STRUCTURES AND PROPERTIES OF SUBSTANCES

STRUCTURES AND PROPERTIES OF SUBSTANCESCHAPTER 4CHEMICAL BONDINGOnly noble gases exist naturally as single, uncombined atoms.All other atoms: combined. CHEMICAL BONDS: Electrostatic forces that hold atoms together in compounds.- In nature, systems of lower energy (more stable) tend to be favoured over system of higher energy (less stable). - Bonded atoms tend to have lower energies (ie. more favourable!)

LEWIS STRUCTURES (Review)Chemical bonding involves the interaction of valence electrons (outermost electrons)

Lets you see exactly how many electrons are involved in the bond helps you keep track of the number of valence electrons. Two ways to show bonding pairs of electrons:

(Dots represent a lone pair (non-bonding pair) of electrons.)

IONIC BONDINGForce of attraction between oppositely charged ions. Occurs between atoms with large differences in electronegativity (why is this? Periodic Table!)Ionic solid: arranged in a specific sequence of repeating units minimum possible energy. Atoms of ionic compounds usually come from s and p blocks.

PPs, pg. 165#1-4PROPERTIES OF IONIC SOLIDSCrystalline with smooth, shiny surfacesHard but brittleNon-conductors of electricity and heatHigh melting pointsMany are soluble in waterLATTICE ENERGY AND IONIC BONDINGLattice energy: the amount of energy given off when ionic crystal forms from the gaseous ions of its elements. Example: MgF2 has lattice energy= 2957 kJ/mol Same amount of energy needed to break up the ionic crystal. Compounds with larger lattice energies have higher melting points.COVALENT BONDINGBalance between forces of attraction and repulsion that act between the nuclei and electrons of two or more atoms. Optimum separation for atoms at which nucleus-electron attractions, nucleus-nucleus repulsions, and electron-electron repulsions achieve a balance. Results in the sharing of pairs of electrons.Formation of new Orbital: overlapping of atomic orbitals. Lower energy levels than original atomic orbitals.

Characteristics of Covalent BondingIn most cases, covalent bonding allows atoms to acquire noble gas configurations. Hydrogen must fill its s orbital.Carbon must fill its 2s and three 2p orbitals.

Bond EnergyEnergy required to break the force of attraction between two atoms in a bond and to separate them.Measures the strength of a bond.

Increase in bond energy due to increase of (-) charge in between the two nuclei. Therefore, nuclei are more attracted to the overlap orbital.

PREDICTING IONIC AND COVALENT BONDSUsing Physical PropertiesCovalent (molecular) compounds:Exist as a soft solid, liquid, or a gas at RT.Low melting and boiling pointsPoor conductors of electricityMay not be soluble in water.

Using Formula of the CompoundTwo atoms with identical electronegativities share electrons equally. Therefore,the bond is _______________________. Very different electronegativities one atom attracts electrons more. For example, sodium chloride sodiums valence electron has a very high probability of being found near sodium. This bond is ________________.Electron is not actually not lost, gained, or transferred. bonding contiuum

PPs 5-8MetalsProperties:Conduct electricity and heat in both solid and liquid states. Malleable and ductile.BondingDifference in electronegativities not large enough to form ionic bonds.Do not have a sufficient number of valence electrons to form covalent bonds with one another. Electrons are shared but different than covalent.Metallic BondMetals composed of densely packed core of cations. Electrons are shared and mobile can move throughout metal.Force of attraction between the positively charged cations and the pool of valence electrons that move among them metallic bond.

Properties of Metals and the Free-Electron ModelConductivity (heat and electricity): electrons can move freely throughout the metallic structure. Malleability and Ductility: metallic bonds are non-directional. Cations can slide over one another. Melting and Boiling Points: Group 1 metals have lower melting and boiling points of Group 2 greater number of valence electrons and larger positive charge = stronger metallic bonding forces. Transition metals: generally high melting and boiling points.

SR, page. 171#1, 2-7. 4.2 Molecular Shape and Polarity

Co-ordinate Covalent BondsWhen one atom contributes both of the electrons to the shared pair. Occurs when a filled atomic orbital overlaps with an empty atomic orbital. Behaves in the same way as any other single covalent bond.

Resonance Structures: More than one Possible Lewis StructureBonds between S and O are identical: two one-and-a-half bonds.Resonance structures are combinations hybrids if its two resonance structures.Does NOT shift back and forth between bonds.Resonance structures: models that give the same relative position of atoms as in Lewis structures, but show different places for their bonding and lone pairs.

PPs, page 177#9-12.Central Atom with an Expanded Valence LevelExpanded valence energy level: bonding in some molecules is best explained by a model that shows more than eight electrons in the valence energy level of the central atom. Experimental evidence suggests that larger atoms can accommodate additional valence electrons because of their size. Sometimes, must violate the octet rule to allow for more than four bonds around a central atom.PPs, pg. 178#14-17Shapes and Polarity of MoleculesLewis structures do not communicate any information about a molecules shape. VSEPR (Valence-Shell Electron-Pair Repulsion) Theory:Bonding pairs and lone pairs of electrons repel one another. Lone pair (LP) will spread out more than a bond pair repulsion is greatest between lone pairs (LP LP)Bonding pairs (BP) are more localized between nuclei, so spread out less. BP-BP repulsions are smaller than LP-LP repulsions.Repulsion between BP and LP is intermediate.

