Chapter 2: Modern Atomic Theory
• Matter consists of atoms
• Atoms consist of three fundamental particles, found in the nucleus and the space around the nucleus
Arrangement of Subatomic Particlesnot to scale!
If an atom has a diameter of about 100 m (a football field), the nucleus would be 1 mm in diameter
Nucleus containsprotons and neutrons.
What holds an atom together?
Coulomb’s Law of electrostatic interactions.
General Behavior: Like charges repel.Opposite charges attract.
What holds an atom together?
Coulomb’s Law of electrostatic interactions.
If we double a charge from +1 to +2, the force will:
1. Double2. Halve3. Quadruple4. Quarter
What holds an atom together?
Coulomb’s Law of electrostatic interactions.
If we double the distance, the force will:
1. Double2. Halve3. Quadruple4. Quarter
What’s wrong with this picture?
Electrons are held near the nucleus by the electrostatic attraction between them, but …
The forces of nature:
How could we possibly know that the nucleus
is small compared to the size of the atom?
Why not think the protons, electrons and neutrons are all mixed together?
The Rutherford Experiment
We’re not just making it up!
Atom Terminology
• All atoms of an element have the same atomic number = number of protons
• In a neutral atom (no charge), the number of positively and negatively charged particles must be equal. number protons = number electrons
• The mass number = protons + neutrons
Isotopes
• Isotopes are atoms of an element with the same number of protons but different numbers of neutrons, and different mass
• Atom symbols are written using the following format:
X = symbol of elementA = mass numberZ = atomic number
ZA X
11H 1
2H 510X 30
66X
Isotopes, continued
• Most elements have multiple isotopes– 1H, 2H (deuterium), 3H (tritium, radioactive)– 79Br, 81Br– 64Zn, 66Zn, 67Zn, 68Zn, 70Zn
• While atomic mass is the mass of an atom, average atomic mass takes into account isotopes and natural abundance
• We use atomic mass units (amu)1 amu = 1.661 x 10–24 g(about the mass of a proton)
Isotopes and Average Atomic Masses
all isotopes
Average atomic mass = (fractional abundance) (mass of isotope)
Isotopes and Average Atomic Masses
all isotopes
Average atomic mass = (fractional abundance) (mass of isotope)B has 2 isotopes: B-10: mass = 10.0129 amu
abundance = 19.91%
B-11: mass = 11.0093 abundance = 80.09%
Example: Boron
What is the average atomic mass of Boron?