The Structure of Substances

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Ionic Compounds

Ionic compounds are giant structures of ions. They're held together by strong forces of attraction between oppositely charged ions, that act in all directions. This type of bonding called - ionic bonding

Ionic compounds:

  • have high melting and boiling points
  • conduct electricity when molten or in solution because the charged ions are free to move about and carry the current
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The Ionic Bond

An ionic bond occurs between a metal & a non-metal. Involves a transfer of electrons from one atom to another.

This forms electrically charged ions, each has a complete outer energy level

Ions have the electronic structure of a noble gas.

  • Atoms that lose electrons become positively charged ions
  • Atoms that gain electrons become negatively charged ions
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Alkali Metals and Halogens

The alkali metals (group 1):

  • have one electron in their outermost shell
  • react with non-metal elements to form ionic compounds where the metal ion has single positive charge

The halogens (group 7):

  • have seven electrons in outer shell
  • react with alkali metals to form ionic compounds where the halide ions have a single negative charge
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Mixtures and Compounds

Mixture consists of 2 or more elements/compounds that aren't chemically combined. Properties of the substances remain unchanged & specific to that substance. Compounds are substances in which the atoms of two or more elements are chemically combined (not just mixed together)

Atoms can form chemical bonds by:

  • sharing electrons (covalent bonds)
  • gaining or losing electrons (ionic bonds)

When atoms form chemical bonds, the arrangements of outer shell of electrons changes - each atom getting a complete outer shell of electrons.

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The Covalent Bond

Covalent bond occurs between non-metal atoms. Strong bond formed when pairs of electrons are shared. Some covalently bonded substances have simple strucutres, others have giant covalent structures, called macromolecules.

Atoms that share electrons usually have low melting and boiling points, because they often form molecules in which there are:

  • strong covalent bonds between the atoms
  • weak forces of attraction between the molecules

These forces are very weak compared to strength of covalent bonds

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Giant Covalent Structures

All atoms in giant covalent structures r linked by strong covalent bonds - have high melting points.

Diamond, form of carbon that has a giant, rigid covalent structure. Each carbon atom forms 4 covalent bonds w other carbon atoms. Diamond has a large number of covalent bonds - hard substaance.

Graphite, form of carbon also has giant covalent structure. In graphite, each carbon atom forms 3 covalent bonds w other carbon atoms in layered structure. Graphite has layers - can slide past each other - so soft & slippery.

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Fullerenes and Metals

Carbon can form molecules- fullerenes, have different numbers of carbon atoms. Structure of fullerenes based on hexagon rings of carbon atoms. Fullerenes can b used to deliver drugs in the body, in lubricants, as catalysts, & in nanotubes for reinforcing materials.

The layers of atoms in metals are able to slide past each other - metals can be bent and shaped.

Metals have a giant structure, electrons in highest enegy level can be delocalised, producing a regular arrangement of positive ions that are held together by electrons using electrostatic attraction. Delocalised electrons can move around freely - allowing metals to conduct heat & electricity

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Alloys and Shape Memory Alloys

Alloy - mixture containing a metal & at least one other element. The added element disturbs regular arrangement of the metal atoms so layers don't slide over each other so easily. So alloys usually stronger and harder than pure metal.

Shape Memory Alloys

Smart alloys belong to group of materials that are being developed to meet demands of modern engineering & manufacturing. These materials respond to changes in their environment. Smart alloys remember their shape, can be deformed but will return to original shape.

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Properties of Polymers

Properties of polymers depend on:

  • what they're made from
  • the conditions under which they're made

For example, low density poly(ethene) and high density poly(ethene) are both made from the monomer ethene. But the polymers have different properties because different catalysts and reaction conditions are used to make them.

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Thermo-softening and Thermo-setting Polymers

Thermo-softening polymers consist of individual polymer chains that are tangled together.

There are weak intermolecular forces between all of the polymer chains in a thermo-softening polymer.

Thermo-setting polymers consist of polymer chains that are joined together by cross-links between them. These polymers don't melt when they're heated.

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Nanoparticles and Nanostructure

Nanoscience - study of v. small structures.

Nanoparticles are tiny particles that can combine to form structures - nanostructures. Nanostructures can be manipulated so materials can be developed that have new & specific properties. The properties of nanoparticles are different to the properties of the same materials in bulk. For example:

  • nanoparticles more sensitive to light, heat & magnetism
  • nanoparticles have a high surface area in relation to their volume
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