1. Atoms: Classification & Representation
4-Ch 1

2. Review: The Atom What is an atom? What are the parts of an atom?
Definition: An atom is the smallest particle of matter. It cannot be divided chemically
What are the parts of an atom?
In the nucleus: Protons and Neutrons
Outside of the nucleus: Electron
Protons have a positive charge, neutrons have no charge, and electrons have a negative charge

3. How Small is an Atom?

4. History of the Atomic Model
Reading: “Scientist Contributions”
Video: “Gold Foil Experiment”
Review: Task Cards

5. The Rutherford-Bohr Atomic Model
Named after two scientists: Ernest Rutherford and Niels Bohr
Rutherford: Discovered that most of the atom is empty space and contains a very small but dense nucleus
Bohr: Electrons surround the nucleus in orbits (pathways) but can move from one orbit to another
Definition: A representation of the atom as a very small nucleus made up of positively charged protons, surrounded by negatively charged electrons moving in defined orbits

6. Representing Atoms (Rutherford-Bohr)
To keep things simple, protons and neutrons are represented as a “ball” in the center (the nucleus)
Electrons are arranged on orbits (also called shells or valences) around the nucleus
Each orbits has a MAXIMUM number of electrons it can support
The number of protons and electrons are EQUAL and we know this number as the ATOMIC NUMBER

7. Example of an Exam Question
Rutherford modified the atomic model after doing experiments where the alpha particles were dispersed by a sheet of gold foil. Considering the statements below, which statements are based on Rutherford’s experiments only?
1-The number of protons is equal to the number of electrons.
2-Protons are concentrated in a small positive space at the center of the atom.
3-Atoms consist of mostly empty space.
4-Electrons are contained in a positive sphere made up of protons.
5-Electrons move about in specific energy levels.
A)1 and 2 B) 1 and C) 2 and 3 D) 2, 3 and 5

8. How Many Electrons? For the first 20 elements: 1st orbit = 2 electrons
2nd orbit = 8 electrons
3rd orbit = 18 electrons BUT after 8 electrons the 4th begins to fill too…
Examples:
Helium: Atomic Number = 2
Aluminum: Atomic Number = 13
Calcium: Atomic Number = 20

9. Example of an Exam Question
Which of the following best represents the aluminum (Al) atom according to the Rutherford-Bohr model?

10. Representing Atoms (Lewis Notation)
A simplified representation of the atom
Only valence electrons are shown
Valance Electrons: the electrons located FURTHEST from the nucleus