basic chemistry notes on covalent bonding, practice questions included
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Covalent Bonding Outcome Ugl y OK Supe r Describe what molecules are Describe covalent bonding as a sharing of electrons Draw electron dot diagrams for a range of molecules Predict the molecular shape of a range of molecules with up to four electron pairs around a central Name covalent compounds Use the electronegativity of elements to predict the polarity of covalent bonds Describe the polarity of covalent molecules Define intramolecular and intermolecular forces Describe dispersion forces and identify molecules containing them Describe dipole-dipole interactions and identify molecules containing them Describe Hydrogen bonding and explain why it results in molecules having higher than expected melting and boiling points Describe the properties of covalent molecular substances by relating them to their structure and Describe the structure, bonding and properties of covalent network lattices including diamond and Describe the structure, bonding and properties of covalent layer lattices including graphite Identify the allotropes of carbon including diamond, graphite, nanotubes and fullerenes Classify compounds as covalent molecular, ionic or metallic Use your knowledge to identify the structure and bonding in a range of compounds and thus explain Molecules Molecule: A group of non-metallic atoms held together by covalent bonds Molecules are electrically neutral Monoatomic Diatomic Polyatomic / Covalent molecular compound UNIT ONE: COVALENT BONDING 1
Transcript
Covalent Bonding
Outcome Ugly OK Super
Describe what molecules are
Describe covalent bonding as a sharing of electrons
Draw electron dot diagrams for a range of molecules
Predict the molecular shape of a range of molecules with up to four electron pairs around a central atom
Name covalent compounds
Use the electronegativity of elements to predict the polarity of covalent bonds
Describe the polarity of covalent molecules
Define intramolecular and intermolecular forces
Describe dispersion forces and identify molecules containing them
Describe dipole-dipole interactions and identify molecules containing them
Describe Hydrogen bonding and explain why it results in molecules having higher than expected melting and boiling pointsDescribe the properties of covalent molecular substances by relating them to their structure and bondingDescribe the structure, bonding and properties of covalent network lattices including diamond and quartzDescribe the structure, bonding and properties of covalent layer lattices including graphiteIdentify the allotropes of carbon including diamond, graphite, nanotubes and fullerenesClassify compounds as covalent molecular, ionic or metallicUse your knowledge to identify the structure and bonding in a range of compounds and thus explain the resulting properties
Molecules Molecule: A group of non-metallic atoms held together by covalent bonds Molecules are electrically neutral
Covalent bonding occurs between non-metals and other non-metals. Non-metals have high electronegativity When non-metals bond electrons are not lost from an atom, rather shared, to make
up a full outer shell in each of the atoms in the molecule. Some examples of covalent molecules are:
Electron dot diagrams (Lewis diagrams)
Can be used to determine the bonds, and then the structure of covalent molecules
Electron dot diagrams for atoms
To draw:o Put the chemical symbol in the centre, then add the outer shell electrons
only. Put the first four electrons singly, and then when you get to the fifth, make a pair etc.
Eg Carbon nitrogen oxygen
The electrons that are unpaired are available to be shared and are known as bonding electrons.
The pairs of electrons are called lone pairs or non-bonding electrons.
Electron dot diagrams for molecules
Rules:o Draw the dot diagrams for each atom in moleculeo Element with least valence electrons (most bonding electrons) is the central
atom – Draw it in the centreo Place other atoms around central atom – Pairs of bonding electrons should
be between atoms.
Each atom in molecule should be surrounded by 8 electrons (except H)
UNIT ONE: COVALENT BONDING 2
Oxygen Chlorine Water
Methane (CH4) Ammonia Ethene (C2H4)
HCN HCl Propane (C3H8)
H2S HOCl H2CO
Plasticine and toothpicks
UNIT ONE: COVALENT BONDING 3
Molecular Shape
Each molecule has its own particular shape or 3-D arrangement due to the number of bonds, lone pairs as well as the repulsion between bonds.
VSEPR – Valence shell’ electron pair repulsion theory is used to predict the shape of molecules
Electron pairs repel each other and try to sit as far away from each other as possible
ShapeNumber of lone pairs
around central atom
Number of bonds around
central atom
Lewis Diagram Shape Diagram Examples
Tetrahedral
Pyramidal
V-shaped
Linear
Planar
Practice time: Draw the shape diagram for the following:CF4 H2S HCl CH2O HCN C2F4 NF3 CHCl3
UNIT ONE: COVALENT BONDING 4
Naming Covalent Compounds
Molecular elements Are elements which form molecules E.g. hydrogen(H2) and oxygen(O2). These are simply known as their elemental name
Molecular compounds (Only have to be able to do molecules with 2 different atoms) Contain more than one type of atom Rules:
o First element in formula is named in fullo Second element is named just like an anion with the suffix -ideo A Prefix is used in front of each atom name and tells you how many atoms of
In a covalent molecule if the atoms are the same they attract the electrons of the bond the same amount as the atoms electronegativities are the same
The bond between atoms is non-polar
In a covalent molecule, if the atoms are different, the atoms will have different electronegativities and so attract the shared electron pair an unequal amount.
