Chemistry part 1a

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  • Created by: Harriet
  • Created on: 31-03-13 18:16

Atoms

Atoms are made up of protons, neutrons and electrons.

  • A proton has a charge of +1
  • an electron has a charge of -1
  • and a neutron has a charge of 0 - it is neutral

This means all atoms have no overall charge (as 0+1-1=0)

The proton and neutron are found in the middle in the nucleus and the electrons are found around the outside.

Elements are made up of only one type of atom.

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The Periodic Table

Atoms with similar properties form columns, called groups. Horizontal rows are called periods.

All atoms in a group have the same amount of electrons in their outer shell, so have similar properties. This means if you know the properties of one atom, you can predict the properties of other atoms in that group.

For example, the group one elements are all metals and react the same way. They all react with water to form an alkaline solution and hydrogen gas, and they all react with oxygen to form an oxide.

The elements in the final column are called the Noble gases (or group 0) all have a full outer shell. this means they are stable and unreactive.

With each atom symbol there are two numbers:

  • The top one is the mass number. It tells us the numer of protons and neutrons added together.
  • The bottom number is the atomic number. It tells us the number of protons (which is the same as the number of electrons)
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Electron Shells

Electrons always occupy shells.                                                               

You have a maximum of 2 elecrons in the first shell, 8 in the second and 8 in the third.

The shells are filled inside to out and a shell must be full before starting a new one.

Atoms want full outer shells. This means they will react with other atoms to get a full outer shell. Noble gases already have full outer shells, which is why they are very unreactive.

Atoms in group 1 (the alkali metals) are the most reactive as they can easily lose one electron to gain a full outer shell.

Atoms in group 7 (the Halogens) are also very reactive as they only have to gain one electron to get a full outer shell.

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Properties

Compounds are 2 or more atoms that are chemically bonded.

If a substance is:

  • malleable - it can easily be hammered into different shapes
  • ductile - it can be drawn into wires
  • brittle - if it's hit with a hammer at room temperature, it will break

Non metals are brittle, poor conductors of elecricity and have low melting points.

Metals are malleable, most are ductile and have high melting points.

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Chemical Bonds - Ionic

Ionic bonding:

  • happens between metals and non-metals
  • involves a transfer of electon(s)
  • ions that are left are held together by electrostatic (+ attracts -) attraction
  • we say we have formed an ionic lattice e.g a metal
  • a lattice is a regular formation of atoms or ions
  • They form ionic compounds which are crystalline solids
  • Solubility in water is good as they are a polar solvent (the ions in an ionic substance attract to opposite charges in water molecules, therefore dissolving).
  • High melting and boiling points as a lot of ionic bonds must be broken in the ionic lattice which requires a lot of energy.
  • Don't conduct heat or electricity when they are a solid, as the ions can't move. They are good conductors when they are moltern, or in an aqueous solution as the ions can move and pass a charge.
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Chemical Bonds - Covalent

Covalent bonding:

  • formed between non-metals
  • electrons are shared between atoms
  • forms simple and giant covalent strucures

Properties:
Generally they are gases at room temperature, therefore they have low melting and boiling points.
Solubility in water is poor. This is because they have no dipoles (positive and negative ends) and are not made of ions. (Some do have dipoles, so would disolve in water, but there are only a few).
Poor conductors of heat and electricity because they have no free electrons or ions.

They have low meling and boiling points as between the mollecules are weak intermolecular forces (Van der Waaln forces). These are very easy to break with a small amount of heat energy.

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Molecules and Ions

 

In covalent bonding (where non-metlas share electrons) molecules are formed.

 

In ionic bonding (with metals and non-metals transfering electrons) ions are formed.

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Limestone

Limestone is mainly calcium carbonate (CaCO3). It is quarried and used to make cement, concrete and glass.

  • Cement - calcium carbonate and clay (and heat)
  • Mortor - calcium carbonate, clay (cement), sand, water (and heat)
  • Concrete - calcium carbonate, clay (cement), sand, gravel, water (and heat)
  • Glass - calcium carbonate, sand, sodium carbinate
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Calcium Carbonat Reactions

You can use CaCO3 in a series of reaction that can be repeated as they are form a cycle.

1) CaCO3 is heated to thermally decompose it, forming calcium oxide (CaO) and carbon dioxide (CO2). Calcium oxide is also known as quicklime. Thermal decomposition is when one substance chemically changes ino at least two new substances when it heated.

2) When calcium oxide is reacted with water you get calcium hydroxide - Ca(OH)2 - also known as slacked lime or lime water. It is alkaline and so can be used to neutralise acids (such as neutralising acidic soil)

3) When calcium hydroxide is reacted with carbon dioxide it forms calcium carbonate again.

*Other carbonates - such as magnesium, copper and zinc - all thermally decompose in the same way (forming an oxide and CO2), but a bunsen burner can't reach the temperate to thermally decompose some group 1 metals.

 

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Acids and Carbonates

Carbonates react with acids to make a salt, carbon dioxide and water.

 

 

This means that limestone is damaged by acid rain.

Other carbonates that react in the same way are magnesium, copper, zinc and sodium.

The type of salt that is produced depends on the acid. For example a reaction with hydrochloric acid would make a chloride salt.

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