1. ATOMS

2. An atom is the smallest whole part of an element.
Democritus, a Greek philosopher, was the first person to realize that all matter is composed of atoms.

3. John Dalton (~1800) proposed the Atomic Theory, which stated that:
All matter is composed of atoms.
Atoms cannot be created or destroyed.
Atoms of elements combine to form compounds.
Each element is made up of different kinds of atoms.

4. Atoms
are made up of:
Electrons, which have a negative charge.
Protons, which have a positive charge.
Neutrons, which have no charge.

5. Atomic number = # of protons
A stable atom has a neutral charge. It has an equal number of positive and negative charges.
Atomic number = # of protons
Atomic mass = proton # + neutron #

6. All atoms of the same element have the same no. of protons
All atoms of the same element have the same no. of protons. The proton determines the position, or element no. on the periodic table.
Hydrogen has 1 proton, so its atomic no. is 1. Hydrogen is element #1 on the Periodic Table of the Elements.

7. How many protons does gold have?79
How many electrons does one atom of gold have? 79
How many neutrons does one atom of gold have?
– 79 = neutrons

8. So how can you have part of a neutron, such as 119.97 ?
Because of ISOTOPES ! An isotope is a form of an element that has the same number of protons, but different numbers of neutrons. The atomic mass on the periodic table reflects the average mass of all of the known isotopes of an element. Each isotope may have different characteristics.

9. 13 26.98 – 13 = 13.98 neutrons 13 How many protons does aluminum have?
How many neutrons does this element have?
26.98 – 13 = neutrons
How many electrons does aluminum have? 13

10. e p n
This is a Bohr model of a lithium atom. The Bohr model is named after Niels Bohr.
Atoms are arranged with protons and neutrons in the nucleus, with electrons orbiting around the nucleus in energy shells.

11. In a Bohr model, each energy level can hold a characteristic number of electrons.
K = 2 K L
L = 8

12. Draw a Bohr model of oxygen:
K = 2
L = 6 e e
8 p
8 n e e e e e
At. # = 8