SCH 3U - VSEPR Theory

Lewis diagrams only show how many bond pairs and lone pairs surround a given atom.  However, all molecules are 3D and we need a method to illustrate the true shape of a molecule.  That is where VSEPR Theory comes in (VSEPR Theory stands for “Valence Shell Electron Pair Repulsion Theory”).

Chemists need to know the shape of molecules (for instance, much drug design is based on the molecule fitting into the site in the body where healing is required).

Thus, we need to convey a sense of 3D in 2D.  Like artists, we will use a form of perspective to indicate which parts “stick out” of the page and which parts “recede into” the page:  wedge (out of page), broken line (into page), straight line (on page). 

We can predict shapes by assuming that each pair of electrons (both bonding pairs and lone pairs) repel all other pairs of electrons.

Notice that there are a few exceptions to the Octet Rule in the examples.  Be, B and Sn are happy to bond covalently and with less than a full octet.  P and S will normally bound following the Octet Rule but when required, they will employ an extended valence to allow 10 or 12 electrons (respectively) around them.  

Below you will find Lewis dot and structural diagrams for each of the examples in the chart.

 

You need to watch this video.  It takes you through all the VSEPR shapes.

 

Also, try this interactive VSEPR generator.  It's really neat!

 

Homework: 

 

Answer Key:

SCH 3U/4C - Polar Covalent Bonding

A polar covalent bond results when electrons are shared unequally (ex. HCl).  In this molecule, a pair of electrons is shared between the H and Cl atoms, but the sharing is not equal.  The Cl atom has a stronger attraction for the shared pair of electrons than does the H atom.  This makes the Cl end of the molecule slightly negative and the H end slightly positive. This is designated by the symbol delta (ẟ).

 ẟ⁺ H - Cl ẟ^- 

 

Electronegativity:  Electronegativity is the quantitative measure of an atom’s electron attracting ability.  These values range from 0.9 to 4.1.  The higher the electronegativity value, the greater the attraction for electrons. 

Two elements with very different electronegativities such as sodium (0.9) and Cl (2.9) are expected to form ionic bonds.  Two elements of slightly different electronegativity such as carbon (2.5) and hydrogen (2.1) are expected to form only slightly polar covalent bonds.  A molecule like H₂, where each atom has the same electronegativity forms a purely covalent compound.  The greater the difference in electronegativity (ΔENeg), the more polar the bond becomes. 

 

Guidelines for Determining Chemical Bond Types

ΔENeg > 1.7  🠊  IONIC BOND

0.5 < ΔENeg < 1.7  🠊  POLAR COVALENT BOND

ΔENeg < 0.5  🠊  PURE COVALENT BOND

*Note that the above information is a guideline.  The only value written in stone is the one for pure covalent bonding.  Be sure to use your common sense.  For instance, if you find the ΔENeg for two non-metals bonding is 1.75, you have a polar covalent bond.  Despite the ΔENeg value falling in the ionic range, we know that two non-metals will bond covalently.

 

TryIt!:  Determine the bond type and provide the Lewis diagram and the Structural diagram for silicon tetrafluoride.  The answer is below.

 

ex. SiF₄

 

Homework: 

HW 1: (i) Determine the chemical bond type for the following molecules.  (ii)  Draw the Lewis diagrams of the molecules.  (iii) Draw the structural diagrams of the molecules (if the molecule contains a polar covalent bond, indicate which end of the bond is slightly negative (δ-) and which end is slightly positive (δ+)).

