Formation of Interstellar Polyatomic Substances

1. Formation of Interstellar Polyatomic Substances Lewis Structures, Chemical Bonding, Formulas, Equations

3. Stability of Atoms The neutral atom is the stable form for only the inert gases All other atoms undergo chemical reactions so as to attain a more stable electron structure (usually that of the inert gas nearest in atomic number, Z)

4. Lewis Symbols Method of �bookkeeping� for valence electrons Lewis symbol -valence electrons represented by dots around the chemical symbol for an element For A-families (Main Group elements) the number of valence electrons = Group (or family) number

6. Chemical Bonding Chemical bonding - Rearrangement of electron structures of 2 or more atoms Bonding occurs so that each atom attains a more stable electron structure Most common stable electron structure for Main Group elements is that of an inert gas, usually 8e (Octet Rule), except 2e for H, He, and Li

7. Ionic Bonding Occurs when a metal combines with a non-metal Sufficient valence electrons are transferred from the metal to the non-metal so that each atom attains an octet (or duet for H, Li, and Be)

10. Covalent Bonding Occurs when non-metal atoms combine Pairs of electrons are shared between non-metal atoms so that each atom attains an octet (or duet for H) A shared pair of electrons is called a bonding pair (see hydrogen molecule)

14. Metallic Bonding Simplest model is Electron Sea Model - metal atoms lose valence electrons to the crystal producing cations of the metal surrounded by delocalized electrons - electrons free to move Delocalized electrons account for electrical and thermal conductivity Represented by empirical formula

16. Network Covalent or Atomic Substances It is possible for many atoms to link up to form a giant covalent structure or lattice The atoms are usually non-metals This produces a very strong 3-dimensional covalent bond network or lattice The diamond is an example - shown on next slide

19. Chemical Formulas Ionic, atomic, and metallic substances are represented by an empirical formula -subscripts indicate the ratio in which the atoms have combined Molecular covalent substances are represented by a molecular formula - subscripts indicate the number of atoms of each type in one molecule of the compound

21. Molecular Weight and Mole The sum of the atomic weights of all the atoms in a formula is called the molecular weight (MW). MW of H2O = 2(1.008 amu) + 16.00 amu = 18.02 amu MW expressed in grams,18.02 g, is the mass of one mole of water or 6.02 x 1023 molecules of water

22. Chemical Equations A chemical equation is a method of representing a chemical change in terms of formulas for stable forms of the substances (all atoms have attained octets or duets of electrons) General form: A + B => C + D, where A and B are formulas for reactants and C and D are formulas for products

23. Writing Chemical Equations Write formulas for stable reactants and products in equation in the general form - called skeleton equation Balance skeleton equation by inserting coefficients so as to obtain the same number of atoms of each element on both sides of the equation using fractions if necessary

24. Writing Equations (con�t) Multiply coefficients by appropriate factor to clear fractions and obtain the lowest set of integers as coefficients Omit coefficients of 1 in final equation Balance: H2 + O2 => H2O 1 H2 + 1/2 O2 => 1 H2O 2 H2 + O2 => 2 H2O

27. Interstellar Substances Atoms in space combined to form both ionic and covalent substances Many of the substances formed in space do not follow the Octet Rule observed on earth due to the relatively small numbers of collisions of atoms possible in space