1. Introduction to Atoms

2. Development of the Atomic Theory
What Do You Think?
Imagine that you have cut a penny in half. Then, you take one piece and half it again. Will this continue forever, or will you come to a point where no more cutting is possible?

3. Development of the Atomic Theory
Democritus was a Greek philosopher who theorized that all matter was made of invisible particles called atoms.
Democritus of Abdera,
about BCE

4. Development of the Atomic Theory
Dalton, a British chemist and teacher, noticed that elements combine in specific proportions to form compounds, and theorized that their atoms combine at the same proportions.
John Dalton

5. Development of the Atomic Theory
Thomson’s experiments using a cathode-ray tube showed that smaller particles make up atoms.
Joseph John
“J.J.” Thomson

6. Development of the Atomic Theory
Rutherford, a former student of Thomson’s from New Zealand, tested his teacher’s theories in his Gold Foil Experiment.
He expected his alpha particles to go straight through the foil, and most of them did.
Ernest Rutherford

7. Development of the Atomic Theory
But some of the particles were deflected or bounced straight back!
This showed that a nucleus with a positive charge makes up the center of an atom.
Rutherford’s Gold Foil Experiment

8. Development of the Atomic Theory
Bohr, a Danish scientist who worked with Rutherford, described the motion of electrons around the nucleus.
Niels Bohr

9. Development of the Atomic Theory
Bohr said that electrons orbit the nucleus at specific energy levels, and can move from one level to another.
Bohr’s Atomic Model

10. Development of the Atomic Theory
The current atomic theory states that there are regions inside an atom where electrons are likely to be found.
These regions are called electron clouds.
Electron Cloud Model

11. The Atom
What Do You Think?
What is the smallest thing you have ever seen? How does it compare to the size of an atom?

12. Parts of the Atom
The nucleus is the small, dense, positively charged center of the atom.

13. Parts of the Atom
Protons are positively charged particles in the nucleus.
Each proton is equal to 1 atomic mass unit.

14. Parts of the Atom
Neutrons are particles in the nucleus that have no charge. Protons and Neutrons are the most massive particles in the atom.

15. Parts of the Atom
Electrons are negatively charged particles found in electron clouds outside the nucleus.
Super small mass compared to a proton.
1800 electrons = 1 proton
Protons and Electrons cancel each other out making atoms neutral.

16. Atomic Number and Isotopes
The number of protons is equal to the number of electrons in atoms.
The atomic number of an atom is equal to the number of protons in the nucleus.
All atoms of an element have the same number of protons.
If atoms have the same number or protons but a different number of neutrons, it is called an isotope.

17. Isotopes Elements that are isotopes are always the same element.
A different number of neutrons gives it a different mass.

18. Isotope Practice
Helium has 2 protons and 2 neutrons. Its atomic mass is 4.
Helium-3 (isotope) has an atomic mass of 3.
How many neutrons does it have?
Carbon has 6 protons and 6 neutrons. Its atomic mass of 12.
Carbon-14 (isotope) has an atomic mass of 14.
How many neutrons does it have?

19. Atomic Mass Every element has an atomic mass.
The atomic mass is the sum of the number of protons and neutrons in the nucleus of an atom.
The mass is the average of the masses of all the isotopes of the element.

20. Forces in Atoms
The forces that are always pushing and pulling on atoms are:
Gravitational force
Electromagnetic Force
Strong Force
Weak Force

21. Forces in Atoms Gravitational Force Electromagnetic Force
Acts between atoms.
Electromagnetic Force
Same charges repel each other.

22. Forces in Atoms Strong Force Weak Force Holds the nucleus together.
In some radioactive elements, this allow some neutrons can turn into protons.