(a)  MgCl₂  (b)  BrCl  (c)  CO  (d)  N₂  (e)  HF  (f)  K₂S

Answers:

 

SCH 3U - Bonding in Ions and Ionic Compounds

 Covalent Bonding in Ions

Charges indicate that there are more or fewer electrons than normal.  For instance, the negative charge on SO₄^2- means that there are two extra electrons to use when bonding.  Conversely, the positive charge on NH₄⁺ means that there is one less electron to use when bonding.

 ex.  NO₃^-

Tips: 

1. Typically, the single element is surrounded by the others (in this case, N will be in the middle and the Os will “click into” the central N.
2. Since both N and O are both non-metals, we expect the bonding to be some form of covalent, so the atoms will share electrons.
3. Since nitrate has a -1 charge, there will be one extra electron (represented by the triangle) available to use in the bonding.
4. Be sure all atoms have satisfied the Octet Rule and have a full outer shell (usually eight electrons).
5. Trial and error is your best bet if you don’t immediately see the answer.  See my thought process below.
6. Always check to ensure the Octet Rule is satisfied and that you have included charges and a bracket where appropriate.

 

 

Covalent Bonding in Ionic Compounds

Ionic compounds that contain polyatomic ions will have both ionic and covalent bonding present.

ex.  KIO₃

Tips

1. First survey the elements – both I and O are non-metals and will bond covalently by sharing electrons.  The K is a metal and will bond to the iodate group using ionic bonding via electron transfer.
2. Always start with the covalent bonding.  Use what you already know – place the I in the middle and click in the Os around it.
3. Then transfer the electron from K to one of the Os.  Never transfer an electron to a central atom – IRL the electron would be blocked from the I by the Os.
4. Always check to ensure the Octet Rule is satisfied and that you have included charges and a bracket where appropriate.

To convert to a structural diagram, write down the element symbols in the same relative positions as in the Lewis diagram.  Replace all shared electron pairs with a dashed line and remove any other electrons

Homework: LDD&SD # 3, 5

SCH 4C - Electromagnetic Spectrum & Flame Tests

Electromagnetic Spectrum

Electromagnetic energy is light energy, which is thought to move in the form of waves.

Visible light, infrared light, ultraviolet light and X rays all have different frequencies (the number of cycles that pass a point per second).

 

Each wave has crests and troughs and the distance which encompasses one crest and one trough is the wavelength (given the symbol λ, measured in nm).

Light has a characteristic speed, 3.0 × 10⁸ m/s

The band of light that the human eye can detect is called the visible spectrum (400 – 700 nm).

Each wavelength of light is associated with a different colour – a rainbow or continuous spectrum.

 

Atomic Structure

Recall that Neils Bohr was the scientist who proposed the planetary model of the atom.

The nucleus is in the centre of the atom and the circular orbitals radiate out from the centre - the further the orbital is from nucleus, the higher the energy of each electron in that shell.

Electrons can move up or down between the shells, but cannot exist between the orbits.

When the electrons are at the lowest possible energy, the atom is in the ground state.

If an electron jumps from a lower energy orbital to a higher energy orbital, the atom is in the excited state.

 

Emission Spectra & Fireworks

When an atom in the excited state loses energy and the electron drops back down to the ground state, the excess energy is lost in the form of coloured light.

Each atom has a characteristic colour, indicative of the size of the jump between orbits.

This is how fireworks get their colours.

Emission (or bright line) spectra is observed that has lines of colour separated by black regions.

Line spectra can be observed using a spectroscope – an instrument that separates light into its components colours using a prism.

 

Flame Tests

Since metal ions produce different colours of light, we can use this knowledge to determine which elements are present in unknown samples by carrying out flame tests.

Flame Test Procedure:

1. Soak a wooden splint in water.
2. Dip splint into the powdered sample.
3. Place sample (on the splint) into a flame.
4. Observe colour produced.

➣ Each element produces a unique colour.  We can compare the unknown's colour to a list  of known colours and determine the identity of the unknown.

This is one example of qualitative analysis.

Homework: 

Section 1.4 Questions, p. 18 # 2-4

Watch this video and answer the following questions.  Send your answers to me on Edsby.  Alert! Alert!  This is for marks!!! 

1. List out the substances (in order of appearance in the video) and the corresponding flame test colour.
2. Identify the elements in the middle two pictures above (the picture found at the end of today's lesson).
   

SCH 4U - Structures & Properties Final Thoughts

I have put together some study notes for you.  These notes do not encompass the entire unit, but rather the section on forces and crystals.

