1. Properties of Atoms and the Periodic Table
Chapter 19
Sections 1 and 2

2. Section 1
Structure of the Atom

3. Structure of the Atom
An element of matter is made up of one type of atom
Atoms are composed of particles called protons, neutrons, and electrons
Protons-positive charge
Electrons-negative charge
Neutrons-no charge
Protons and neutrons are in centrally-located nucleus surrounded by a cloud containing electrons

4. Quarks
Scientists hypothesize that electrons are not composed of smaller particles
Protons and neutrons are made up of smaller particles called quarks
The search for the exact composition of protons and neutrons is ongoing

5. Atomic Models
Many different models of the atom were proposed throughout the years
Aristotle argued that atoms didn’t even exist
John Dalton’s provided evidence that atoms existed
His model was a simple sphere
Joseph Thomson proposed the “plum-pudding” model
Ernest Rutherford’s model included a nucleus
Niels Bohr suggested that electrons travel in fixed orbits around the nucleus
Dalton’s Model
Thomson Model
Rutherford Model
Bohr Model

6. The Electron Cloud Model
By 1926, scientists developed the electron cloud model of an atom
This is the currently accepted model
The electron cloud is the area around the nucleus where its electrons are most likely to be found
The cloud is 100,000 times larger than the diameter of the nucleus

7. Section 2
Masses of Atoms

8. Atomic Symbols All elements are represented by a chemical symbol
The symbols consist of one capital letter or a capital letter and one or two lowercase letters
Some symbols are the first letter of the elements name
Oxygen O
Some are derived from Latin
Silver Argentum Ag
Some are name in honor of scientists
Some are named for where they were discovered
The symbols are recognized worldwide

9. Atomic Number
The number of protons in an atoms is known as the atomic number
The number of protons in an atom is what gives an atom its identity
Carbon will always have 6 protons
Atomic number = 6
Neon will always have 10 protons
Atomic number = 10
The atomic number is on the periodic table and is used to identify each element

10. Atomic Mass The nucleus contains most of the mass of an atom
The mass of a proton and a neutron are very similar
1.67 x g
The mass of a proton (or a neutron) is 1,836 times greater than that of an electron
Because the mass of proton or neutron is awkward to use, a standard measurement is used.
Each proton or neutron has a mass of approximately 1 atomic mass unit (1 amu)
Carbon has 6 protons and 6 neutrons
6(1) + 6(1)= 12 amu (mass of a carbon atom)
Why didn’t we use the mass of an electrons?

11. Mass Number
The mass number of an atom is the sum of the number of protons and neutrons
Remember: This is because each proton or neutron has a mass of 1 amu
If you know the mass number and the atomic number of an atom, you can determine the number of neutrons
# of Neutrons = Mass Number – Atomic number
Example: How many neutrons are in an Carbon atom that has a mass number of 14?
What is the atomic number of Carbon? 6
14 – 6 = 8 There are 8 neutrons in the atom.

12. Isotopes All atoms of an element will have the same number of protons
But not all the atoms will always have the same number of neutrons
Atoms of the same element that have different numbers of neutrons are called isotopes
Isotopes are usually identified by their mass number
Carbon-12 (6 protons, 6 neutrons)
Carbon-14 (6 protons, 8 neutrons)
Boron-10 (5 protons, 5 neutrons)
Boron-11 (5 protons, 7 neutrons)
How many neutrons does Oxygen-18 have?

13. Average Atomic Mass
Because not all the atoms of an element have the same mass, an average mass is used
To figure out the average mass, we need to know how many of each isotope is present in a sample
Scientists do this by using weighted averages
For example:
80% of all Boron atoms are Boron-11
20% of all Boron atoms are Boron-10
.80(11) + .20(10) = 10.8 amu