Molecular Structure and Properties of Compounds Chemistry 30 Unit 2

Covalent Bonding Is the ____________________ It occurs between a: nonmetal and nonmetal semimetal and nonmetal semimetal and semimetal We already know how to name covalent compounds (By adding prefixes and ending the last element in “ide”.)

How do Covalent Bonds Form? Atoms share electrons with other atoms in order to complete their shell. __________________: Atoms can have a maximum of 8 valence electrons in their outer shell. _________ – The attraction btwn atoms (2 shared electrons) _______________- a pair of electrons that are left on their own around a central atom. ___________________– the one that has the most atoms attached to it (usually the one with the lowest electronegativity; exception is hydrogen, it is never the central atom).

How to Draw Lewis Dot Structures with Molecules Count the total number of valence electrons for the molecule Choose a central atom (usually the one with the lowest electronegativity – will always be carbon if it’s there) Place electrons around atoms so that the octet rule is satisfied If the molecule is an ______________, place brackets around the entire structure and write the charge on the outside of the bracket.

Example: CH 4 H C H H H H H H C H C H H H H A bond is indicated by a dash/line

Example: OH - O H First, count up the total number of valence electrons of all the atoms. H Oxygen has 6 valence electrons Hydrogen has 1 valence electron That equals 7 valence electrons. However, we must take a look at the charge. In this case, it is -1, which means that there is one extra electrons, giving us a grand total of 8 valence electrons. O Now put brackets around the molecule and add the charge on the outside.

Double and Triple Bonds Double bonds occur when there are 4 shared electrons in one spot. Ex: CO2 Triple bonds occur when there are 6 shared electrons in one spot. Ex: CO

Ionic vs. Covalent Bonding Ionic: Transfer of electron(s) from one atom to another. Example is Sodium Chloride, NaCl Cl Na + - H C HHH Covalent: The atoms share electrons. Example is Methane, CH4 H H H C H C H H H H Cl Na

MOLECULAR GEOMETRY

VSEPR Theory __________________ __ Most important factor in determining geometry is relative repulsion between electron pairs. A Molecule adopts the shape that minimizes the electron pair repulsions.

VSEPR charts Use the Lewis structure to determine the geometry of the molecule Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Electron arrangement establishes the bond angles Geometry of the molecule can depend on either the regions of electrons (Electron Pair Geometry) or on the number of atoms (Molecular Geometry). Geometry of the molecule can depend on either the regions of electrons (Electron Pair Geometry) or on the number of atoms (Molecular Geometry). Charts look at the CENTRAL atom for all data! Charts look at the CENTRAL atom for all data! Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region) Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region)

Electron Pair Geometry Count up the total number of regions of bonds and # of lone pairs around the central atom (If double bond or triple bond, it counts as 1) # of Regions of Electrons Electron Pair GeometryBond Angle 2Linear180 3Trigonal Planar120 4Tetrahedral109.5, 107, 104.5

Electron Pair Geometry Examples BeH2 has two regions of electrons, therefore it is linear. CO3 2- has three regions of electrons, therefore it is trigonal planar. H2O has four regions of electrons, therefore it is tetrahedral.

Molecular Geometry Depends on Electron Pair Geometry as well as the number of atoms around the central atom Count up the number of atoms that are connected to the central atom Electron Pair Geometry Bond Angle # of Atoms around Central Molecular Geometry Linear1802Linear Trigonal Planar1202Bent 1203Trigonal Planar Tetrahedral104.52Bent 1073Trigonal Pyramidal Tetrahedral

Molecular Geometry Examples CO2 has two atoms around C, therefore it is linear. NO2 - has two atoms around N, therefore it is bent. H2O has two atoms around O, therefore it is bent.

Bond Angles The angle between atoms Depends on Electron Pair Geometry and Molecular Geometry

Linear Electron Pair Geometry 180° Bond Angle

______________Electron Pair Geometry 120°

Tetrahedral Electron Pair Geometry _______________________________________ Molecular Geometries

Exceptions to the Octet Rule Sometimes there are exceptions, and an atom doesn’t need to satisfy the octet rule (there aren’t enough electrons)…. Sometimes an atom exceeds the octet rule… But don’t worry about these!

Bonding between molecules or atoms in solids or liquids Recall that molecules are farthest apart in gases, but closest together in solids.