TryIt!:  answer shown after success criteria

Success Criteria:

• atomic theories - be familiar with the scientists and their contributions
• quantum mechanical model of the atom:
• be able to draw and state the characteristics of the orbitals (s, p, d, f) and suborbitals
• be able to create the superposition of e clouds representing the e probability density for atoms
• be able to create the e configuration (OBN, SN, NGN) for atoms and ions
• be able to explain the organization of the periodic table, magnetism, ions and anomalous e configuration
• bonding - be able to draw the Bohr diagram, Lewis diagram, VSEPR diagram (and name/angles), as well as calculate electronegativity difference (with predicted bond type) and molecular dipole prediction for a given molecule
  
• hybridization - be able to show the 3 lines for hybridization of the central atom in a molecule and then draw the orbital overlap diagram for bonding
• list, describe and provide examples for all intermolecular forces (cov, ion, met, HB, DD, LF)
• list, describe and provide examples for all intramolecular forces (cov, ion, met)
• be able to list/explain the properties and characteristics for all crystal types (cov, ion, met, mol)
• be familiar with all carbon allotropes

 

Answer to TryIt!

 

SCH 4U - Covalent Network Crystals

Covalent Network Crystals

Covalent network crystals (also referred to as “covalent crystals” or “network crystals”) consist of atoms held together in large networks or chains by covalent bonds (a covalent network crystal is essentially one large molecule).

Covalent crystals are found in three categories, depending on the orientation of bonding with the crystal lattice:  3D, 2D or 1D.  We will discuss them all below:      

 

3-D Covalent Network Solids

In diamond, each C is covalently bonded to four other carbon atoms.  This 3-D structure contributes to diamond’s amazing hardness.  

SiO₂ (quartz) is not as hard because of different sized atoms, which results in weaker bonds.

Metalloids, like Si and Ge, are also found as covalent crystals.

Another 3-D covalent crystal is silicon carbide (SiC, known as carborundum).

Properties of 3D Covalent Network Solids

Mechanical Strength

• extremely hard (will cleave) due to very strong covalent bonds

Electrical Conductivity

• insulators because there are no free electrons or ions to carry electricity

Melting Point

• extremely high since strong covalent bonds are difficult to break

Interaction with Light

• usually transparent because there are no free electrons to interact with the light

 

2-D Covalent Network Solids

Graphite, which is based on the benzene ring (C₆H₆), is composed of stacked sheets consisting of interlocking benzene rings without hydrogen.

Mica (Si₂O₅) is the other substance that is a 2-D covalent network crystal.

In most molecules the electrons are localized on the atom (i.e. the σ- and 𝛑-electrons are associated totally with the two atoms forming the bond).  In benzene (and hence, graphite), the electrons are delocalized (the electrons are semi-mobile and can move about causing resonance structures).  Therefore, graphite has (a) very strong covalent bonds linking C atoms in the plane of the sheets and (b) weak London forces between the sheets, holding them together.

Let's take a closer look at the bonding in the six carbon benzene ring to see how this works.  We will focus on one of the six carbon rings that is found in the graphite sheet.

Since the p orbitals can overlap in two different manners (creating the resonance structures), the electrons are considered to be delocalized.  This means that they are not completely locked into place on the parent atom. 

 

Properties of 2-D Covalent Network Solids

Mechanical Strength

• graphite is a soft solid that cleaves in planes (because the London forces are weak); can be used as a lubricant

Melting Point

• very high since the covalent bonds require a large amount of energy to break
  

Electrical Conductivity

• excellent in graphite since it has delocalized electrons to carry the electricity

• poor in mica since there are no delocalized electrons

Interaction with Light

• opaque and shiny in graphite due to the interaction of light with the delocalized electrons

• transparent in mica since there are no delocalized electrons to interact with the light

 

1D Covalent Network Solid

There is only one example of a 1D covalent network solid: asbestos.  Asbestos mainly consists of alternating silicon and oxygen atoms bonded to each other. Asbestos, due its 1D structure is fibrous in nature.

