A chemical compound is formed when two or more elements combine in a chemical reaction.Compounds cane be broken down chemically into simpler substances, and in many cases, directly to their elements. e.g. electrolysis of water.Hydrated substances contain molecules of water in definite proportions, usually locked into a crystal structure e.g. washing soda (hydrated sodium carbonate). Water bound in this way is described as water of crystallisation, it can be driven off by heating to give the corresponding anhydrous compound.Water of crytallisation is water chemically combined in definite proportions in a crystalline compound.
Compound -- Two or more elements chemically combined Elements are chemically combined by forming bonds. Bonds involve electrons being lost, gained, or shared....Two types: Ionic and Covalent. Octet rule -- A useful rule to predict the bonding between elements but there are a lot of exceptions. 'Atoms on reaction tend to reach an electron arrangement with eight electrons in the outermost energy level'Exceptions:Beryllium and boron atoms have too few electrons to be able to reach eight in the outer level.The d-block elements (between groups II and III) do not usually obey the octet rule.The rule works in some cases for sulfur and phosphorous, but nor inn others.Hydrogen and lithium atoms tend to reach the electronic structure of helium, but they are unable to gain the large number of electrons required to obtain an octet.4. The valency of an element is the amount of bonds it forms in a compound.e.g. oxygen = 2 (needs two more electrons to have a full outer shell)carbon = 4hydrogen = 1
Noble gas electron configurationThe elements of group 0 of the periodic table (the noble gases), are found to be very stable and unreactive. This is because they have stable arrangement of electrons ( a full outer shell).The uses of helium and argon are related to their chemical unreactivity. Helium is a much safer alternative to hydrogen for use in weather balloons and blimps. Both have very low densities, but unlike hydrogen, helium is not flammable. Electric light bulbs are filled with argon to prevent the tungsten filament from evaporating or reacting. Incandescent light bulbs are being replace by energy-saving light bulbs throughout the EU with Ireland being the first country to have made the change.
The valency of an atom is the number of bonds an atom of the element forms when it reacts.Valency gives a measure of the power of the atom.Group I --> 1Group II --> 2Group III --> 3Group IV --> 4Group V --> 3Group VI --> 2Group VII --> 1Group 0 --> 0
Ionic bonds are typically found in compounds containing metals combined with non-metals. Metals, on reaction, lose one or more electrons and become positively charge ions called cations.e.g. sodium has one electron in its outer shell and upon losing this electron, becomes positively charged. The sodium ion is smaller than the sodium atom. In general, a cation is smaller than the corresponding atom.
Non-metal atoms, on reaction, gain one or more electrons becoming negatively charged ions called anions.e.g. chlorine has seven electrons in its outer shell and therefore gains one electron to achieve a full outer shell. The chloride ion is then negatively charged and bigger than the chlorine atom. In general, an anion is larger than the corresponding atom.
An ionic bond is the electrostatic force of attraction between oppositely charged ions.A) Electrons are transferredB)Electrons are given from a metal to a non--metal.C) Ions form (charged atom)Ionic bonds are represented by dot and cross diagrams:
-- Crystal structureIonic bonding does not result in the formation of molecules. The oppositely charged ions attract eachother to form a rigid, three-dimensional crystal lattice. Each ion in the lattice is surrounded by others of opposite charge.
Characteristics of ionic compoundsIonic compounds are generally solids with high boiling points and high melting points because a lot of energy is needed to seperate the ions from eachother. They cannot conduct electricity in the solid state because the ions are held tightly in the crystal and are not free to move. However, they conduct electricity when molten or in aqueous solution as the ions are free to move - they are electrolytes. Soluble in water (like dissolves in like - highly polar). The water molecules are attracted to the ions causing them to separate.
Everyday examples:Salt tablets (taken to replace salt in the body lost through sweat)Table salt (flavouring food, preservative)Brine (a solution of sodium chloride, has long been used for curing bacon, in effect a preservation process)Flouride compounds (sodium flouride - prevents tooth decay - added to water and sometimes toothpaste.)
