1. Atomic Structure Objectives: History of an Atom Atomic Models
Modern Atomic Theory
Ions
Mass of an Atom
Mass Number
Isotopes
Atom Energy
Wave – Mechanical Model
Electrons

2. The History
Democritus – a Greek philosopher that lived in 450 B.C. Said that all matter is composed (made) of tiny parts
He developed a definition of an atom – smallest particle of an element that retains the chemical properties of that element

3. History Continued
John Dalton – Lived in 1803 and developed the first Atomic Theory of Matter.
1. Each element is made of very small parts called atoms
2. All atoms of a given element are identical but they differ from any other element
3. Atoms are not created nor destroyed in a chemical reaction
4. A given compound always has the same proportion and same type of atoms

4. Atomic Models
J. J. Thompson – Lived in 1897 and developed the “Plum Pudding Model”.
1. Atoms themselves are neutral
2. Atoms have electrons that have a
negative charge
3. Atoms have particles with a
positive charge

5. History Continued Rutherford – Lived in 1909 and
wondered about the JJ Thomson’s model. He used
the Gold Foil
Experiment to study how
alpha particles
interact with thin metal foil.

6. Gold Foil Experiment
Rutherford aimed a beam of high speed alpha particles at a piece of really thin (Au) Gold Foil.
He found that almost all particles passed through with no detection! Only 1 in 8000 did!
He then proposed that all atoms positive charge as well as the mass is concentrated in a small center core at the center of the atom. He called the center a nucleus and considered most of an atom empty space.

7. Modern Atomic Theory - Parts
Atoms are made of three parts:
Protons
Neutrons
Electrons
These are subatomic particles

8. Nucleus
The nucleus is at the center of the atom and contains only protons and neutrons.
Protons have a positive charge (+)
Neutrons do NOT have a charge
(Zero charge)
Both weigh 1 amu (atomic mass units.)

9. Electrons Electrons: Move in space around the the nucleus
Weighs 1/1836 amu (VERY small)
Electrons have a negative charge (-)
The outer most electrons are called valence electrons
*Note: the number of electrons in an atom can change, which changes the chemical properties of an atom.

10. Neutral Atoms
Atoms are neutral (overall charge of 0) unless a change in electrons takes place
The number of protons = the number of electrons in a neutral atom
Protons and electrons have equal but opposite charges
Protons have significantly more mass than electrons

11. Atomic Number
Atoms contain a unique positive charge located in the nucleus
The number of protons makes each element unique
Atomic number = the number of protons
The number of protons NEVER change.

12. Mass of an Atom Mass number is measured in Atomic Mass Units (AMU).
AMU = # of Protons + # of Neutrons
Note: The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes

13. Ions Ion – when an atom gains or loses an electron.
1. Anion – an atom gains electrons and becomes negatively charged ex) O-2
2. Cation- an atom loses electrons and becomes positively charged Ca+2

14. Isotopes
Isotopes are atoms with the same number of protons but different number of neutrons but have the same chemical properties.
The more neutrons, the greater the atomic mass.
ex) N-14 has 7 neutrons
N-15 has 8 neutrons

15. Average Atomic Mass
Shows the average atomic mass of all occuring isotopes.
Ex)90% of C-12
10% of C-13
Make percentages into decimals
Multiple the decimal times the atomic mass.
(.90)(12) + (.10)(13) = 12.1 amu

16. Bohr Model
Electrons are arranged in shells (rings). Each shell represents a different energy level and is assigned a quantum number (n).
Shows exactly where the electrons are located – The Bohr model is only good for hydrogen (H). It is a basic model but helped to form the current model for electron structure.
There can only be 2 electrons in the first shell, and 8 in the second shell

17. Wave – Mechanical Model
Shows the relative location of electrons
1. It is represented by a cloud of negative charges around the nucleus
2. The cloud mass is most dense where there is a high probability of finding an electron

18. Ground State vs. Excited State
First energy level is n=1.
This is the energy level
closest to the nucleus.
When an electron absorbs
energy (heated up)
it becomes excited
and jumps to higher energy
levels, n=2, n=3, n=4, etc.
This is called the
“excited state”.

19. Spectral Lines
Energy is absorbed by an atom so that an electron jumps from the ground state to the excited state
When the electron returns to the ground state, radiation (energy) is emitted and gives off light. This can be used to identify an element.