Formula Writing

Week Five October 11-October 15, 2010

Lecture

Valence, oxidation numbers, nomenclature, and formula writing

Combining Capacity of Elements

1.     The early 1800’s saw Berzelius working with the concept of combining capacities of elements.

2.     The mid 1800’s saw the concept of “valence” come into use. The valence of an element is the number of atomic weights of hydrogen, sodium, or chlorine that combine with 1 atomic weight of the element. OR The valence of an element is the number of atoms hydrogen, sodium, or chlorine that combine with 1 atom of the element.

3.     Valence, then, is a number that describes the combining capacity of an element.

4.     Examples: HCl   CaCl2   AlCl3   NaH   CH4    CCl4    Na2O    CaO    Al2O3

5.     Fixed valences- about 1/5 of all elements have fixed valences

a.     sodium, calcium, and aluminum

b.     write the formulas for the hydride -1, chloride -1, oxide -2, and sulfide -2 of each

6.     Variable valences-about 4/5 of all elements have variable valences

a.     mercury, copper, iron are examples

 

 

Oxidation Numbers

1.     The oxidation number is related to valence. It is used to designate the positive or negative nature of the atoms in a compound.

2.     Ionic compounds are formed when atoms lose or gain electrons. If electrons are lost, the atoms of the element acquire a positive charge. If electrons are gained, the atoms of the element acquire a negative charge.

a.     In ionic compounds, positive oxidation numbers are used for the combining capacity of the elements that become positive ions while negative oxidation numbers are used for the combining capacity of the elements that become negative ions

b.     The oxidation number is the same as the ionic charge on the ion.

                                                             i.      NaCl   Na = +1  Cl = -1

                                                           ii.      BaO    Ba = +2    O = -2

c.      Polyatomic ions are groups of atoms , two or more of which carry a positive or negative charge, which means that the ion has more or less electrons than would be had by the corresponding neutral atoms of the group.

3.     Covalent compounds are formed when atoms share electrons. The less negative element has a positive oxidation number while the more negative element has a negative oxidation number. For example   HCl      H = +1    Cl = -1

4.     The oxidation number of an element in its elementary form as an atom or as a molecule (Cu, Fe, H2, O2, Al) is always 0.

 

 

Writing Compounds

1.     The Summation Zero Rule

a.     The positive oxidation numbers and the negative oxidation numbers of the atoms present in a neutral compound always add up to zero.

b.     H3(PO4)   H = (3 X +1)   O = (4 X -2)   P must equal -5

2.     The Summation Charge Rule

a.     In a polyatomic ion, the sum of all of the positive and negative oxidation numbers of the elements must equal the charge on the ion (ionic charge)

b.     PO4 has an ionic charge of -3. Since O is -2 and there are 4 of them, that equals -8, meaning the phosphorous must be +5.

Nomenclature of Chemical Compounds

 

1.     Chemical compounds are given systematic names, which reflect the formula of each compound.

2.     Trivial names have to be memorized since they have no relationship at all to the formula- water, ammonia, sugar

3.     See handout

 

 

The Periodic Table Introduction

1.     When the elements are arranged in order of increasing atomic number, every ninth element exhibits very similar physical and chemical properties.

2.     They can be placed in vertical columns which are labeled I through VII and ending with O for the last column.

3.     The first member of each group is the least like the other members of its group.

4.     Elements of Group I are called the Alkali Metals

a.     They are soft with low densities and low melting points

b.     They are the most electropositive elements with an oxidation number of +1

5.     Elements of Group II are the Alkaline-Earth Metals

a.     They are harder with higher melting point

6.     Elements of Group III are all metallic but boron can go either way

7.     Groups I, II, and III are metallic and are more strongly so within each group as we go down the group.

8.     Elements of Group VII are called Halogens

9.     Elements of Group O are called Inert Gases

10.                        Groups VI and VII elements are all non-metals and become more strongly so as we proceed up each group.

 

 

Chemical Reactions and the Periodic Table

Group I elements lose 1 electron when reacting and have a fixed oxidation number of +1.

Group II elements lose 2 electrons when reacting and have a fixed oxidation number of +2.

Group III elements lose 3 electrons when reacting and have a fixed oxidation number of +3.

Group VI elements take on 2 electrons when reacting and have a fixed oxidation number of -2.

Group VII elements take on 1 electron when reacting and have a fixed oxidation number of -1.

Group O elements have a complete outer shell and do not react.

Group I and Group II elements react with Group VI and Group VII elements to form ionic compounds (atoms are given up and taken).

Group IV through Group VII tend to form covalent compounds.

Covalent compounds are more truly covalent when the two elements are in the same group, when the two elements are closer together in the same group, and when different groups are involved, when the two groups are closer together.

The maximum positive oxidation number of an atom of an element corresponds to its group number.

The maximum negative oxidation number of a non-metal element corresponds to its group number minus 8.

 

 

 

Formulas and Names of Binary Metal-Nonmetal Compounds

1. The name of the metal is first (ie: NaCl, sodium chloride)
2. The name of the nonmetal has -ide added (ie: NaCl sodium chloride)
3. IF the metal has more than one possible charge
   1. With the Stock Method you must indicate which ion using the charge in roman numerals (ie: FeCl2 Iron (II) chloride).
   2. Alternatively the common name may be used if the metal has more than one possible ion. Here use the Latin root and then add -ous for the lower charge. -ic for the higher charge.
      1. FeCl2 ferrous chloride
      2. FeCl3 ferric chloride
   3. More examples showing the two different systems:

 

Formulas and Names of Binary Nonmetal-Nonmetal Compounds

1. Systematic Nomenclature:
   1. For names start with element to the left side on the periodic table
   2. add -ide to the second element
   3. use Greek prefixes for number of atoms: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca
   4. Example:
      1. CO carbon monoxide
      2. CO2 carbon dioxide
      3. N2O5 dinitrogen pentoxide
2. Common names: -ous and -ic (-ic has greater charge, OR has fewer atoms). Examples:

 

Polyatomic Compounds.

1. Names of Polyatomic Ions
   1. ammonium ion NH41+
   2. -ide ions
      1. CN1- cyanide
      2. OH1- hydroxide
   3. Oxyanions
      1. -ate ate more oxygen.

2. Sometimes oxyanions have an extra hydrogen

3. If more than two possibilities:

2. Naming compounds with polyatomic ions
   1. Positive charge species on left (using Stock method or common name)
   2. Negative charge species on right (using name of polyatomic ion)