1. Atomic Structure & Past Atomic Models
Chapters 5 & 13

2. Atoms Atoms are the basic unit of matter
Atoms are so small, that billions can fit into a period at the end of a sentence
Atoms aren’t solid, they are made of various subatomic particles

3. Atomic Structure
Nucleus: massive center of the atom, holds the protons and neutrons
Proton: positively charged particle, mass = 1amu

4. Atomic Structure Neutron: has NO charge (neutral), mass = 1amu
Electron: orbits the nucleus, negatively charged, very small mass

5. Atomic Structure
Energy Level: where an electron can be found (e- orbital, e- shell)
Ion: an atom with a charge, more or less e- than a neutral atom

6. Atomic Structure Atomic Number: # of protons
Mass Number: # of protons plus number of neutrons (always whole number)
Isotope: atom with more or less neutrons but the same # of protons, mass # is different, atomic # will stay the same

7. Atomic Structure
Atomic Mass: the weighted average of all the isotopes of an element (usually a decimal)
The mass number of the most abundant isotope is usually the atomic mass rounded to a whole number

8. Atomic Structure
To find the number of neutrons without a mass number, round the atomic mass to the nearest whole number, then subtract the # of protons

9. Atomic Structure
A neutral atom will have the same number of electrons as it does protons

11. Atomic Structure Fill in the Chart: (assume neutral) Name Symbol
Mass #
# P
# e #N
carbon 6 8 He 4 2 14 7

12. Atomic Structure Name Symbol Atomic # Mass # # P # e #N carbon C 6 148
helium He 2 4
oxygen O 16
nitrogen N 7

13. “Formulas” to Remember
Atomic number = # protons
Mass number = # protons + # neutrons
# neutrons = mass # - # protons
Charge = #protons - # electrons
# electrons = # protons - charge

14. Average Atomic Mass
Average atomic mass is the weighted average of all the isotopes of that element
To calculate, you need to do a weighted average calculation
Multiply the mass numbers by the percent abundances and add them all together – do NOT divide by the total number!!

15. Average Atomic Mass Avg. Atomic Mass calculation:
Suppose we have element X, with two isotopes, X-200 (85%) and X-202 (15%). What is the atomic mass of X?
(200)(.85) + (202)(.15) =
= amu

16. Average Atomic Mass amu is the unit of atomic mass
1 amu is equal to 1/12 of a carbon-12 atom
We cannot use grams as our unit when speaking of a single atom, but we will use grams when speaking of a much larger quantity of atoms (mole) later on

17. Early Models & Atomic Theory
Democritus – thought atoms were indivisible & indestructible
Lacked experimental support
4th century B.C.

18. Early Models & Atomic Theory
John Dalton – ( )
Dalton’s Atomic Theory
All elements composed of tiny, indivisible atoms
Atoms of same element are identical. Atoms of 1 element are different from another element

19. Early Models & Atomic Theory
Dalton’s Atomic Theory (cont)
Atoms of different elements can either physically mix or chemically combine in simple, whole number ratios to form compounds

20. Early Models & Atomic Theory
Dalton’s Atomic Theory (cont)
Chemical Rxn’s occur when atoms are separated, joined or rearranged.
Atoms of 1 element cannot change into another element by a chemical rxn.

21. Early Models & Atomic Theory
Dalton’s Theory………
Was tested experimentally
Dalton formulated hypotheses to explain his observations

22. Early Models & Atomic Theory
Dalton’s Theory is mostly accepted today
Except: we now know atoms CAN be divided - into subatomic particles
AND – not all atoms of the same element are exactly identical (ions & isotopes)

23. A Limerick of 5 Hypotheses
The Atom:
A Limerick of 5 Hypotheses
They make up all the matter you’ll find
And each element has its own kind
They wont mix in a fraction; (For a chemical reaction)
For reactions, they’re just realigned!
But Dalton, that Brit, Thought they could not be split.
Now we know they have 3 bits combined!

24. Early Models & Atomic Theory
JJ Thompson ( )
discovered electrons using a cathode ray tube
passed electric current through gases at low pressure

25. Early Models & Atomic Theory
A glowing beam formed between the 2 electrodes
Called the cathode ray

26. Early Models & Atomic Theory
The cathode ray is attracted to metal plates that have a positive charge
Negatively charged plates repelled the ray

27. He called these particles electrons
Atomic Structure
The ray must be negatively charged particles moving at a high speed
He called these particles electrons

28. Atomic Structure
JJ Thomson’s model of the atom is called the plum-pudding model – the electrons were randomly placed throughout the atom

29. Rutherford Ernest Rutherford discovered the nucleus
He had a thin piece of gold foil & bombarded it with alpha particles (helium nucleus)
Most particles passed through the foil like he expected
Some were deflected slightly

30. Other particles bounced back at him!
He was as surprised as if he had sent a bowling ball at a tissue and it came back to hit him
He figured that there must be a dense positive part of the atom, but most of the atom is empty space

32. Bohr’s Model
Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus

33. Bohr’s Model
The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of NRG to move from one to another

34. Review Dalton – thought atoms were solid and indivisible
JJ Thomson – discovered the electron, & made the plum-pudding model

35. Review Rutherford – discovered the nucleus
Bohr – proposed electrons in orbitals around nucleus

36. Evolution of Atomic Models: Summary
First were Dalton’s atomic 5 traits.
Then came Thompson’s e- Pudding update
Nucleus found by Rutherford.
But Bohr’s orbits, in other words,
Won him free beer forever, how great!