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Forces Of Attraction

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  • 1. Forces of Attraction
  • 2. Introduction
    • Atoms aggregate to form molecules and lattice
    • Molecules aggregate to form condensed phases of matte
  • 3. 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.
  • 4. 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
  • 5. Intramolecular Forces
    • Three types
    • Ionic
    • Covalent
    • metallic
  • 6. 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
  • 7.
    • Electronegativity- the ability of an atom to attract (pull) electrons to itself.
    • Electrostatic force depends on the charge on the ions.
  • 8. 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)
  • 9. 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
  • 10.
    • Two types of bonds are formed
    • Either sigma bonds
    • Or pi bonds
  • 11. 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
  • 12. 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.
  • 13. 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
  • 14. 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.
  • 15.
    • The other atom must have an available empty orbital to accommodate this electron pair
    • example, boron trifluoride, aluminium trichloride.
    • can bond with say ammonia.
  • 16. 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
  • 17. What influences the strength of the bond?
    • Availability of electrons
    • - More available delocalized electrons, the stronger the electrostatic attraction, the stronger the metallic bond.
  • 18.
    • Size of the charge on metal ion
    • Larger charge size, stronger the metallic bond. Explain.
    • example Al and Na
    • hard metal and soft metal
  • 19. 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
  • 20.
    • 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.
  • 21.
    • Ductility- Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking
  • 22.
    • 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
  • 23.
    • Density
    • Fusibility
    • Conductivity- Conductivity is the property that enables a metal to carry heat or electricity
    • Contraction
    • Expansion
  • 24. 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
  • 25.
    • Polarized bonds have dipole moment.
    • Dipole- separation of charge within molecules
  • 26. Intermolecular Forces
  • 27.
    • 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
  • 28. characteristics
    • These forces are electrical – result from mutual attraction or mutual repulsion.
    • Generally very weak forces of attraction
    • Responsible for the states of matter
  • 29. Characteristic Features
    • Attractions exerted by one molecule of a molecular substance on another, such as the force of attraction between water molecules in ice.
  • 30.
    • 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.
  • 31.
    • 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.
  • 32. Types of I.M.F
    • Ion-dipole
    • Dipole-dipole
    • London dispersion forces
    • Hydrogen bonds
  • 33. 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
  • 34.
    • 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).
  • 35.
    • 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.
  • 36.
    • 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.
  • 37. Permanent Dipole
  • 38. 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.
  • 39.
    • 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
  • 40.  
  • 41. 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.
  • 42.
    • 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.
  • 43.
    • 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
  • 44. Dipole-Induced Dipole
  • 45. 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.
  • 46. 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.
  • 47.
    • There are fluctuations in electron density occurring constantly.
    • Creating an induced dipole-induced dipole force of attraction between pairs of atoms.
  • 48.
    • 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.
  • 49. Induced Dipole- Induced Dipole
  • 50. 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
  • 51. Hydrogen Bonding
    • Combination of forces
    • - normal covalent bond
    • - dipole-dipole interaction
  • 52. 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
  • 53.  
  • 54. Hydrogen Bonding in water
  • 55. Summary I.M.F
  • 56. Summary of Forces of Attraction
  • 57.  
  • 58.
    • http://www.chem.ufl.edu/~itl/2045/lectures/lec_g.html

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