1. Notes 4-1
Intro to Atoms

2. What’s an atom?
Smallest particle of an element

3. Atomic Theories
There have been many theories of proposed models of the atom.
Some were completely wrong!
And some were partially correct or close to correct.
We will look at some of the most known theories and models.

4. Dalton’s Atomic Theory
Dalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces.
He was credited with coming up with the earliest model of the atom
Most of this theory is still accepted today #2

5. Thomson’s Model
Thomson suggested that atoms had negatively charged electrons embedded in a positive sphere.
This is the first model that included smaller particles of the atom
His model is similar to blueberries in a muffin

6. Rutherford’s Model Developed the first model that included a nucleus
Similar to a peach with a pit, the pit = the heavy mass of protons in the nucleus, the fruit = the empty space made up of electrons

7. Rutherford’s Gold Foil Experiment
Turn to page in your text book
Let’s read about Rutherford’s model/ experiment
Who wants to read first?
Everyone else, follow along….

8. Rutherford’s Gold Foil Experiment
Rutherford was surprised that a few particles were deflected strongly. This led him to propose an atomic model with a positively charged nucleus. #5

9. Bohr’s Model
Said electrons could only have a certain amount of energy, so they must move in certain orbits
Similar to planets orbiting the sun (sun = nucleus), or the rings/layers of an onion

10. Cloud Model
Says Bohr’s model is wrong, electrons do not orbit the nucleus like planets do the sun
Instead, electrons can be found anywhere in a cloud-like region around the nucleus, rapidly orbiting in every direction

11. Later Atomic Models
Through the first part of the twentieth century, atomic models continued to change.

12. Modern Atomic Model Accepted since the 1930s
At the center of the atom is a massive nucleus, containing positively charged protons and neutrally charged neutrons (no charge)
Surrounding the nucleus is a cloud-like region of moving negatively charged electrons
# protons in atom = # electrons in atom
# neutrons can change

13. Particles in an Atom
An atom is composed of positively charged protons, neutral neutrons, and negatively charged electrons. Protons and neutrons are about equal in mass and are measured in atomic mass units (amu). An electron has about 1/2,000 the mass of a proton or neutron.
#18

14. Atomic Number
Every atom of a given element has the same number of protons in its nucleus
This is known as the element’s atomic number
For example, Helium’s atomic number is 2, so every atom of helium has 2 protons in its nucleus. If the element has more than 2 protons, it is NOT helium!
Carbon’s atomic number is 6. How many protons does an atom of carbon have?

15. Atomic Number
The atomic number is usually found above the letter on the periodic table
Look at helium below
The atomic number tells the number of protons in the nucleus. #7

16. Isotopes: Atoms of elements have same number of protons, but different numbers of neutrons
Atoms of all isotopes of carbon contain six protons and six electrons, but they differ in their number of neutrons. Carbon-12 is the most common isotope.

17. Mass number 6 How do you know? Because if the mass number is 12,
An isotope is identified by its mass number
Mass number = #protons + #neutrons
Example: Carbon-12, the mass number is 12. We know carbon has 6 protons by looking at its atomic number on the periodic table. How many neutrons does carbon-12 have? 6
How do you know?
Because if the mass number is 12,
12 - 6protons = 6neutrons

18. Problem:
The atomic number of nitrogen is 7. How many protons, neutrons, and electrons make up Nitrogen-15?
Protons = 7
Neutrons = 8
Electrons = 7