Ionic, Covalent and Metallic Structures

Ionic, Covalent and Metallic Structures

IONIC COVALENT eg sodium chloride (salt) PURE substances have different STRUCTURES depending on the type of BONDING they have METALLIC eg copper SIMPLE eg carbon dioxide, water GIANT eg diamond, graphite The structure of a substance decides what its PHYSICAL PROPERTIES will be.

IONIC Positive sodium ion Na+ Negative chloride ion Cl- Ionic substances are compounds of metals and non-metals (eg sodium chloride, copper oxide, magnesium sulphide etc) They are made of IONS: atoms which have lost or gained electrons giving them a positive or negative CHARGE The + ions and – ions STRONGLY ATTRACT each other to make a regular crystal structure

Strong ionic bond Because of the very STRONG BONDS between the IONS, ionic compounds have HIGH MELTING & BOILING POINTS Sodium chloride melts at over 800°C

H2O MELT DISSOLVE - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + As ionic compounds are made of CHARGED IONS, they can CONDUCT ELECTRICITY but ONLY if the ions can MOVE. If it is MOLTEN the ions can move If it is DISSOLVED the ions can move 800°C 20°C

- - - - - - - + + + + + + + + MOLTEN IONIC COMPOUND MOLTEN ionic compounds CONDUCT ELECTRICITY

H20 molecule Ions free to move around When salt is put in water, H2O molecules pull the ions apart to make a solution. This lets the ions move around.

PURE WATER SOLID SALT SALT SOLUTION DISSOLVED ionic compounds also CONDUCT ELECTRICITY

Ionic Bonding Questions • List two compounds that contain ionic bonds. (Grade D) • Describe three properties of ionic compounds. (Grade C) • Explain why ionic compounds often form giant structures – maybe draw a diagram (Grade B)

H atom O atom Whole thing = H2O molecule SIMPLE MOLECULAR SUBSTANCES These are substances like carbon dioxide CO2, water H2O and methane CH4 which are always made of simple molecules whether they are SOLIDS, LIQUIDS OR GASES

MOLECULES ONLY WEAKLY ATTRACT EACH OTHER VERY STRONG bonds BETWEEN ATOMS (so molecule is very hard to break up) WEAK bonds BETWEEN MOLECULES (so molecules are easy to separate)

This means simple molecular substances have LOW melting and boiling points Liquid oxygen boiling at -183°C Solid oxygen at -240°C

As the bonds between the molecules are weak, simple molecular substances are weak and soft when solid. As the molecules are NOT CHARGED simple molecular substances DON’T CONDUCT ELECTRICITY when solids, liquids or gases.

Carbon atom Only STRONG bonds GIANT MOLECULAR SUBSTANCES In these materials strong covalent bonds join atoms together with other atoms of the same type to make giant structures, rather than little groups. Every C atom joined to 4 others DIAMOND (this is only part of the structure - the same pattern carries on in every direction)

SILICA (Silicon dioxide SiO2) has a similar structure to diamond Every Si atom joined to 4 O atoms Silica is the main substance in ROCKS. Pure silica is called QUARTZ

Because all the atoms in Giant Structures are joined by STRONG BONDS they: • Have HIGH melting / boiling points • Are usually HARD and STRONG Because all the atoms in Giant Structures are UNCHARGED, they will not conduct electricity.

GRAPHITE – a special case WEAK BONDS STRONG BONDS Common form of carbon found in soot, charcoal, pencil leads etc Carbon atoms each joined to 3 others with STRONG bonds to make hexagonal sheets of atoms The sheets of atoms are joined to other sheets by WEAK bonds

As the bonds between the layers of atoms are weak, they can easily slide over each other As the C atoms are only bonded to 3 others, the extra electrons form clouds of ‘free electrons’ between the layers

GRAPHITE - Properties The STRONG BONDS between the ATOMS mean it has HIGH MELTING and BOILING POINTS The WEAK BONDS between the LAYERS mean it is SOFT and SLIPPERY as the layers SLIDE over each other easily (used in pencils and as a solid lubricant) The FREE ELECTRONS between the layers mean that graphite CONDUCTS ELECTRICITY (used as sliding contacts in electric motors)

Covalent Bonding Questions • List one covalent compound that has a simple structure and one that has a giant structure. (Grade C) • Describe two properties of simple covalent compounds and two properties of giant covalent compounds. (Grade B) • Explain why graphite and diamond have different properties (Grade A)

METALS In a metal the atoms are held together by strong bonds in regular structures. This means most metals have high melting and boiling points and are hard and strong

As they have LOST a few electrons, the atoms become POSITIVE IONS Free (“delocalised”) electrons In a metal the atoms LOSE SEVERAL OF THEIR OUTER ELECTRONS which drift around between the metal ions as FREE ELECTRONS.

The large number of free electrons makes all metals are GOOD CONDUCTORS of electricity AND heat. The regular structure means the layers of atoms can fairly easily slide over each other without breaking the bonds (though not as easily as graphite) and so metals are MALLEABLE (bend rather than snap)

Metallic Bonding Questions • List two elements that contain metallic bonding. (Grade D) • Describe three properties elements that contain metallic bonding. (Grade C) • Explain why metallic structures conduct electricity OR are malleable (Grade B)

SUMMARY - Descriptions METALLIC Strong malleable solids Don’t dissolve eg copper IONS ONLY IONIC Crystals Dissolve in water eg sodium chloride (salt) IONS + FREE ELECTRONS SIMPLE Covalent Usually Gases eg CO2, H2O ATOMS joined into GIANT MOLECULES GIANT Covalent Hard strong solids Don’t dissolve eg diamond (graphite – special case) MOLECULES ONLY

SUMMARY - PROPERTIES

Ionic, Covalent and Metallic Structures