Classifying Materials

The Periodic Tables and Electron Shells

The periodic table is a table of all the known elements.

It contains all the known elements in order of ascending atomic number and arranged into groups with similar properties. 

Elements in a group have the same number of outer electrons.

  • The elements in each group aall have the same number of electrons in their outer shells e.g. Group 1 has one outer electron, Group 2 has two outer electrons. 
  • This is why they have similar properties, and why they are arranged in this way. 
  • When only half the elemnts were known, the periodic table was made by looking at the properties of the elements and arranging them into groups. These are the same groups we use today. 
  • The properties of the elements are decided on how many electrons they have. Atomic number is very significant because it is equal to the number of electrons each atom has. 
1 of 13

The Periodic Tables and Electron Shells

Electron Shells are what chemistry is all about

The fact that electrons form shells around atoms is the basis for the whole of chemistry. If they were just in a cloud around the nucleus, there would be no chemical reactions. However they do form shells and the electron arrangement in an atom determines its chemical behaviour.

Electrons always follow the same pattern when filling shells

Electron shell rules:

  • Electrons always occupy shells.
  • The lowest energy levels are always filled first- these are the ones closest to the nucleus.
  • Only a certain number of electrons are allowed in each shell: 1-2, 2-8, 2-8.
  • Atoms are much stable when they have full electron shells such as the noble gases.
  • In most atoms the outer shell is not full and this makes the atom want to react to fill it.
2 of 13

Electron Shells

Electron configurations are not hard to work out:

For nitrogen:

  • The periodic table tells you nitrogen has seven electrons.
  • The first shell can only take two electrons and the second shell can take a maximum of 8 electrons.
  • So the electron configuration must be 2,5.

3 of 13

Ionic Bonding

A shell with just one electron is not stable

  • All the atoms on the left hand side of the periodic table have just one or two electrons in their outer shell.
  • They want to get rid of these electrons because they will only have full shells left, so they will be stable.
  • When they get rid of these electrons they are called ions.
  • Ions want other ions with an opposite charge.

A nearly full outer shell wants electrons

  • On the other side of the periodic table, the elements in Group 6 and 7, have outer shells which are nearly full. 
  • They want to gain extra electrons to fill the shell up so they can become stable. 
  • When the do, they become ions and they can latch on to the ion that gave up electrons. 
4 of 13

Ionic Bonding

Giant ionic stuctures have a high melting point

  • Ionic bonds always produce giant ionic structures.
  • The ions form a closely packed regular lattic arrangement.
  • There are very strong chemical bonds between all the ions.
  • A single crytstal of salt is one giant ionic lattice, with is why they tend to be cuboid in shape.
  • They have a high melting and boiling point.This is due to the very strong chemical bonds between all the ions in a giant structure.
  • The dissolve to from solutions that conduct electricity. When dissolved, the ions seperate and are all free to move in the solution. These free-moving charged particles allow the solution to carry electrical current.
  • They conduct electricity when molten. When the substance melts, the ions are free to move and so they carry electric current.
5 of 13

Ions and Formulas

Groups 1+2 and 6+7 are the most likely to form ions

  • Remember, atoms that have lost or gained an electron are ions.
  • The elements that most readily form ions are those in Groups 1, 2, 6 and 7.
  • Group 1 and 2 elements are metals and they lose electrons to form +ve ions or cations.
  • Group 6 and 7 elements are non-metals. They gain electrons to form -ve ions or anions.
  • When cations react with anions they form ionic bonds.
  • Only elements at opposite sides of the periodic table will form ionic bonds where one of them becomes a cation and one becomes an anion.
6 of 13

Covalent Bonding

Covalent bonding is sharing electrons

  • Sometimes atoms want to make covalent bonds by sharing electrons with other atoms.
  • This way both atoms feel that they have a full outer shell, which makes them stable.
  • Each covalent bond provides one extra shared electron for each atom.
  • Each atom involved has to make enough covalent bonds to fill up its outer shell.

Example: Hyrdogen atoms only have one electron. They only need one more electron to fill up their shell, so they often from single covalent bonds to achieve this.

