Structure and Bonding

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Structure and Bonding

Giant Covalent Structures

Graphite Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity. Diamond Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity. Silica Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms. The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica. Buckminster fullerene Buckminster fullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.

Properties of giant covalent structures Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC. Variable conductivity - Diamond does not conduct electricity. Graphite contains free electrons, so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.

Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes of carbon, and of silica (silicon dioxide).

Atoms into ions

Forming ions Ions are charged particles formed when atoms, or groups of atoms, lose or gain electrons to obtain full outer shells. Ions form when a metal reacts with a non-metal. In the reaction: the metal atoms lose electrons to form positively charged ions the non-metal atoms gain electrons to form negatively charged ions The outer electron from a sodium atom transfers to the outer shell of a chlorine atom.An electron transfers from the outer shell of a sodium atom to the outer shell of a chlorine atom For example, sodium reacts with chlorine to form sodium chloride. The transfer of electrons can be modeled using dot and cross diagrams. The electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses. The dots and crosses do not need to be coloured. Sodium ions and chloride ions form in the reaction.

There is a quick way to work out what the charge on an ion should be: The number of charges on an ion formed by a metal is equal to the group number of the metal The number of charges on an ion formed by a non-metal is equal to the group number minus eight Hydrogen forms H+ ions

Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions

Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases. Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic forces of attraction between oppositely charged ions are called ionic bonds.

Giant Metallic Structure

Metallic bonding Metals consist of giant structures of atoms arranged in a regular pattern. The electrons from the outer shells of the metal atoms are delocalised, and are free to move through the whole structure. This sharing of delocalised electrons results in strong metallic bonding.

Metals tend to have high melting and boiling points because of the strength of the metallic bond. The strength of the bond varies from metal to metal and depends on the number of electrons which each atom delocalises into the sea of electrons, and on the packing.

Covalent Bond

For example, a hydrogen molecule forms when a hydrogen atom shares its outer electron with another hydrogen atom. Dot and cross diagram of a hydrogen bonding

A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells (highest occupied energy levels) of the atoms. An atom that shares one or more of its electrons will fill its outer shell. Covalent bonds are strong, and a lot of energy is needed to break them.

States of Matter

MELTING When a solid is heated, the particles are given more energy and start to vibrate faster. At a certain temperature, the particles vibrate so much that their ordered structure breaks down. At this point the solid melts into liquid. The temperature at which this change from solid to liquid happens is called the melting point. Each solid has a set melting point at normal air pressure. At lower air pressure, such as up a mountain, the melting point lowers. FREEZING Lava is liquid rock, which erupts through a volcano at temperatures as high as 1,500ºC (2,732ºF) through a volcano. However, the red-hot lava cools as it meets the Earth’s surface, and turns back into solid rock again. This change from liquid to solid is called freezing or solidifying. It is the opposite process to melting. BOILING When a liquid is heated, the particles are given more energy. They start to move faster and further apart. At a certain temperature, the particles break free of one another and the liquid turns to gas. This is the boiling point. The boiling point of a substance is always the same; it does not vary. INVISIBLE STEAM Water boils when it reaches its boiling point of 100ºC (212ºF). This is the temperature at which water turns to steam. Steam is an invisible gas. When it reaches the lid it cools back to a liquid. EVAPORATION Even without boiling water in a kettle, some of the liquid water changes to gas. This is evaporation. It occurs when a liquid turns into a gas far below its boiling point. There are always some particles in a liquid that have enough energy to break free from the rest to become a gas. CONDENSATION Dewdrops are often found on a spider’s web early in the morning after a cold night. Water that is present as a gas in the air cools down and changes into tiny drops of liquid water on leaves and windows. This change from gas to liquid is called condensation.

There are five known phases, or states, of matter: solids, liquids, gases, plasma and Bose-Einstein condensates. The main difference in the structures of each state is in the densities of the particles.