UNIT ONE: COVALENT BONDING 5
A bond in which this occurs is said to be a polar bond
The atom which attracts the electrons more strongly has a slight negative charge - The atom which attracts the electrons less strongly has a slight positive charge + We call these different sides of the molecule dipoles and show them with the symbol
- and +.
Polar or non-polar molecule?
Non-polar
A molecule which does not contain polar bonds is non-polar. Examples: H2 O2
Draw in the molecules
A molecule which contains polar bonds and has a symmetrical shape that causes
the polar bonds to cancel out are non-polar. The shape of the molecules are always symmetrical The polar bonds all have to be the same for them to cancel out and make the
molecule non-polar
Example: CH4
Other examples: Draw out some shape diagrams of other molecules that have polar bonds but are non-polar molecules
UNIT ONE: COVALENT BONDING 6
All these bonds have to be the same for the molecule to be non-
A molecule containing polar bonds that do not cancel Molecules that are not symmetrical, contain polar bonds and/or lone pairs are
always going to be polar
NH3 H2O
In some molecules, even though the shape might be symmetrical the bonds are not all the same size of polarity, therefore the molecule is polar
Other examples: Draw out some shape diagrams of other molecules that have polar bonds and are polar molecules.
UNIT ONE: COVALENT BONDING 7
Intermolecular Attractions The covalent bonding within each molecule is an intramolecular force. Intermolecular forces are the forces between molecules holding them together.
Intramolecular force vs Intermolecular forces diagram
If a compound has a low boiling point it is because the intermolecular forces are weak. Eg CH4 is a gas because it has weak intermolecular forces
There are three main types of intermolecular force.
Dispersion Forces These are the weakest type of intermolecular forces. All covalent molecular substances have dispersion forces between their molecules. Caused by the attractions between the protons and electrons of adjacent molecules. Electrons are always moving and at any one point they may be found on one side of
an atom or molecule. This causes a temporary dipole which results in an attraction between particles of a substance.
The size of the dispersion forces depends on o the size of the molecule (and therefore the number of protons and electrons) o the shape of the molecule – the closer the molecules can get together the
stronger the attraction will be
Dipole-Dipole Interactions Polar molecules attract each other because the positive side of one molecule will
attract the negative side of another. Opposite dipoles are attracted to each other Dipole-dipole attractions are much stronger than dispersion forces.
UNIT ONE: COVALENT BONDING 8
Example Explain the difference in boiling points of HCl (-83.7oC) and Ar (-186oC) considering both contain 18 electrons
3. Hydrogen bonding Is a special case of dipole-dipole interactions. Occurs only between two molecules in which hydrogen is bonded to nitrogen,
oxygen and fluorine. These three elements are very electronegative and strongly attract the shared
electrons in their covalent bond with hydrogen, therefore they make the hydrogen very positive and themselves (N, O or F) very negative.
The Hydrogen bond is between the N, O or F of one molecule and the H of another molecule
Note: It is the bond between the dipoles of the different molecules which is the hydrogen bonding. The bond between the H and N, F or C atom is COVALENT.
Hydrogen bonds are stronger than other dipole- dipole bonds and result in materials with higher melting and boiling points.
Covalent Network Lattices Some non-metals form giant structures with no individual molecules. They consist of many atoms covalently bonded to each other forming a three
dimensional network lattice. Every crystal is like one very large molecule with continuous bonding throughout the
structure These substances have very high boiling points (diamond = 5000 C sublimation) and
are very hard due to the high number of strong covalent bonds
Diamond each carbon atom is bonded to other carbon atoms (covalent bonds) continuous 3D structure Is very brittle Conductivity: It does not have free moving charged particles therefore……
Solubility: No attraction between carbon atoms and water molecules, covalent bonds too strong, therefore . . . ..
Diamond is the hardest substance known due to its bonding pattern and strong covalent bonds
Use:
Silica
SiO2 silicon dioxide Each silicon atom is bonded to oxygen’s and each O is bonded to two
silicon’s. 3D structure Is very brittle Conductivity: It does not have free moving charged particles therefore……
Solubility: No attraction between Si and O atoms and water molecules, covalent bonds too strong, therefore
each carbon atom is bonded to other carbon atoms (covalent bonds) the remaining valence electron is delocalized between layers. the bonding between each layer is weak and thus is easily broken results in 2D layers which can slide over each other Conductivity: Contains delocalized electrons (free moving charged particles) – these
are what hold the layers together therefore …..
Melting and Boiling point: High melting point but is soft enough to sketch with (pencil)
Solubility: No attraction between carbon atoms and water molecules, covalent bonds too strong, therefore . . . ..
Uses:
Allotropes Graphite and diamond are both made from carbon and are said to be allotropes of
carbon. Allotropes are different forms of the same element. Other allotropes of carbon include buckminsterfullerines (buckyballs) and
