1. Atomic Structure
Chemistry
Chapter 4

2. The Atom
The smallest particle of an element that still behaves like that element
Ex) The smallest bit of gold that still has the intensive properties of gold.
Started with the philosophy of Democritus
Atoms are indivisible and indestructible!

3. Dalton buys the Democritus hype.
John Dalton uses experiments to come up with 4 ideas about the atom
Elements are composed of indivisible atoms.
All atoms of the same elements are identical. Atoms of different elements must be different.
Atoms of different elements can be combined together in ratios to form compounds.
Chemical reactions occur when atoms are separated, attached, or rearranged. But atoms aren’t changed, only rearranged.

4. What are atoms made of?

5. What are atoms made of?
Pens!

6. The Parts of the Atom
Three particles have been discovered to be part of the atom
Protons
Electrons
Neutrons
PENs!
Okay, not really, but hopefully it will help you remember.

8. Thomson’s Discovery
Thomson used electricity to create a stream of particles (cathode ray tube)
He could deflect their flow with positively charged plates
The stream was pulled towards positively charged objects
Must be negatively charged!
Thompson had discovered that atoms MUST have a negative particle.
Electrons

10. More about the electron…
The mass must be very small (1/1840th of the mass of hydrogen)
The charge must always be the same
Regardless of the element
Must be part of all elements

12. The electrons get friends!
Rutherford shot positively-charged alpha particles at gold
Most went straight through
But some bounced straight back, or sideways
There must be a solid positively charged “kernel” at the center
“The nucleus” made of protons
Chadwick discovers that there is also uncharged particles that make it heavier
“Neutrons!”
Have basically the same mass as protons

13. The atom gets a “shape” Basically all empty space
Nucleus at the center
Very, very, very small
Positively charged protons
Neutrally charged neutrons
99.995% of the mass
Electrons revolving around the outside
Zooming near the speed of light, with very little mass
Require more space than the nucelus

14. If time permits

15. Atoms!
Dalton was right that different elements would have different atoms.
But some different atoms are actually the same elements!
What determines the element? THE NUMBER OF PROTONS!
Oxygen is oxygen because it has 8 protons.
Nitrogen is nitrogen because it has 7 protons.

16. Atomic Number The number of protons is called the “atomic number”
Elements are separated and arranged specifically by their atomic number
No two atoms with different atomic numbers can be the same element!
EVER!
EVVVVVVVVVVVVAAAAR!!!!!!
For neutral atoms, the protons and electrons are equal

17. Mass Number Mass number is how “heavy” the nucleus of an atom is
The number of protons plus neutrons
The two sub-atomic particles that have a large mass
Protons + Neutrons = Mass
Neutrons = Mass – Protons
The mass number doesn’t affect what element it is
EX) Hydrogen can have a mass number of 1, 2 or 3, but it’s atomic number is ALWAYS 1!
Mass number

18. Okay, lets try some! I have 14 protons. I also have 15 neutrons.
What element am I?
I also have 15 neutrons.
What is my mass?

19. Okay, lets try some! I have 14 protons. I also have 15 neutrons.
What element am I? Silicon!
I also have 15 neutrons.
What is my mass? 29
Therefore I am Silicon-29

20. How about another one? My mass is 14, my atomic number is 6.
What element am I, and how many neutrons do I have?

21. How about another one? My mass is 14, my atomic number is 6.
What element am I, and how many neutrons do I have?
I am CARBON-14, and I have 8 neutrons
14-6 = 8!

22. Isotopes The number of neutrons can be different in an element.
Atoms of the same element MUST have the same number of protons
The number of neutrons can be different in an element.
Ex) Carbon-12
6 protons, 6 neutrons
Ex) Carbon-14
6 protons, 8 neutrons
Isotopes of elements have different numbers of neutrons, and therefore different numbers of neutrons
But Isotopes are still the same element, just different masses

23. Consider the following…
Mass number is just a COUNT of the number of subatomic particles that have mass.
Ex) 9 protons + 10 neutrons is 19 masses
It gives you a very close, but general idea of the mass of an atom.
But it is not exact.

24. Atomic Masses Units (amu)
The mass of protons and neutrons is not always exactly 1
(but really close)
The only one that they are is Carbon-12, and everything else is based off that.
Neutrons are (usually) a little heavier than 1, protons less
So we calculate based on “atomic mass units”, which is based off of Carbon-12=12.000
Then we calculate the mass of all the other atoms (sorry)
Mass =

25. So, atomic mass is affected by…
The element
The number of protons compared to neutrons
The number of electrons (barely)
But as a general rule it is the protons plus the electrons-ish

26. (Average) Atomic Mass
We can’t fit ALL the isotopes on a periodic table
and we don’t need to because the isotopes are usually all mixed together with each other.
So, what we do is take the AVERAGE amu, and use that as the mass of an element.

27. Calculating average AMU
You could find every single atom in the universe and get the mass, then divide by the number of atoms.
Or you could determine what fraction of the atoms are which isotope, and determine the average.
“Weighted average”
Average AMU (with isotopes A, B, and C) =
(% of A)(mass of A) + (% of B)(mass of B) + (% of C)(mass of C)
% MUST BE WRITTEN AS A DECIMAL
EX) 65% = 0.65
This is where the significant digits of the AMU come from.

28. Calculating average AMU practice
Carbon has 3 major isotopes.
Actually, it has 16 known, but 13 are very, very, very rare
Carbon 12
AMU: (repeating)
98.928%
Carbon 13
AMU:
1.071%
Carbon 14
AMU:
~ %
Don’t forget to turn % into decimal before doing math!