4 - Chemical Bonding and Structure

• Atoms have a neutral charge as their negative electrons and positive protons balance out.
• Ions are formed when atoms gain or lose electrons.
• The charge of an atom can be changed by adding or removing electrons.
• Cations are atoms that lose electrons and therefore have a positive net charge.
• Negative ions, also known as anions, are atoms that gain electrons and therefore have a negative net charge.
• Ionic bonds form as a result of the electrostatic attraction between oppositely charged ions.
• An electrostatic attraction is the attraction of oppositely charged particles/objects.
• Ionic bonds only occur between metals and non-metals.
• The metal will have a positive charge, while the non-metal will have a negative charge.
• The opposite charges create an electrostatic attraction between the ions, causing them to form a neutral lattice.
• The charge of the ions will cancel each other out.
• When metals react with non-metals, electrons are transferred from the metal to the non-metal forming an ion.
• Ionic compounds are balanced as the negative and positive charges balance each other out.
• The formation of an ionic compound can be represented using electron shell diagrams.
• When sodium and chlorine react together to form sodium chloride, the electron that a sodium atom loses to form a stable sodium ion is gained by a chlorine atom to form a stable chlorine ion.
• Most elements try to reach a noble gas configuration by either losing or gaining electrons.
• To find the ionic charge of an element, the periodic table can be used.
• Group 1, 2, 3 elements form ions with charges of 1+, 2+ and 3+ respectively.
• Group 15, 16, 17 elements form ions with charges of 3 - , 2 - , 1 - respectively.
• Elements in group 14 can lose 4 electrons but can also gain 4 electrons.
• The shape of a molecule is determined by the geometric arrangement of bonding pairs and lone pairs.
• Two conditions must apply if a molecule is to be a dipole (polar): it must have polar bonds and the partial charges must be distributed asymmetrically across the molecule.
• A molecule that contains polar covalent bonds will form a dipole.
• A polar molecule must have dipoles distributed so overall there is a positive and negative end of the molecule.
• Covalent compounds have two types of structures: simple covalent and giant covalent (network covalent) structures.
• Simple covalent structures contain only a few atoms held together by strong covalent bonds such as carbon dioxide where one atom of carbon is covalently bonded with two atoms or oxygen.
• Molecular covalent substances have low boiling points and are usually liquids and gases, due to the weak intermolecular forces (not intramolecular)
• These structures are also non-conductive because they do not have any free electrons or an overall electric charge.
• Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds.
• Silicon and carbon form giant covalent structures.
• The atoms are usually arranged into giant regular lattices, which are extremely strong structures because of the many bonds involved (such as graphite or diamond).
• Giant covalent structures have very high melting points because a lot of strong covalent bonds must be broken.
• Giant covalent structures also vary in conductivity, as some contain free electrons while some do not.
• Carbon can be found in three forms, all of which vary in their structure, and are called allotropes of carbon.
• Graphite has a hexagonal layer structure, and although it has covalent bonds between the carbon atoms, it has weak Van Der Waals forces between the layers themselves, allowing the bonds to be shattered easily and the layers to slide over each other easily.
• The covalent bonds within the layers of graphite are very strong, but the dispersion forces between the layers are weak.
• The structure of graphite is described as a covalent layer lattice.
• Graphite has good conductivity as there are delocalized electrons between the hexagonal layers and electrons are free to move parallel.
• In diamond, each carbon atom is joined to four other carbon atoms, forming a giant covalent structure.
• Diamond has a tetrahedral structure held together with strong covalent bonds.