1. Warm Up
1.What did Rutherford’s scattering experiment show? Need to list 2 important findings. 2. What did Thomson discover? Describe the atomic model he constructed to explain this. 3. Describe Dalton’s water and electricity experiment. How did this show that atoms probably exist?
What is the current model being used today?
Describe the model?

2. Atomic Structure

3. What is an atom?
Atom: the smallest unit of matter that retains the identity of the substance

4. Atomic Structure Atoms have 2 regions
1) Nucleus: the center of the atom that contains most of the mass of the atom.
2) Electron cloud: surrounds the nucleus & takes up most of the space of the atom.
Nucleus
Electron
Cloud

5. What’s in the Nucleus? In the nucleus we find:
Protons: positively charged subatomic particles
Mass of 1 amu
Neutrons: neutrally charged subatomic particles

6. What’s in the Electron Cloud?
In the electron cloud we find:
Electrons: the subatomic particle with a negative charge and relatively no mass
Mass of ~ 1/1836 amu

7. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1
9.11 x 10-28
Electron cloud
Proton (p+) +1
1.67 x 10-24
Nucleus
Neutron
(no)

8. How do we know the number of protons in an atom?
Atomic number (#)= # of protons in an atom
Ex: Hydrogen’s atomic # is 1
hydrogen has 1 proton
Ex: Carbon’s atomic # is 6
carbon has 6 protons
**The number of protons identifies the atom-it’s an atom’s fingerprint.

9. How do we know the number of neutrons in an atom?
Mass #: the # of protons plus neutrons in the nucleus
# of neutrons = mass # - atomic #
Example
Li has a mass # of 7 and an atomic # of 3
Protons = 3 (same as atomic #)
Neutrons= 7-3 = 4 (mass # - atomic #)

10. Mass # vs. Atomic Mass
Mass # ? = The Atomic mass on the periodic table rounded either up or down

11. How do we find the number of electrons in an atom?
Most atoms are neutral (have no overall charge)
Because the only charged subatomic particles are the protons and electrons… they must balance each other out in an electrically neutral atom.
Therefore..
# Electrons = # Protons *
* (in a neutral atom..)

12. Examples He has a mass # of 4 and an atomic # of 2
p+ = 2 no = 2 e- = 2
Cl has a mass # of 35 and an atomic # of 17
p+ = 17, no = 18, e- = 17

13. How exactly are the particles arranged?
Bohr Model of the atom: electron configurations
All of the protons and the neutrons
The 3rd ring can hold up to 8 e-
The 1st ring can hold up to 2 e-
The 2nd ring can hold up to 8 e- 13

14. What does carbon look like?
Mass # = 12 atomic # = 6
p+ = 6 no = 6 e- = 6
6 p and 6 n live in the nucleus 14

15. These are called isotopes.
Dalton’s 1st postulate was wrong. Atoms of the same element can be different (they can have different # of neutrons)
Thus, different mass numbers.
These are called isotopes.

16. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

17. Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

18. We name the isotope based on its mass number
Naming Isotopes
We name the isotope based on its mass number
carbon-12
carbon-14
uranium-235

19. Isotopes
Elements occur in nature as mixtures of isotopes.

20. Atomic Mass How heavy is an atom of oxygen?
It depends.. b/c there are different oxygen isotopes.
We are more concerned with the average atomic mass.
This is determined based on the abundance of each isotope
We don’t use grams for this mass because the numbers would be too small.

21. Measuring Atomic Mass Instead we use the Atomic Mass Unit (amu)
defined as one-twelfth the mass of a carbon-12 atom.
Each isotope has its own atomic mass, thus we determine the average from percent abundance.

22. Composition of the nucleus
Atomic Mass
Atomic mass is the average of all the naturally occurring isotopes of that element.
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%
Carbon =