1. More about Elements

2. Energy Levels
Electrons are found orbiting the nucleus within things called energy levels
The outer most of these levels contain valence electrons
Atoms want to have 8 valence electrons to be stable

4. 1 8 3 4 5 6 7 2

5. Isotopes Atoms of the same element with different mass numbers
They still have the same number of protons and electrons, but their number of neutrons are different
The element chlorine has 2 isotopes
Chlorine – 35
Chlorine – 37
The atomic mass is an average of the mass numbers of all the isotopes
Chlorine 17 Cl
35.453

6. Most Abundant Elements
Astronomers have identified the 2 most abundant elements in the universe are hydrogen and helium
All the other elements account for less then 1% of all the atoms in the universe