Module 5

• Octet Rule: This is a chemical rule of thumb based on the observation that atoms of main group elements (representative elements) tend to combine in such a way that each atom has eight electrons in its valence shell.
• Octet Rule: This rule explains why atoms combine to form molecules and ions as a tendency for an atom to achieve electronic stability like the inert gases — that is, to have eight electrons in its outermost shell.
• Hydrogen - the exception in octet rule
• The chemical reactivity and the bonds that hold atoms together in molecules and crystals are due to the number of outer electrons (valence electrons) in any element.
• Atomic number - the basis for ordering of the elements
• Periodic Law: The order of the elements shows a periodic recurrence of properties of elements, such that elements with similar properties are aligned in vertical columns.
• Atomic size - determined by the covalent radius
• covalent radius - half the distance between the nuclei of identical atoms when they are joined by a single covalent bond
• Ionization Energy (IE) or Ionization Potential - the minimum energy needed to remove the outermost electron from the neutral atom in the gaseous state
• Electron Affinity - the energy released when a neutral atom in the gaseous state accepts an electron in its outermost shell to become a negatively charged ion (anion).
• Electronegativity - the measure of the ability of an atom in a chemical compound to attract electrons from another atom in a compound.
• Metallic Property - the tendency of an atom to lose electrons during chemical combination
• Nonmetallic Property - the tendency of an atom to gain electrons during chemical combination
• The periodic properties

  1. Electron affinity
  2. Ionization Energy
  3. Electronegativity

  increases in the periodic table from left to right.
• The periodic properties

  1. Electron affinity
  2. Ionization energy
  3. Electronegativity

  decreases in the periodic table from top to bottom.
• When the atoms of elements combine, chemical bonds are formed.
• Chemical bonding refers to interactions that account for the association of atoms into molecules, ions, crystals, and other substances.
• Through chemical bonding that compounds are formed from elements.
• A Lewis structure is a structural representation of a molecule where dots are used to show the position of electrons around the atom(s) and lines or dot pairs are used to represent covalent bonds between atoms.
• The types and ratio of atoms that combine, and how the atoms are connected to one another in a molecule, and the nonbonding pairs of electrons, if present, can be deduced from the Lewis structure.
• Metals easily give away their valence electrons due to their low ionization energy, whereas the nonmetals keep them due to high ionization energy.
• During combination, the metal transfers its valence electrons to the nonmetal, and because atoms are electrically neutral, the metal, after giving away its valence electrons, becomes positively charged, and the nonmetal, after accepting the electrons, becomes negatively charged.
• Ionic bonding - bond between metal and nonmetal
• In ionic bonding, both the metal and nonmetal achieve electronic stability like the inert gases by electron transfer.
• In ionic bonding, the bond that holds the two oppositely charged ions is called an ionic bond.
• metal + nonmetal = ionic compounds
• Examples of ionic compounds are sodium chloride (NaCl, table salt) and ferric oxide (Fe2O3, red iron oxide, rust).
• Atoms of nonmetals combine by sharing electrons.
• The atoms of nonmetals are of high ionization energy and high electronegativity that do not give away their electrons. That is why during combination they simply share their valence electrons.
• The sharing of electrons of nonmetals generates a chemical bond called covalent bond, and the compound formed is called a covalent compound or a molecular compound.
• Examples of covalent compounds are O2 , H2O, glucose, and sucrose.
• Water, the liquid of life, is considered one of the most important chemical compounds on earth.
• Water is a covalently bonded molecule composed of two hydrogen atoms and one oxygen atom.
• Water is formed when two hydrogen atoms combine with one atom of oxygen by sharing their valence electrons.
• There are two covalent bonds (bonding electrons) and two nonbonding pairs (lone pairs) in water.
• A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically.
• Water is a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The polarity is a result of the difference in the electronegativity of oxygen atom and hydrogen atom.
• Electric dipole - a separation of positive and negative electric charge
• Because the amount of charge separated in such dipoles is usually smaller than a fundamental charge, they are called partial charges, denoted as δ+ (delta plus) and δ− (delta minus).
• In a polar bond, the dipoles do not cancel out due to an asymmetrical distribution of charge resulting in a net dipole.