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Forces Of Attraction

Forces Of Attraction






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    Forces Of Attraction Forces Of Attraction Presentation Transcript

    • Forces of Attraction
    • Introduction
      • Atoms aggregate to form molecules and lattice
      • Molecules aggregate to form condensed phases of matte
    • Aggregation of atoms, appositively charged ions and molecules is a consequence of
      • Electrical forces exerted on the electrons of one particle by the nucleus (or nuclei) of the other
      • www.wpi.edu/academics/Depts/chemistry/course/general/concept6.html.
    • Two broad categories of forces of attraction
      • Intramolecular- forces that exist within molecules or fundamental/formula units
      • (forces that hold atoms or ions in a compound)
      • Intermolecular- forces of attraction that exist between the molecules in a compound
    • Intramolecular Forces
      • Three types
      • Ionic
      • Covalent
      • metallic
    • Ionic or Electrovalent Bond
      • Electrostatic force of attraction between ions of opposite charge
      • Ions are formed from atoms as a result of electron transfer from one atom to another
      • Formed as a result of a large difference in electronegativity of atoms
      • Formed between metals and non-metals
      • Electronegativity- the ability of an atom to attract (pull) electrons to itself.
      • Electrostatic force depends on the charge on the ions.
    • Properties
      • Crystalline solids- rigidity and strength
      • High melting and boiling points
      • Conduct electricity in molten and aqueous state
      • They are hard
      • They are brittle
      • Soluble in polar solvents such as water (solute-solvent interactions)
    • Covalent Bonds
      • Formed between atoms with a small difference in electronegativity
      • Formed by the overlapping of atomic orbital.
      • Molecular orbital result
      • Electrons are shared between nuclei of the two atoms
      • The attraction between the shared electrons and the nuclei that holds the molecule together
      • Two types of bonds are formed
      • Either sigma bonds
      • Or pi bonds
    • Sigma bonds
      • Three possible ways for a sigma bond to be formed
      • Overlapping of two S-orbital
      • Overlapping of an S and a P-orbital
      • Head to head overlapping of two P-orbital
    • Pi bonds
      • The side to side overlapping of two P orbital.
      • Electrons in this bond are delocalized
      • The electron density is above and below the plane of the sigma bond.
      • These bonds make a compound reactive
      • example in alkenes with the c-c double bond.
    • Properties of Covalent Compounds
      • Liquids and gases at room temperature
      • Relatively low boiling point.
      • Do not conduct electricity
      • Insoluble in polar solvent
      • Soluble in non-polar solvent
    • Dative (Coordinate) covalent Bond
      • One atom donates both electrons to form a covalent bond
      • Recall- in a normal covalent bond each atom donates an electron to be shared.
      • The atom donating the electrons must have at least one lone pair of the electrons.
      • The other atom must have an available empty orbital to accommodate this electron pair
      • example, boron trifluoride, aluminium trichloride.
      • can bond with say ammonia.
    • Metallic Bonds
      • Positive ions surrounded by a sea of mobile (delocalized) electrons.
      • Strong electrostatic force of attraction binds the system together
      • Attraction between valence electrons and metal ion
    • What influences the strength of the bond?
      • Availability of electrons
      • - More available delocalized electrons, the stronger the electrostatic attraction, the stronger the metallic bond.
      • Size of the charge on metal ion
      • Larger charge size, stronger the metallic bond. Explain.
      • example Al and Na
      • hard metal and soft metal
    • Properties of metals
      • Hardness- Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion
      • Brittleness- Brittleness is the property of a metal that allows little bending or deformation without shattering
      • Malleability- A metal that can be hammered, rolled, or pressed into various shapes without cracking or breaking or other detrimental effects is said to be malleable.
      • Ductility- Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking
      • Elasticity- Elasticity is that property that enables a metal to return to its original shape when the force that causes the change of shape is removed.
      • Toughness
      • Density
      • Fusibility
      • Conductivity- Conductivity is the property that enables a metal to carry heat or electricity
      • Contraction
      • Expansion
    • Polar Covalent Bonds and Dipole Moments
      • Polar bonds- formed between atoms of different electronegativity (EN)
      • - example, chlorine-carbon bond
      • * chlorine is more EN than carbon
      • * chlorine attracts the shared electron pair
