PHY SCI

• Electronegativity (x)


  Measure of the ability of an atom to attract bonding pairs of electrons
• Electronegativity


  Tendency of an atom to pull electrons towards itself in a bond
• Electronegativity difference


  Determines whether a bond is polar or nonpolar
• Linus Pauling


  • Developed a numerical scale of electronegativity of selected representative elements in the periodic table
• The general trend of electronegativity values increases across the periods (i.e. from left to right) and decreases within the groups (i.e. from top to bottom)
• Electron Affinity


  Energy released when an electron is incorporated in an atom
• Electron affinity refers the tendency of individual atoms to attract electrons while electronegativity refers to the same tendency but in a covalent bond
• Noble gases (group 8A) do not usually have electronegativity values since they do not form in bond formation
• Heavier noble gases have been discovered to form covalent compounds at low temperature and high pressure
• Neil Bartlett was able to observe the formation of the very first noble gas compound, xenon tetrafluoride (XeF4)
• Electronegativity values are determined from thermochemical data, and the values vary from one element to another depending on the element's atomic size, number of inner shell electrons, and nuclear charge
• Electronegativity difference (∆x)


  Important mathematical quantity that describes the polarity of a covalent bond
• The electronegativity difference in a bond composed of two similar atoms is zero
• In some cases, even if the bond is composed of two different atoms their electronegativity values may be close to one another leading to a small electronegativity difference
• Most of the time, bonds formed between two different atoms have significant electronegativity difference. In these cases, the electrons are attracted more towards one of the nuclei
• Different periodic trends

  • Ionization energy
  • Electron affinity
  • Electronegativity
• Electronegativity


  Measure of the ability of an atom to attract bonding pairs of electrons
• Linus Pauling was an American chemist who developed a numerical scale of electronegativity of selected representative elements in the periodic table
• The higher the electronegativity value of a specific element, the greater is the attraction of the atoms of that element to the bonding pair of electrons
• Electronegativity difference


  Describes the polarity of the bond they can form
• Polarity


  Measure of the degree of inequality in the attraction of electrons between atoms in a molecule
• The H-Cl bond is POLAR and is characterized by an asymmetrical distribution of electron clouds. The Cl-Cl bond is nonpolar and is characterized by a symmetrical distribution of electron clouds
• Nonpolar bonds


  Described by symmetrical electron clouds between the nuclei
• Partially negative end


  Designated with δ-
• Partially positive end


  Designated with δ+
• In HCl bond, the electrons are more attracted towards Cl, resulting in a partially negative charge (δ-) at this end. As a consequence, H acquires a partially positive charge (δ+)
• Other ways of representing bond polarity is the electrostatic potential map which represents a bond with its electron density and use colors to represent partial charges. In this diagram red and orange colors are used to represent electron rich (𝛿-) areas, whereas blue areas represent electron poor (𝛿+) regions
• Electronegativity Difference and Bond Polarity


  • Nonpolar – 0 to 0.4
  • Polar – 0.4 to 1.7
  • Ionic – greater than 1.7
• Dipole moment


  Quantity that describes the polarity bond in a molecule
• Polar molecule


  Dipole moment do not add up or do not cancel out; a net dipole moment is present
• Polar bonds in asymetrical arrangement: bent (H2O), trigonal pyramidal (NH3), T-shape (CIF3)
• Non Polar molecule


  Dipole moment cancel out
• Symmetrical linear, trigonal planar, square planar, tetrahedral are examples of non-polar molecules
• Polar covalent bond


  A shared pair of electrons is held more closely to one atom
• Non-polar covalent bond


  The electrons are shared equally
• Polarity has direct effect on solubility and miscibility
• Liquids are miscible with liquids of similar polarity
• Polar compounds will dissolved other polar compounds, but not non-polar compounds
• The greater the polarity


  The greater the attraction between molecule, the greater energy to separate them into gaseous form, the higher the boiling point
• Molecular Polarity


  Depends on the polarity of the bonds and the shape of the molecule