7 of 13

Covalent Bonding

Covalent bonding is sharing electrons

  • Sometimes atoms want to make covalent bonds by sharing electrons with other atoms.
  • This way both atoms feel that they have a full outer shell, which makes them stable.
  • Each covalent bond provides one extra shared electron for each atom.
  • Each atom involved has to make enough covalent bonds to fill up its outer shell.

Example: Hyrdogen atoms only have one electron. They only need one more electron to fill up their shell, so they often from single covalent bonds to achieve this.

8 of 13

Giant Covalent Strutures

Giant Covalent Structures

  • These are similar to ionic structures except there are no charged ions.
  • All the atoms are bonded to each other by strong covalent bonds.
  • They have a very high meltion and boiling point.
  • They don't conduct electricity- not even when molten (except graphite).
  • They're usually insoluble in water.
  • Important examples are diamond and graphite which are bothe made only from carbon atoms.

Diamond:

  • Diamonds are sparkly, colourless and clear. 
  • Each carbon atom forms four covalent bonds in a very rigid giant covalent structure, which makes diamond the hardest natural substance. This makes it ideal for cutting tools. 
  • All thos strong covalent bonds give diamond a very high melting point. 
  • It doesn't conduct electricity because it has no free electrons.
9 of 13

Simple Molecular Covalent Structure

Simple molecular substances

  • The atoms form very strong covalent bonds to form small molecules of two or more atoms.
  • By contrast, the forces of attraction between these molecules are very weak.
  • The result of these inter-molecular forces is that the melting and boiling points are very low, because the molecules are easily parted from each other.
  • Most molecular substances don't conduct electricity, simply because there are no ions.
  • Molecular substanes are gases or liquids at room temperature.
  • Most molecular substances don't conduct electricity, simply beacause there are no ions.
  • You can usually tell a molecualr substance from its physical state.

(http://1.bp.blogspot.com/-Rvvxk2vun60/VW9pGYDvzlI/AAAAAAAAASA/7DWXoNhU4oM/s1600/bondin4ytyy.gif)

10 of 13

Atomic Structure

The Nucleus 

  • In the middle of the atom. Contains protons and nuetrons.
  • Has an overall positive charge.
  • Almost the whole mass of the atom is in the nucleus.

Electrons

  • Move around the nucleus in energy levels called shells.
  • Have a negative charge.
  • Tiny compared to the nucleus but they cover a lot of space as tehy move around.

Number of protons= number of electrons: Neutral atoms have no charge overall. 

11 of 13

States of Matter

Solids

Strong forces of attraction between particles. Holds up them in fixed positions in a regular lattic arrangement.The particles don't move from their postitons. Solids keep a definite shape and volume. Particles vibrate about their positions- the hotter the solid becomes, the more they vibrate.

Liquids

Some force of attraction between particles. Free to move, but tend to stick together. Don't have a definite shape and can fill a container. Particles constantly moving with random motion. When heated they move faster so liquids expand slightly when heated.

Gases

Nearly no force of attraction between particles. Travel in straight lines- only interact when they collide. No definite shape or volume. Will fill any container. When particles bounce off the walls of the container they exert pressure on the container. Particles move constantly with random motion. Gases either expand when heated or the pressure increases.

12 of 13

Elements, Compounds and Mixtures

Elements consist of one type of atom only

Copper, aluminium, iron, oxygen and nitrogen are all elements.

Compounds are chemically bonded

A compund is a substance that is made of two or more different elements which are chemically bonded together.

  • For example, carbon dioxide is a compound formed from a chemical reaction. One carbon atom reacts with two oxygen atoms to form a molecule of carbon dioxide, with the formula C02.
  • It's almost impossible to seperate the original elements.
  • Properties of compounds are usually different to the original elements.

Mixtures are easily seperated- not like compounds

  • Unlike in a compund, theres no bonds between the different parts of a mixture.
  • Air is a mixture of gases, mainly nitrogen, oxygen, carbon dioxide and argon. The gases can all be seperated out easily.
13 of 13

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Classifying Materials resources »