C3 - Structure and Bonding

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States of Matter

Solids

  • Particles are: packed closely together, in a fixed arrangement, and vibrate constantly.

Liquids

  • Particles are: close together in a changing, random arrangement, and they move very quickly

Gases

  • Particles are: much further apart in a random arrangement; they move very quickly.
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Particle Theory

All matter is made up of particles.

  • The particle theory describes the movement and arrangement of particles.
  • The particles are represented by small, solid spheres.

Limitations to the particle model are:

  • It assumes particles are solid spheres with no forces between them

However, particles:

  • can be atoms, ions or molecules
  • vary in size
  • can contain many atoms
  • are not solid or spherical
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Melting Points

A solid turns in to a liquid at its boiling point.

  • As temperature increases, the particles in a solid vibrate faster.
  • The temperature at which enough energy is transferred to a solid for the forces between its particles to break is its boiling point.
  • As the particles break away from their fixed position and start to move around, the particles become liquid state.
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Boiling Points

A liquid turns to a gas at its boiling point.

  • As temperature increases, the particles in a liquid move faster.
  • As more energy is transferred to the subtsance, some of its particles evaporate before the boiling point is reached.
  • The boiling point is the temperature at which bubbles of gas form within a liquid and rise freely to the surface.
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States of Matter 2

Substances with higher melting points and boiling points have stronger forces between their particles.

Each change of state is reversible.

  • Gases condense to form liquids
  • Liquids freeze to form solids

Under certain conditions, some solids turn straight into a gas when heated. This process is called sublimation.

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Covalent Bonding & Giant Covalent Structures

Covalent bonds are formed when atoms of non-metals share pairs of electrons with eachother to stabilise their electonic structures.

Many substances containing covalent bonds consist of simple molecules, but some have giant covalent structures.

  • Every atom is joined to other atoms by strong covalent bonds.
  • These substances have very high melting and boiling points.
  • E.g. diamond
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Ionic Bonding & Compounds

Ionic compounds are formed when metals react with non-metals.

  • The oppositely charged ions formed are held together by strong forces of attraction, which act in all directions.
  • Ionic bonds form a giant lattice.

Ionic compounds have high melting points:

  • It takes a lot of energy to break the many strong ionic bonds, operating in all directions, that hold a giant ionic lattice together
  • All ionic compounds are solid at room temperature

Ionic compounds will conduct electricity when molten or dissolved in water:

  • water molecules can split up the lattice;
  • their ions become mobile and can carry charge through the liquid
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Intermolecular Forces

Intermolecular forces: attraction between the individual molecules in a covalently bonded substances.

Intermolecular forces increase with the size of the molecules:

  • so larger molecules have higher melting and boiling points.
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Polymers

Polymers have very large molecules. They are made up of many small molecules that are covalently bonded to each other to form long chains.

Polymers are solids at room temperature because their intermolecular forces are relatively strong.

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Simple Molecules Summary

Simple molecules:

  • do not conduct electricity - there is no overall charge on the simple molecules in a compound (e.g. sucrose)
  • have low melting and boiling points
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Diamond

Diamond is a form of carbon with a giant covalent structure.

Every carbon atom forms strong covalent bonds with four other carbon atoms. 

Diamond:

  • is very hard
  • has high melting and boiling point
  • does not conduct electricity
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Graphite

In graphite, each carbon atom forms three strong covalent bonds with other carbon atoms. 

They form hexagonal rings, which are arranged in giant layers

  • There are no covalent bonds between the layers. 
  • Between the layers there are only weak intermolecular forces, so the layers can slide over each other quite easily; 
  • this makes graphite soft and slippery.

Graphite conducts electricity; one electron from each carbon atom is delocalised. 

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Fullerenes

Fullerenes have strutures where carbon atoms join together to make large hollow shapes. 

The structure of fullerenes is based of hexagonal rings of carbon atoms. However, fullerenes can also have pentagonal or heptagonal rings of carbon atoms. 

Fullernes have important uses such as,

  • drug delivery into the body 
  • catalysts
  • lubricants

Cylindrical fullerenes called carbon nanotubes can also be produced. The nanotubes have a very high length to diamteter ratio

Carbon nanotubes have very useful properties, such as high tensile strength that makes them useful to reinforce composite materials for making tennis rackets. They have delocalised electrons, giving high electrical conductivity, so they are used in the electronics industry.  

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Graphene

Graphene is a single layer of graphite. It is a layer of hexagonal rings, one carbon atom thick. 

Graphene is useful in electronics and composites because: 

  • its an excellent conductor of electricity
  • it has a very low density
  • it is incredibly strong for its mass
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Metallic Bonding & Giant Metallic Structures

The atoms in a metallic element are all the same size. They form giant structures in which layers of atoms are arranged in regular patterns

When metal atoms pack together, the electrons in the highest energy level delocalise and can move freely between atoms. 

This produces a lattice of positive ions in a 'sea' of moving electrons. 

The delocalised electrons strongly attract the positive ions and hold the giant structure together. 

Pure metals can be bent and shaped because the layers of positively charged ions in a giant metallic structure can slide over each other. 

Alloys are harder than pure metals because the regular layers in a pure metal are distorted by atoms of different sizes in an alloy. 

Delocalised electrons in metals enable electricity and thermal energy to be transferred through a metal easily. 

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