Ap chemistry – chapter 2: atoms, molecules & ions
The Atomic Theory of Matter
• Democritus (460–370 BC): All matter can be divided into indivisible atomos.
• Dalton: proposed atomic theory with the following postulates:
1. Elements are composed of atoms.
2. All atoms of an element are identical. *Sort of wrong.......___________________________
3. In chemical reactions atoms are not changed into different types of atoms. Atoms are neither
created nor destroyed.
Law of ____________________________________________________
4. Compounds are formed when atoms of elements combine.
Law of ____________________________________________________
a.k.a. Law of Constant Composition (See Ch. 1 Notes)
• Law of multiple proportions: If two elements, A and B, combine to form more than one compound, then
the mass of B, which combines with the mass of A, is a ratio of small whole numbers.
Atomic Structure
Electron:
- JJ Thompson is credited with their discovery.
- Streams of negatively charged particles were found to emanate from cathode tubes.
Millikan Oil-Drop Experiment
• Goal: find the charge on the electron to determine its mass.
Discovery of the Nucleus:
- Ernest Rutherford shot α particles at a thin sheet of gold foil and observed the pattern of scatter of
the particles.
- Rutherford postulated a very small, dense nucleus with the electrons around the outside of the
atom.
- Most of the volume of the atom is empty space.
Protons were discovered by Rutherford in 1919
Neutrons were discovered by James Chadwick in 1932
(The mass of an electron is so small we ignore it.)
Atomic Numbers, Mass Numbers & Isotopes
12
6
C
- Elements are symbolized by one or two letters.
- Atomic Number – number of protons
All atoms of the same element have the same number of protons
- Mass number - number of protons plus neutrons
Isotopes:
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
11
6
C
12
6
C
13
6
C
14
6
C
Atomic Mass of an Atom:
The actual mass can be calculated for any atom:
• Actual Mass (g) = (Mass#)(1.67x10-24g)
• This can be converted to an atomic mass by relating the mass to carbon-12
Mass of one atom in amu = Mass of a mole of atoms in grams!
*Scientists defined the mole so this would work!
Atomic Mass of an Element:
A weighted average of atomic masses of all the isotopes of an element.
Use the following data to calculate the atomic mass for the element Magnesium
Isotope
Atomic Mass of Isotope
Abundance
Mg - 24 23.982628 µ
78.600 %
Mg - 25 24.963745 µ
10.11 %
Mg - 26 25.960802 µ
11.29 %
The Periodic Table
- A systematic catalog of elements.
- Elements are arranged in order of atomic number.
- The rows on the periodic chart are periods.
- Columns are groups.
- Elements in the _____________________________ ___________________________________.
Nonmetals are on the right side of the periodic table (with the exception of H).
Metalloids border the stair-step line (with the exception of Al and Po).
Metals are on the left side of the chart.
Chemical Formulas
- The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule
of the compound.
Types of Formulas:
• Empirical formulas give the ______________________________________________________________ of each
element in a compound.
• Molecular formulas give the _____________________________________________________________ of each
element in a compound.
Example: Molecular Formula: C6H12O6
Empirical Formula:
Ions:
When atoms lose or gain electrons, they become ions.
 Cations are ________________________________
 Anions are _________________________________
Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.
*See handout on naming and writing formulas!
Molecules:
Molecular compounds are composed of molecules and almost always contain only nonmetals.
*See handout on naming and writing formulas!
Diatomic Molecules: These seven elements occur naturally as molecules containing two atoms.
Acids:
Compounds containing hydrogen with something that looks like a negative ion (for now at least…..)
*See handout on naming and writing formulas!
Organic Molecules:
• Organic molecules contain carbon.
•
Alkanes: the most simple organic compounds. They have chains of carbon with only single bonds
•
surrounded by hydrogen.
They are named by the number of carbons in the chain + the suffix -ane
•
Other classifications of organic compounds have multiple bonds or other atoms or groups of atoms
replacing a hydrogen.
•
Alcohol has an –OH group instead of a –H off one of the carbons in an alkane
They are named with the same root as the alkanes + the suffix -ol
Starting with propanol they include a number in front of the name indicating which
carbon the –OH is bonded to