Electron-Group ArrangementsWhen all electron groups are bonding pairs, a molecule will have one of these shapes. If there are lone pairs, variations of these result.

Molecular GeometryShape of molecule depends on electron pairs.

Predicting Molecular Shape

PPs, 185#18-22Molecular Shape and PolarityRecall:For diatomic molecules: bond polarity is also the molecules polarity.Dipole: term used to describe the charge separation for an entire molecule.

Molecular Polarity/Molecular Shape Table:

A: central atomX: more electronegative than A.Y: more electronegative than X.

PPs, page 188# 23-26SR, page 189.# 1-3, 5, 6

4.3 Intermolecular Forces in Liquids and SolidsIntramolecular Forces: forces exerted within a molecule or polyatomic ion. Intermolecular Forces: forces of attraction and repulsion that act between molecules or ions.also called van der Waals forces.Dipole-dipole forcesIon-dipole forcesInduced dipole forcesDispersion forcesHydrogen bonding

Dipole-Dipole ForcesPolar molecules (dipoles) in liquid form orient themselves so that oppositely charged ends are near to one another.

Polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would.More energy required to separate polar molecules than non-polar of similar molar mass.Higher melting and boiling points.Ion-Dipole ForcesForce of attraction between an ion and a dipole.Reason why most ionic solids are soluble.

Induced Intermolecular ForcesInduced by chargeIon-induced dipole force: When an ion in close proximity to a non-polar molecule distorts the electron density of the non-polar molecule. Molecule becomes temporarily polarized two species are attracted to each otherDipole-induced dipole force:Charge on polar molecule induces the charge on a non-polar molecule.

Dispersion (London) ForcesShared pairs of electrons in covalent bonds are constantly vibrating.Bond vibrations cause momentary, uneven distributions of charge. Act between any particles. Factors of Magnitude:Number of electrons: larger molecules, more uneven distribution of charge. Raise boiling point.Shape of molecule: sphere has less surface area than linear molecule. More linear molecules have higher boiling points than spherical.

Hydrogen BondingStrong form of dipole-dipole attraction that exists between a hydrogen atom in a polar-bonded molecule that contains bonds such as H-O, H-N, H-F, and an unshared pair of electrons on another small, electronegative atom such as O, N, or F. Small, electronegative atom can be on its own, but is usually bonded in a molecule.H-bond is about 5% as strong as a single covalent bond strength in numbers.Ex// DNA

Hydrogen bonding and properties of waterWhy is water less dense in a solid state than in a liquid state?Water molecules align in a specific pattern so that hydrogen atoms of one molecule are oriented toward the oxygen atom of another molecule.

If water molecules behave as most molecules do, lake water would freeze from the bottom up floating ice insulates the water beneath it.

Polar covalent compounds are soluble in waterEx// alcohols (O-H bonds), ammonia (N-H bonds), small mass amines (N-H), etc.

INTRAMOLECULAR AND INTERMOLECULAR FORCES

Bonding in solidsCrystalling solids: organized particle arrangements with distinct shapes: gemstones, etc.Amorphous solids: indistinct shapes: particle arrangements lack order: glass and rubber.

Crystalline solids: 5 typesAtomic MolecularNetworkIonicMetallicAtomic solidsMade up of individual atoms held together by ONLY by dispersion forces noble gases.Very low melting and boiling points.Molecular solidsMade up of molecules.Mainly dispersion forces.Low melting and boiling points.Can be made up of polar molecules, with higher melting and boiling points. Network solidsCarbon-Based Network SolidsAllotrope: different crystalline or molecular forms of the same element that differ in physical and chemical properties.

Read more about network solids on page 192.

IONIC CRYSTALSArray of ions, arranged at regular positions in a crystal lattice. Different and distinct arrangement depending on ions involved: charge and size.Anion is usually larger than the cation, and so it determines the arrangement of ions in the crystal lattice (oranges and kiwis).

AMORPHOUS SOLIDSMolecules that are not arranged in an orderly, crystalline structure. Glass: heat silica sand to melting point (1700 degrees C), add limestone and soda ash, and allow to cool 4000 years ago!

Crazy intermolecular and intramolecular forcesKEVLAR: Thin, lightweight fabric that can withstand startling impacts and piercing forces. Interlaced strands of fibre to make a net.Capable of absorbing large amounts of energy (heat, kinetic, etc.)Bulletproof vests, protective gloves.

Strands of KEVLAR are twisted to makeincrease density and thickness and strandsare woven very tightly.

http://www.youtube.com/watch?v=L-ZbBIhLDUw&feature=relatedaramidsPolymers containing aromatic and amide groups. Strength comes from: intramolecular forces in the aromatic groups that limit bond rotation in the strait chains amide linkage - intermolecular hydrogen bonding between strait chains.

superconductorsa material with no resistance to electrical current none of the energy is given off as heat.Even good conductors, such as copper, give off wasted heat as electric current passes through them resistance.Superconductors are perfect electrical conductors. Future of superconductors: once a current is induced in a circuit, the current continues to flow indefinitely without diminishing.Ability to completely repel magnetic field lines magnet over a superconductor will hover in mid-air.

SR, page 208# 1-3, 7-9. (If you can not answer these questions, look through the section!)