not enough energy to overcome the attraction between the particles so more bonds form between them. At the melting point, so many bonds have been formed that the particles are held in place. The liquid becomes a solid.

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Nanoparticles

Nanoparticles have a diameter between 1 nm and 100 nm. These are particles that contain only a few hundred atoms.

Nanoparticles have a large surface area to volume ratio. As particles decrease in size, the size of their surface area increases in relation to their volume. This causes the surface area to volume ratio to increase.  This is an important factor as it can affect the way that a paticle behaves. This can cause the properties of a material to be different depending or whether it's in bulk.

surface area to volume ratio = surace area

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Uses of Nanoparticles

Finding new ways to use nanoparticles is a really important area of scientific research. Here are some uses that have already been developed:

  • They have a huge surface area to volume ratio so they could help make new catalysts.
  • Nanomedicine - the idea that tiny particles such as fullerenes are absorbed more easily by the body than most particles, this means that they could deliever drugs right to the cells where they're needed
  • some nanoparticles conduct electricity so they can be used in tiny electric circuits for computer chips 
  • silver nanoparticles have antibacterial propertie so they can be added to polymer fibres that are then used to make surgical masks and wound dressings and they can also be added to deodorants
  • nanoparticles are also being used in cosmetics for example they're used to improve moisturises without making them really oily 

The effects of nanoparticles on health aren't fully understood. Some people are worried that product containing nanoparticles have been made avaible before they know the long term effects on the body and so products containing them should be labelled clearly.

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