The Periodic Table 1. 2. The elements in columns have very similar chemical properties. The first column, the alkali metals are extremely reactive in air and are seldom found uncombined. 3. Elements in the second column, the alkaline earths are considerably less reactive. 4. Elements in the first 12 columns are considered metals. 5. The non-metals include all the elements included in the columns listed above the diagonal defined by the elements of B, Si, As, Te and At. 6. The last column contains the inert gases. While most elements in the periodic table are found combined with each other, the inert gases are found uncombined and monatomic. 7. The number at the top of each square is called the atomic number. This number defines the number of protons in the nucleus of each atom. The nucleus, which consists of protons positive charged particles) and neutrons (neutral particles) make up better than 99% of the mass of an atom. The sum of the number of protons and neutrons is called the atomic weight. This number is found at the bottom of each square in the periodic table. Atoms that have the same number of protons but different number of neutrons are called isotopes. Isotopes have the same chemistry but different masses. The atomic weights listed in the periodic table are weighted averages of all the isotopes of each element. In a neutral atom, the number of electrons that surround the nucleus equals the atomic number. The volume the electrons occupy make up more than 99+% of the volume of an atom. 8. The chemistry of the elements in the periodic table is governed by the number of electrons that surround the nucleus. Much of the chemistry of the elements can be understood in terms of their desire to mimic the electronic environment found in the inert gases. He 2 electrons Ne 2) 8 electrons Ar 2) 8) 8 electrons I. Types of bonding found within molecules (molecule: two or more atoms bound together to form a single entity) 1. metallic bonding: typically found in various metals; usually characterized by good conductivity; bonding viewed as metal ions immersed in a sea of electrons. ex: metals such as sodium, iron, magnesium 2. ionic bonding: typically found in salts; usually characterized by good conductivity when dissolved in water, bonding viewed as transfer of electrons from a metal to a non-metal to form a cation (+) and anion (-). ex: table salt: NaCl; NaF 3. covalent bonding: typically found in compounds formed from non-metals; non-conducting, organic molecules. ex: sugar, cinnamaldehyde (oil of cinnamon); eugenol II. Examples of ionic and covalent bonding 1. Ionic bonding that results from the transfer of one or more electron(s) . Na + .. :Cl : . Na+ .. + : Cl : .. Examples of compounds with ionic bonding KCl CaCl2 NaI 2. Covalent bonding that results from sharing of two or more electrons (note that a sharing of two electrons is denoted by a line _ , sharing of four electrons by two lines (=)). Case A. each partner contributes equally H . + .. :Cl : . .. H __Cl : .. Note: two elements may contribute equally but the electrons may not be shared equally. Examples of covalently bound molecules where each partner contributes equally H2 NH3 C2H4 H2O C2H6 C2H2 H2S O2 C3H8 CH4 N2 C3H6 Case B. Unequal contribution of electrons .. S: .. - + .. .. .. _ :O .. S = O . + 2 :O: . Case C. A combination of both ionic and covalent bonding .. :O : . 2 Na . + 2 :O: . + . . C. . 2 Na+ -2 .. O =C .. :O .. : Rule 1. In order to understand bonding, remember hydrogen wants a total of 2 electrons in the electron hotel and most of the remaining elements of interest to us want a total of eight. See if you can draw structures for each of the following compounds that satisfy this rule. CO2 C3H6 NaHCO3 C2H7N C2H6O C2H6S Rule 2. The geometry of groups about a central atom (a group is either a pair of electrons not involved in bonding or another atom; this other atom may be a single atom or have other groups attached to it) Central atoms with two groups attached: Central atom with three groups attached: Central atom with four groups attached: Predict the geometry of the following molecules: BCl3 NH3 CO2 CH4 Exercises 1. Try to make models for each of the following molecules. First draw a structure that satisfies Rules 1 and 2. Then make the model. More than one correct answer is possible for some compounds. CH4O C2H6 H2S C3H8 C2H6O CH5N Summary of Rule 2. i. Central atoms containing two groups (groups as defined in Rule 1) are linear; ii. central atoms with three groups are planar and triangular (the type of triangle depends on the nature of the groups; iii. central atoms with four groups attached are tetrahedral.