1. Development of the Atomic Theory

2. History of the Atom
Democritus argued that atoms were the smallest particles of matter.
The atom comes from the Greek word atomos that means “not able to be divided”.

3. John Dalton Dalton proposed the first atomic theory.
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.

4. J.J. Thomson
Thomson discovered negatively charged particles called electrons.
Designed the “plum pudding” model.

5. Ernest Rutherford
Rutherford discovered that atoms contain a small, dense, positively charged center called the nucleus.

6. Niels Bohr
Bohr suggested that electrons (which have a negative charge) moved around the nucleus at certain fixed distances.
Electrons
Nucleus
Electrons

7. According to the current atomic theory, electrons are most likely to be found in the electron cloud around the nucleus.
Electron Cloud

8. Dmitri Mendeleev
Russian chemist
Created the periodic table

9. Matter

10. What is Matter? Matter is anything that has mass and takes up space.
The basic building blocks of all matter are atoms.

11. An atom is the smallest particle into which an element can be divided and still be the same substance.

12. Molecules
A particle made of two or more atoms bonded together forms a molecule.
For example, two hydrogen atoms join with one oxygen atom to form a water molecule.

13. Elements
An element is the simplest form of matter that cannot be changed into another simpler form by ordinary means.
Examples of elements: Oxygen, Helium, Gold, Silver….

15. Compounds
A substance made of two or more elements chemically combined form a compound.
Example: NaCl, H2O, CO2

16. Parts of the Atom
Proton: a positively (+) charged particle of the nucleus.
Neutron: a neutral particle of the nucleus.
Electron: a negatively (-) charged particle around the nucleus.

17. Electron (Shell) Cloud (levels or orbitals)
Neutron (neutral)
Nucleus
(98% of the mass of atoms)
Proton (+)
Electron (Shell) Cloud (levels or orbitals)

18. Atomic Mass Unit (amu): the SI unit for the masses of particles in atoms.
Atomic Number: the number of protons in the nucleus of an atom.
Mass Number/Atomic Mass: the number of protons and neutrons in an atom.
Atomic Mass: the weighted average of the masses of all the naturally occurring isotopes of an element.