A covalent bond is formed when two atoms share a pair of electrons.Each shared pair is regarded as one covalent bond.Shared pairs that form covalent bonds are called bonding pairs. Pairs of electrons not involved in bonding are called lone pairs or non-bonding pairs.Covalent bonding is illustrated using a dot and cross diagram.
Covalent bonds result in the formation of molecules Covalent bonds may be between atoms of the same element (O2) or atoms of different elements (H2O) Covalent bonds form between non-metals.
Double and triple covalent bondingSome atoms form covalent bonds by sharing two/three pairs of electrons.Double covalent bond:
Triple covalent bond:
The strengths of covalent bonds can be studied by measuring the energy needed to break the bonds. These bond dissociation energies show that double covalent bonds are stronger than a single bond but not twice as strong, similarly, triple bonds are not three time stronger than single bonds. This indicates that not all the bonds in a multiple bond are equally as strong.
When a single covalent bond is formed between two atoms, atomic orbitals containing a single electron from each atom overlap. The new molecular orbital formed contains a pair of electrons which constitute a single covalent bond. This bond is called a sigma bond when it involves two s orbitals, an s and a p orbital, or two p orbitals which overlap end-on to eachother. A sigma bond is always formed between two atoms in a molecule if they are covalently bonded.In double or triple covalent bonding, sideways overlap is possible between two p atomic orbitals, each containing one electron. In this case a pi bond is formed.e.g. O2. Like all double bonds consists of one sigma bond and one pi bond. Triple bonds e.g. N2, consist of one sigma bond and two pi bonds. Sigma bonds are stronger than pi bonds.
Polar and non-polar covalent bondingIn a pure covalent bond / non-polar bond, electrons are shared equally. e.g. molecules containing only one type of atom like H2 (atoms attract the shared electrons in the covalent bond equally)In covalent bonds containing atoms of different elements, two factors dictate the extent to which the shared electrons are attracted by different nuclei: The size of the atom (smaller atoms have a greater attraction than larger atoms with a similar charge because they can get closer to the shared pair of electrons). The nuclear charge (atoms with a bigger charge in the nucleus wiill have a greater attraction for the shared electrons than atoms of a similar size with a smaller charge). When electrons are shared unequally, polar covalent bonds are formed. The atom with the lesser share of electrons will become slightly positively charged, the other atom becoming slightly negatively charged
Characteristics of covalent compounds Since the intermolecular forces are weak, most molecular substances are liquids/gases at room temperature. They have low melting and boiling points They do not conduct electricity as they consist of neutral molecules rather than electrically charged particles such as ions. They are not soluble in water. However polar covalent compounds have some ionic character and are more likely to form aqueous solution.
Examples:Polar solvent = waterNon-polar solvent = cyclohexanePolar = glucoseNon-polar = petrol and oil
Electronegativity is a measure of the relative attraction an atom has for the shared pair of electrons in a covalent bond.
The most commonly used scale of electronegativity is the Pauling scale which runs from 0 to 4. The higher the electronegativity value an atom has, the better it is at attracting the shared electrons towards itself.3 factors are responsible for the variation in electronegativity values in atoms of different elements:The nuclear charge (the attraction of the nucleus for electrons)The atomic radius (the distance from the nucleus of the outer shell electrons)The screening effect by which inner shell electrons shield valence shell electrons from the full force of the nucleur charge.
Predicting the nature of bonds:The greater the difference in electronegativity values between two bonding atoms, the greater the degree of polarity.0 / very small (pure covalent or non-polar covalent) e.g. 0.01greater than 1.7 (ionic) less than 1.7 (polar covalent) e.g. 0.38 - weak polar covalent or 0.96 or 1.24 - strong polar covalent
see experiment copy for cyclohexane experiment!!!
Ionic and Covalent Bonding