 

Allotropes of Carbon

We have already discussed diamond and graphite, but there is a third allotrope (physical form) of carbon, discovered in 1985, C₆₀, aka, Buckminsterfullerene (or Bucky Ball for short).

Some scientists believe that carbon nanotubes (relatives of Bucky Ball) could be used to create a space elevator.

Homework # 31-37

   

SCH 3U/4C - Ionic & Covalent Bonding

Atom:  An atom is the smallest particle of an element that retains all the chemical properties of that element.  It is electrically neutral.  ex.  Na, H, K, S, Fe, Ag, Sn

 Molecule:  A molecule is any electrically neutral group of atoms that is held together tightly enough to be considered a single particle.   ex. H₂O, CO₂, H₂SO₄, CH₄, N₂, O₂, C₂H₄

 Chemical Bonds:  A chemical bond is a strong force of attraction holding atoms together in a molecule, resulting from the sharing or transfer of electrons.  Molecules are more stable than the isolated atoms from which they are formed.  Only valence electrons are involved in bonding.

 Stable Octet:  There is a special stability of an atom with 8 valence electrons.  All noble gases, except He (which is happy with its 2 electrons), have a stable octet and so are very stable and relatively inert with respect to bonding with each other or with other elements.

 

Ionic Bonding

When a metal (low ionization energy) reacts with a non-metal (high ionization energy, high electron affinity), electrons are transferred from the metal to the non-metal.  The atoms of the non-metal, having gained electrons become negatively charged ions, while the atoms of the metal, having lost electrons, become positively charged ions.  These oppositely charged ions attract each other and an ionic bond is formed.  The resulting molecule is electrically neutral.

 

Isoelectronic:  When an atom has the same electron arrangement as another atom, the two atoms are referred to as isoelectronic.  In the first example below, Na⁺ is isoelectronic with Ne and Cl^- is isoelectronic with Ar. 

TryIts!:  Provide the Lewis diagram for the bonding of (a) sodium chloride, (b) calcium oxide, (c) aluminum sulfide.  Answers are below.

Covalent Bonding

When electrons are shared between two atoms, a covalent bond is formed.  

 

Structural Diagram:  The structural diagram resembles the Lewis dot diagram, except that single lines are used to represent each shared pair of electrons.

Octet Rule: When atoms combine, the bonds are formed in such a way that each atom finishes with an octet of valence electrons (except hydrogen).

Single Covalent Bond

A single bond is formed when one pair of electrons are shared between two atoms.

TryIts!:  Provide the Lewis diagram and the Structural diagram for (a) chlorine gas, (b) water. Answers are below.

Double Covalent Bond                

A double bond is formed when two pairs of electrons are shared between two atoms.

TryIt!:  Provide the Lewis diagram and the Structural diagram for oxygen gas. Answer is below.

Triple Covalent Bond

A triple bond is formed when three pairs of electrons are shared between two atoms.

TryIt!:  Provide the Lewis diagram and the Structural diagram for nitrogen gas.  Answer is below.

Coordinate Covalent Bond           

A coordinate covalent bond is a bond in which both electrons of the shared pair come from only one of the bonded atoms.

TryIt!:  Provide the Lewis diagram and the Structural diagram for POCl₃. Answer is below.

 

 Homework:   

• Draw the Lewis diagram showing the ionic bond.  For each ion formed, state with which noble gas atom the ion is isoelectronic.

(a)  KF  (b)  SrCl₂  (c)   BaS   (d)  Cs₂O  (e)  Ca₃P₂   (f)   MgCl₂  (g)  K₃N   (h)  Na₂S  (i)    SrI₂

• Draw the Lewis diagram and structural diagram for each of the following molecules.

(a)  H₂ (b) NH₃  (c) CH₄  (d) F₂  (e) HCN  (f) CCl₄  (g) CO₂  (h) NI₃ 

• Lewis Dot Diagrams & Structural Diagrams (LDD & SD) #1bcde, 2befhi - if you are in SCH4C don't do this part of the HW - the rest of you, get at it!
  

Answer Keys (TryIts! & HW):