Unit 4

Formulas and Equations

Textbook Chapter 2, 6, & 8

Review Book Topic 2

• Formulas use chemical symbols and numbers to show what elements and how many atoms of each are involved in each compound

Chemical Symbols

• Each element has been assigned a one-, two- or three- letter symbol for its identification

• First letter is ALWAYS capitalized, additional letters are lowercase

• Only recently discovered, unnamed elements are given three- letter symbols

• Some symbols show a relationship

– Ex. Carbon ~ C

Sodium ~ Na (Latin – natrium)

• Symbols are assigned by IUPAC

– International Union of Pure and Applied Chemists

• Roots used for naming elements:

0 : nil 1 : un 2 : bi 3: tri 4 : quad

5 : pent 6 : hex 7 : sept 8 : oct 9 : enn

• Ex. Element #109

– Un-nil-enn-ium

(1)-(0)-(9)

• Ex. Element #114

Chemical Molecules

• Monatomic molecules – uncombined elements, written without a subscript

– Ex. Neon gas – Ne

Argon gas – Ar

• Diatomic molecules – elements can exist in nature as two identical atoms bonded together

– Ex. Hydrogen – H2

(F, O, N, Cl, Br, I)

Chemical Formulas

• Chemists have identified over 10 million compounds

• Compound – two or more elements that are chemically combined (bonded together) in definite proportions by mass

– Ex. H2O, C6H12O6, H2O2

• No two compounds have identical properties

• Chemical formula – shows the kinds and numbers of atoms in the smallest representative unit of the substance

– If monatomic: use chemical symbol (ex. Kr)

– If diatomic or a compound: use chemical symbols of elements involved, and subscripts to represent # of atoms present (ex. F2 or O3 or NaCl)

– Types of formulas: molecular, empirical, structural

• Subscript – smaller number after an element symbol that indicates how many atoms of that element are in the molecule

– Ex. H2O means there are 2 H and 1 O atom

• Coefficient – number in front of a molecule’s formula indicating how many molecules are present

– Ex. 2H2O means there are 2 water molecules

• Molecular formulas – shows the kinds and numbers of atoms present in a molecule of a compound

– Subscript written after the symbol indicates the # of atoms of each element

• If only 1 atom, subscript of 1 is omitted

– Show composition but NOT molecular structure

• Empirical formula (“formula unit”) – shows the lowest whole number ratio of ions in a compound

– Ex. MgCl2

• For every 1 Mg+, there are 2 Cl-

– Ex. H2O and H4O2

• Both have a ratio of 2 H : 1 O

• Molecular formulas can be seen as a multiple of an empirical formula

– Ex. Glucose: C6H12O6 (molecular)

CH2O (empirical)

6(CH20) = C6H12O6

• Structural formula – shows the physical organization of the atoms in a molecule

• Law of definite proportions – in any compound, the masses of the elements involved are always in the same proportions

– Ex. NaCl always has 1 Na (23 amu) and 1 Cl (35 amu) = 58 amu total for one NaCl

– Ex. H2O always has 2 H (total 2 amu) and 1 O (16 amu) = 18 amu total for one H2O

– Proportions of mass equals the ratio proportions of the number of atoms of each element in the molecule

• Law of multiple proportions – whenever two elements form more than one compound (ex. H2O and H2O2), the different masses of one element (ex. O versus O2) that combine with the same mass of the other element (H2) are in the ratio of small whole numbers

– Ex. We have two compounds, each with 2 g of element B. Compound 1 has 5g element A, compound 2 has 10 g element A

Atoms, Compounds and Ions

• Atoms and compounds are electrically neutral

– (# p+ = # e-)

• Ions have a net charge, either (+) or (-)

– (# p+ ≠ # e-)

– (+) ions attract (-) ions in a ratio that produces a neutral compound