      • to itself.
      • * C-Cl bond is polarized (delta negative
      • and delta positive
      • Polarized bonds have dipole moment.
      • Dipole- separation of charge within molecules
    • Intermolecular Forces
      • What are the forces between one chlorine molecule and another?
      • Intermolecular forces- forces between molecules or ions and influence their properties.
      • Molecular polarity gives rise to the forces of attraction between molecules
    • characteristics
      • These forces are electrical – result from mutual attraction or mutual repulsion.
      • Generally very weak forces of attraction
      • Responsible for the states of matter
    • Characteristic Features
      • Attractions exerted by one molecule of a molecular substance on another, such as the force of attraction between water molecules in ice.
      • Attractions between molecules of one substance and molecules of another, as when two liquids are mixed, or a molecular solid such as sugar is dissolved in a liquid.
      • Attractions between atoms of the noble gas elements, helium through radon.
      • Attraction between molecules of one substance and ions of another, as when an ionic compound dissolves in a liquid.
    • Types of I.M.F
      • Ion-dipole
      • Dipole-dipole
      • London dispersion forces
      • Hydrogen bonds
    • Ion-Dipole Forces
      • Result from electrical interactions between an ion and the partial charges on a polar molecule.
      • Dipolar molecule- a substance with both a positive and negative ends
      • In the presence of ions dipolar molecules orient themselves with positive end of dipole near the anion and negative end near cation
      • Magnitude of interaction depends on charge.
      • example: NaCl in water (ionic substance in dipolar water molecules).
      • Dipole-Dipole forces- Intermolecular forces that operate between neutral molecules having molecular dipole moments are called dipole-dipole forces
      • Result from interactions among dipoles on neighbouring molecules.
      • The more polar the substance the , the greater the strength of its dipole-dipole interactions.
      • The stronger the I.M.F that must be overcome for a substance to boil or melt.
    • Permanent Dipole
    • Inductive Forces and Dispersion
      • Inductive forces arise from the distortion of the charge cloud induced by the presence of another molecule nearby.
      • Distortion arises from the electric field produced by the charge distribution of the nearby molecule.
      • These forces are always attractive but shorter ranged than electrostatic forces.
      • If a charged molecule (ion) induces a dipole moment in a nearby neutral molecule, the two molecules will stick together, even though the neutral molecule was initially round and uncharged
    • Dipole-Induced Dipole
      • What would happen if HCl is mixed with argon, which has no dipole moment?
      • - The electrons on an argon atom are distributed homogeneously around the nucleus of the atom.
      • - Electrons are in constant motion.
      • Argon close to a polar HCl molecule, the electrons can shift to one side of the nucleus to produce a very small dipole moment that lasts for only an instant.
      • Distorting the distribution of electrons around the argon atom, the polar HCl molecule induces a small dipole moment on this atom.
      • A weak dipole-induced dipole force of attraction between the HCl molecule and the Ar atom is created
    • Dipole-Induced Dipole
    • Induced Dipole- Induced Dipole
      • Some atoms are perfectly symmetrical.
      • No dipole exist
      • Some forces must exist
      • Atoms and Molecules such as; the noble gases, the halogens etc.
      • Electrons are in constant motion.
    • Example: Helium atom
      • Movement of the electrons around the nuclei of a pair of neighboring helium atoms can become synchronized so that each atom simultaneously obtains an induced dipole moment.
      • There are fluctuations in electron density occurring constantly.
      • Creating an induced dipole-induced dipole force of attraction between pairs of atoms.
      • This force is relatively weak in helium.
      • Atoms or molecules become more polarizable as they become larger because there are more electrons to be polarized.
    • Induced Dipole- Induced Dipole
    • Hydrogen Bonding
      • Hydrogen Bonding- a special kind of dipole-dipole force that occurs when a hydrogen atom is bonded to one of the very electronegative atoms, F, O, or N.
      • Electronegative atom must have at least a lone pair of electrons
    • Hydrogen Bonding
      • Combination of forces
      • - normal covalent bond
      • - dipole-dipole interaction
    • Hydrogen Bonding
      • The H-F, H-O, and H-N bonds are very polar, because the electronegative atom draws the bonding electron pair strongly to itself. This leaves the hydrogen nucleus exposed
    • Hydrogen Bonding in water
    • Summary I.M.F
    • Summary of Forces of Attraction
      • http://www.chem.ufl.edu/~itl/2045/lectures/lec_g.html