Chapter 6: Ionic and Molecular Compounds

Transcription

1 Chapter 6: Ionic and Molecular Compounds 1. Recall that the group number indicates the number of valence electrons for any element in that group. Each noble gas has eight valence electrons (except helium, which has two). 2. An electron arrangement of eight valence electrons is unusually stable. 3. The octet rule: Most atoms react to achieve a total of eight valence electrons in their valence shell. 4. Atoms gain or lose electrons to achieve a valence octet. 5. If an atom gains or loses electrons to form a valence octet, the new particle would have an unequal number of protons and electrons and would have a net charge. 6. An ion is an atom that has gained or lost electrons and has a charge. Feb 3 9:56 AM Group 7A elements, like chlorine, have seven valence electrons, gain an electron to have an octet, and form an ion with a 1 charge (ions have 17 protons and 18 electrons). An anion is any atom that contains a net negative charge given that it has an unequal number of protons and electrons. NOTE: When chlorine forms an ion it has the same number of valence electrons and total electrons as argon. Feb 3 11:28 AM 1

2 Group 1A elements, like sodium, have one valence electron. In order to have an octet, the elements in this group must lose one electron to form ions with a 1+ charge (ions have 11 protons and 10 electrons). A cation is any atom that contains a net positive charge, given that it has an unequal number of protons and electrons. NOTE: When sodium forms an ion it has the same number of valence electrons and total electrons as argon. Feb 3 11:28 AM Trends in Ion Formation 1. How do we determine which atoms will gain electrons to form anions and which atoms will lose electrons to form cations? 2. Metals lose electrons to form cations and nonmetals gain electrons to form anions. 3. The main group metals in Groups 1A, 2A, and 3A form cations. Main group nonmetals in Groups 5A, 6A, and 7A form anions. The main group metals form cations with charges of 1+ to 3+, whereas the main group nonmetals form anions with charges of 1 to 3. Feb 3 11:29 AM 2

3 Electron Configuration of Cl atom: Electron Configuration of Cl ion Electron Configuration of Na atom: Electron Configuration of Na+ ion Feb 3 11:29 AM Ionic Compounds Ionic compounds consist of positive and negative ions. have attractive forces between the positive and negative ions called ionic bonds. have high melting and boiling points. are solid at room temperature. Salt Is an Ionic Compound Sodium chloride, or table salt, is an example of an ionic compound. Formulas of Ionic Compounds The chemical formula of an ionic compound represents the element symbols and subscripts, which represent the lowest whole number ratio of ions. In the formula of an ionic compound, the sum of positively and negatively charged ions is always zero. the charges are balanced. Feb 3 11:29 AM 3

4 Charge Balance for NaCl, Salt In NaCl, a Na atom loses its valence electron. a Cl atom gains an electron. the symbol of the metal (sodium) is written first, followed by the symbol of the nonmetal (chlorine). Charge Balance In MgCl 2 In MgCl2, a Mg atom loses two valence electrons. two Cl atoms gain one electron each. subscripts indicate the number of ions needed to give charge balance. the symbol of the metal (magnesium) is written first, followed by the symbol of the nonmetal (chlorine). Feb 3 11:48 AM Crisscross method: quick way to determine formula for ionic compound When using the crisscross method, remember: Transfer the NUMBER but NOT the sign of the charge from one element to the other Reduce to the lowest whole number ratio Type I: Naming Binary Ionic Compounds 1. Determine the oxidation # Al +3 Cl 1 2. Make sure the compound is NEUTRAL no charge because same # electrons will be donated & accepted. 3. Use the crisscross method Al +3 Cl 1 4. Reduce to lowest whole number ratio More Examples: Na 1+ Cl 1 Na 1 Cl 1 NaCl sodium chloride Ca 2+ F 1 CaF 2 CaF 2 calcium fluoride Li 1+ O 2 Li 2 O 1 Li 2 O lithium oxide Practice with this website! Feb 3 12:26 PM 4

5 Here are two examples where you must reduce Practice Magnesium oxide Lithium sulfide Potassium phosphide Aluminum oxide Feb 3 12:26 PM MgCl 2 magnesium chloride K 2 O potassium oxide Al 2 O 3 aluminum oxide InF 3 indium fluoride Rb 3 N rubidium nitride Cs 4 C cesium carbide What is the rule for naming binary ionic compounds? Select the correct formula for each of the following ionic compounds. 1. Na + and S 2 A. NaS B. Na 2S C. NaS 2 2. Al 3+ and Cl A. AlCl 3 B. AlCl C. Al 3Cl 3. Mg 2+ and N 3 A. MgN B. Mg 2N 3 C. Mg 3N 2 Feb 3 12:26 PM 5

6 Type II: Naming Binary Ionic Compounds containing metals with more than one charge 1. Transition metals form more than one ion. For example, iron will form the Fe 2+ and Fe 3+ cation. Copper will form the Cu + and Cu 2+ cation. Roman numerals are used to indicate the charge. 2. The metal always goes first. The name of the compound is [name of metal] + [(roman numeral)] + [non metal root] + [ide]. Feb 3 12:26 PM Practice with transition metals Iron(II) oxide Iron(III) oxide Gold(I) sulfide Gold(III) sulfide SnS SnS 2 CuO Cu 2 O Feb 3 1:52 PM 6

7 Polyatomic ions are a special group of ions formed when two or more nonmetal elements interact with one another. The ions formed have a single charge. Feb 3 1:52 PM Criss cross rule applies use parenthesis when using polyatomic ions Na 1+ PO 4 3 Na 3 (PO 4 ) K 1+ SO 4 2 K 2 SO 4 Mg 2+ CO 3 2 after reducing Mg(CO 3 ) Al 3+ NO 3 1 Al(NO 3 ) 3 NH 4 1+ CO 3 2 (NH 4 ) 2 CO 3 Naming compounds with polyatomic ions: Na 3 PO 4 sodium phosphate NH 4 Cl ammonium chloride K 2 SO 4 potassium sulfate Mg CO 3 magnesium carbonate Al(NO 3 ) 3 aluminum nitrate Feb 3 1:52 PM 7

8 Type III: Molecular Compounds (NM NM) 1. A molecular formula identifies all the components in a molecule of a covalent compound. 2. Table sugar, sucrose (molecular formula C 12 H 22 O 11 ) has 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. 3. Molecular formula only tells us the number of atoms in a molecule not how they are joined or what the structure is. Feb 3 1:52 PM Naming Molecular Compounds 1. name the first element 2. use the stem of the second element; add ' ide" 3. precede each name with a Greek number prefix to indicate the number of each kind of atom. ( if the first element has only atom no prefix is needed) Feb 4 7:16 AM 8

9 Name the following compounds. PH 3 H 2 O SO N 2 O 3 Write the formula for following compounds. 1. sulfur dioxide 2. dihydrogen monoxide 3. phosphorus pentafluoride 4. carbon tetrachloride Feb 4 7:16 AM Electron Dot Formulas of Molecular Compounds Bonding pairs the shared electrons Lone pairs the none bonding pairs of electrons Feb 4 7:09 AM 9

10 State the number of valence electrons, bonding pairs and lone pairs in each of the following electron dot formulas Feb 4 7:16 AM Naming Hydrocarbons Alkanes Hydrocarbons having single bond functional groups are classified as alkanes where the carbon atoms of the molecule are arranged in chains. These formulas all fit the CnH2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon. Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen). Feb 4 7:40 AM 10