Physical and chemical properties depend on the type of bonds involved Ionic compounds typically have ___________boiling points and melting points than molecular compounds, due to the strength of the ionic attraction. Recall that sodium chloride is a solid at room temperature, while carbon dioxide is a gas. vs

Properties of Ionic Compounds ___________ melting/boiling point Dissolve in water Form crystals when solid Conduct electrical current

Why do Ionic Compounds have High Melting Points? Recall that ionic compounds form from oppositely charged ions. This creates strong bonds! Thus a lot of energy is needed to separate the atoms.

Properties of Covalent (Molecular) Compounds Due to weak intermolecular forces, are generally liquids and gases. Conduct little to no electricity Generally have low melting points and boiling points

Properties of Molecular Compounds Vary Covalent bonds differ in terms of how the bonded atoms share the electrons. The number and type of atoms joined together determine the molecular properties. The electrons which make up the covalent bond are being pulled, like a tug-of-war, toward each nucleus.

Nonpolar Covalent Bonds Recall that a magnet has a north and south pole. When the atoms in the bond pull equally, the bonding electrons are shared equally and there are no ‘north or south poles’ formed in the bond. We call this bond _________________. Polarity increases _________________________and therefore increases boiling and melting points. The diatomic elements are nonpolar covalently bonded (e.g. hydrogen) and thus are gases at room temperature (except bromine).

Polar Covalent Bonds Formed when the electrons are shared unequally between atoms Is a result of ________________________ Electronegativity: the ability of an atom to attract electrons when the atom is in a compound (aka how hard it pulls in the tug-of-war) The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge

Describing Polar Bonds In hydrochloric acid (HCl), hydrogen has an electronegativity of 2.1 and chlorine has 3.0. These values are significantly different, so the covalent bond is polar. Chlorine pulls the electrons closer towards itself and becomes slightly negative, leaving hydrogen slightly positive as shown:

Describing Polar Bonds Water is also a polar molecule (elecronegativities H: 2.1, O: 3.5) This explains why most ionic compounds are soluble (can dissolve) in water:

Determining Bond Type Using the electronegativity chart, we can determine which bond type will occur:

Attractions Between Molecules How do the strengths of intermolecular attractions compare with ionic and covelent bonds?  Intermolecular (attraction between molecules) are _____________ than either ionic or covalent bonds. However, these interactions still impact physical properties. They include: Van der Waals forces: Dipole interactions Dispersion forces Hydrogen bonding

van der Waals Forces The two weakest interactions between molecules Named after Dutch chemist Johannes van der Waals Includes: _________________ forces: Weakest of all forces; occurs in all molecules Caused by the motion of electrons Very weak, very temporary attraction between slightly charged regions of a molecule and its neighbours _________________ interaction: Attraction between the slightly charged regions of polar molecules:

Hydrogen Bonding Attractive forces in which a __________________ atom covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom In other words, it is a dipole interaction that involves hydrogen and an electronegative atom (N, O, F, Cl) This is a relatively strong attraction which serves to increase the melting and boiling point of the substances affected by it

Hydrogen Bonding is Responsible for: Surface Tension Ice floating Helical structure of DNA

Intermolecular Attractions and Molecular Properties Recall that the physical properties of a compound depend on the type of bonding it displays – in particular, whether it is ionic or covalent. A great range of physical properties occurs among covalent compounds. The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions. A few solids that consist of molecules break our rules – they will not melt unless at extremely high temperatures or will not melt at all

Network Solids Aka network crystals Solids in which all of the atoms are covalently bonded to each other Melting a network solid would require breaking covalent bonds throughout the solid E.g. diamond

Physical Properties The greater the strength ___________ the bonds (INTRAMOLECULAR FORCES) and _________ the molecules (INTERMOLECULAR FORCES) of a substance, the more energy you need to break those bonds (i.e. to change state by melting or vaporization)

That explains why you see such a variety in physical properties:

Lesson Check! Intramolecular Forces Worksheet

Melting and Boiling Points Dependent on intermolecular forces The stronger the intermolecular force is, the more energy is required to melt or boil a solid or liquid Therefore, intermolecular forces raise the melting and boiling points

Properties of Covalent (Network) Compounds Network = connected in many ways to molecules around it Have high melting and boiling points Cannot conduct electricity

Properties of Metal Compounds High melting points Very good electrical conductors Crystal arrangement Malleable (ability to keep shape without breaking) Dense Shiny