1. Atomic Structure

2. Atom
Definition – smallest particle of an element that retains the chemical identity of that element
Term proposed by Democritus in 450 BC
Democritus proposed that all matter is composed of tiny, indivisible particles
Rejected by Aristotle because Democritus could not explain how particles “stuck together”

3. Dalton’s Atomic Theory of Matter- 1803
Based on observations made by scientists during the 1700’s, John Dalton proposed a theory that could explain the properties of matter
Four postulates:
Each element is made of small particles called atoms
All atoms of a given element are identical, but different from those of any other element
Atoms are neither created nor destroyed in a chemical reaction
A given compound always has the same relative numbers and kinds of atoms

4. Are the four postulates all true?
What do we know about atoms that means we might need to re-state one of the postulates?

5. Postulate Number 2 – not all atoms of the same element are the same
You can have different Isotopes of the same element
Example: Iodine-125 and Iodine-127 are both iodine, but are different from each other. Iodine-125 is radioactive and Iodine-127 is not.

6. History of Discovering about Atomic Structure
Dalton thought atoms were hard and round like tiny marbles
Early 1800’s –
Franklin- two kinds of electric charges
(positive and negative)

7. Electrons were First Particles Found
An English scientist discovered electrons and named them. An Amercian scientist found their mass.
Mass of an electron to be 9.11 x grams

8. Protons were next particles found
Atoms are electrically neutral so the negative electrons must be balanced by an equal number of positive particles
An English scientist found these particles and called them protons.
He also discovered these positively charged particles were all concentrated in the center of the atom – called it the nucleus

9. Modern Atomic Theory
Atoms are composed of protons, neutrons, and electrons
At first, scientists visualized electrons orbiting the nucleus much like planets orbiting the sun

10. Last particle found was the neutron
Neutrons have a mass approximately equal to that of a proton
Neutrons have no charge
Neutrons help “glue” the nucleus together

11. Fundamental Subatomic Particles
Particle Location Charge Mass (amu)
Proton Nucleus
Neutron Nucleus
Electron Outside
Nucleus

12. Moseley’s Discovery
Found that atoms of each element contain a unique positive charge in their nucleus
Conclusion: atom’s identity comes from the number of protons in the nucleus
Called “Atomic Number”
Neutral atoms – positive charges must equal negative charges
Conclusion – for neutral atoms, no. of electrons equals no. of protons

13. Information in Periodic Table
Atomic Number (sometime not in center)
N Chemical Symbol
Nitrogen
Atomic Mass

14. Mass Number
Hydrogen -1
The 1 after the hydrogen is the mass number. Used
to help keep track of what isotope you have.
Definition
Sum of the neutrons and protons
Always a whole number
Another way to represent
Mass Number
H H H
Atomic Number

15. Mass of an Atom
Atomic Mass Unit – approximately the weight of one neutron or proton
Used because the weight in grams is very small
1 a.m.u. = 1.66 x g.

16. Element’s atomic mass
Weighted average of all of the isotopes of an element 7 N
Nitrogen
Atomic Mass
Nitrogen contains mostly Nitrogen-14 with a small amount of Nitrogen-15

17. Practice Find sodium (Na) on the periodic table I gave you
Find the atomic number. What does that number mean?
Find the atomic mass. What does that number mean?

18. How many electrons does one sodium atom have?
I have one atom of sodium represented by 46V 23
How many protons does it have?
How many neutrons does it have?