Introduction to atoms and molecules

1
Introduction to atoms and molecules

• Chapter 2-1 2-5
• Chapter 5-7 and 5-9
• Chapter 4-5 4-6

2
Key concepts in this unit

• Daltons atomic theory
• The electron, proton, and neutron
• Isotope, atomic number, mass number
• Molecules and molecular formulas
• Introduction to ions
• Nomenclature for inorganic compounds

3
Concept of the atom

• Democritus (ancient Greek) -- ?????? (atomos)
  indivisible particle of a substance
• Democritus ideas were promptly lost for the next
  2000 years or so.

4
Daltonbirth of the atomic theory (1803)

• Daltons postulates
• Each element is composed of extremely small,
  indivisible particles called atoms.
• All atoms of a given element are identical.
  Atoms of different elements are different, and
  have different properties, including mass.

5
Daltons atomic theory (cont)

• Atoms of an element are not changed into
  different types of atoms in a chemical reaction
  (i.e., you cant turn lead into gold). Atoms are
  not created or destroyed in chemical reactions.
• Compounds are formed when atoms of more than one
  element combine in small, whole number ratios.
• A given compound always has the same relative
  number and kind of atoms.

6
Daltons theory explains laws

• law of constant composition
• law of conservation of matter
• law of multiple proportions

7
Law of multiple proportions

• Elements combining in more than one proportion
  will do so in multiples of whole numbers.
• Examples
• H2O vs. H2O2.
• NO NO2 N2O

8
Fundamental particles

• Masses of fundamental particles
• Atomic mass unit
• 1 amu 1.66053873 ? 10-24 g

9
Atoms

• Atoms vary by atomic number (Z), which indicates
  ________________________.
• Atoms contain the same number of electrons and
  protons (they have no charge).
• Atomic number increases from left to right on
  periodic table.
• The atomic weight of the atom is indicated below
  the symbol (in amu).
• Why are these weights not whole numbers?

10
Isotopes(chap 5-7, 5-9)

• Atoms of an element have same number of protons
  in the nucleus.
• Isotopes of an element vary in number of
  neutrons, but not in number of protons.
• Atomic symbols (carbon-12)
• C carbon
• Subscript number atomic number number of
  protons
• Superscript number mass number, number of
  protons neutrons
• What is the atomic symbol for carbon-14?

11
Isotope symbols
12
Average atomic weight

• The average atomic weight (the one in the
  periodic table) is a weighted average.
• Not all isotopes of an atom exist in the same
  amounts, and this must be taken into account.

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• Example 1 Lets assume carbon has only 2
  isotopes, 13C and 12C. The natural abundance of
  13C is 1.0 . What is the average atomic mass?
• Example 2 Cl has 2 isotopes, 35C and 37C. The
  average atomic mass of Cl is 35.5 amu. What is
  the natural abundance of the two isotopes?

14
Molecules

• A molecule is ____________________________________
  _______________________________.
• In some cases, an atom is also a molecule.
  Example He, Ne, Ar.

15
Some elements exist in nature as molecules.

• Diatomic molecular elements
• Polyatomic molecular elements
• Allotropes

16
Formulas

• shorthand for writing and describing molecular
  composition.
• Chemical formula
• Structural formula

17
Ionic compounds

• Ionic compounds are composed of
  _________________________________.
• Cation
• Anion
• Ionic compounds do not form molecular units, but
  are _______________ containing a defined ratio of
  cations and anions, forming a _________.
• Formula unit

18
Nomenclature for inorganic compounds
19

• The charge on an ion is designated by a number
  written above and to the right of the symbol
• Examples
• Na1 (or Na)
• Al3
• F-
• O2-

20
Periodic groups and ions

• Normally (but not always) atoms in the following
  groups form ions of the designated charge.
• This commonly occurs such that the of electrons
  in the ion are equal to the of electrons in the
  neighboring noble gas. We will discuss why this
  is so in quantum mechanics.

21

• Transition metals often form more than one type
  of cation (i.e., different charges, such as Fe,
  Fe2, Fe3.)
• We have methods of writing formulas and names to
  indicate what charge the ions take.

22
Identifying ionic compounds

• Generally (but again, not always) ionic compounds
  are formed by combining a metal and a nonmetal.
  Molecular compounds are generally formed by
  combining nonmetals.
• The sum of the charges in an ionic compound must
  equal zero. The subscript of each ion indicates
  how many of that ion are in the formula.
• Example what is the formula for the ionic
  compound formed when Al3 combines with O2-?

23
Nomenclature rules

• Cations
• Cations from metal atoms take the name of the
  metal.
• Examples
• Cations from transition metals have the charge
  indicated by a Roman numeral.
• Examples
• Note there is an older method of naming
  transition metal cations. We will not use the
  older method in this course, but you should be
  aware of it in case you ever run into it. (p.141)
  
• Cations from nonmetal atoms end in ium.
• Examples
• Table 2-3 (p. 55) and Table 4-11 (p. 142) contain
  the names and symbols of common cations that you
  should know.

24
Anions

• monoatomic anions are named by replacing the
  ending of the element with ide.
• Examples

25

• polyatomic anions containing a variable number of
  oxygens. These occur in ternary salts.
• The common anion is named using the suffix
  ate.

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• The suffix or prefix of the name changes when the
  number of oxygens is increased or decreased

Note that the CHARGES on a family of oxyanions
doesnt change when the number of oxygens varies.
The oxidation number of the central atom does
change. Well discuss this later.
27
Examples of ternary salts

• sodium sulfate
• iron (II) oxide
• ammonium chloride
• lithium carbonite

28

• oxyanions with an H or 2 H added are indicated
  using the word hydrogen, or dihydrogen.
• examples
• (Sometimes the addition of a hydrogen is
  indicated by the prefix bi-. For instance, HCO3-
  is the bicarbonate anion, as in NaHCO3, sodium
  bicarbonate. Use of either bi or hydrogen is
  correct).
• Table 2-3 (p. 55) and Table 4-11 (p. 142) contain
  the names and symbols of common anions that you
  should know. You should also be able to properly
  name the relatives of the common oxyanions.

29
Acids

• For the moment, an acid is an ionic compound
  that, when dissolved in water, yields a H.

Double check the formula for any ionic compound
must ZERO.
30
Binary molecular compounds

• The atom farthest to the left in the periodic
  table is usually written first.
• For elements in the same group, the atom
  appearing lower in the table is usually written
  first.
• the name of the second element in the formula
  name is given an ide ending.
• Greek prefixes indicate the number of each type
  of atom appearing in the molecule. (p. 142) You
  should know these prefixes.