1. Atomic Structure
Chapter 4

2. Studying Atoms
4.1

3. Ancient Greek Models of Atoms
Democritus
Believed that all matter was made of small particles that could not be divided
Called the particles atoms (atomos means uncut)
Aristotle
Did not think there was a limit to number of times matter could be cut

4. Aristotle’s View

5. John Dalton Studying gases
Concluded gas was individual particles because of the way they exerted pressure
Measured the masses of elements, discovered all compounds have fixed compositions
Modeled atoms as solid spheres

6. Dalton’s Theory
All matter is made up of individual particles called atoms, which can’t be divided.
All elements made of atoms
Atoms of same element have same mass, atoms of different elements have different masses
Compounds have more than one type of element
Atoms of different elements always combine the same way in compound

7. JJ Thomson Used electric current to study atoms
Thomson thought atoms had charged particles in them
He set up a tube with positive plate and a negative plate, the beam of light was either repelled or bent towards the plates
First evidence that atoms are made of even smaller particles

8. Thomson

9. Thomson’s Model
Had evenly scattered positively and negative charges in the atom
Called this the “plum pudding model”
Think chocolate chip ice cream

10. Ernest Rutherford
His assistant aimed a beam of particles through a slit so that they would hit gold foil, he though that many would be so small they would go in a straight line and hit a screen that would light up
Many particles were deflected though

11. Rutherford

12. Rutherford’s model
Concluded that the positive charge of the atom is not evenly distributed
Concentrated in a very small, central area- called the nucleus
Nucleus = a dense, positively charged mass at center of atom
Negative charges (e-) traveled randomly outside the nucleus

13. The structure of the atom
4.2

15. Parts of the atom
Proton
Neutron
Electron

16. Sub-Atomic Particles
Protons = Positively (1+) charged particle in the nucleus
Neutrons = neutral particle in nucleus
Electron = negatively (1-) charge particle in nucleus
Protons, electrons and neutrons can be distinguished by mass, charge and location in atom

17. Atomic number Atoms of an element always have the same number of p+
Different elements have different atomic numbers
Atomic Number = the # of protons in an atom
Can find on the periodic table
Equals the number of electrons (atoms have no charge so + = -)

18. Mass number Mass Number = sum of protons and neutrons
Number of Neutrons = Mass number – Atomic Number

20. Isotopes
Atoms of the same element but have different numbers of and neutrons
Atomic mass is different because number of neutrons is different

21. Property
Ordinary Water
Heavy Water
Melting Point
0.00 ⁰C
3.81 ⁰C
Boiling Point
100 ⁰C ⁰C
Density
g/cm3
g/cm3

22. Modern Atomic Theory
4.3

23. Bohr’s Model Atom had nucleus with p+ and N (agreed with Rutherford)
Electrons moved at constant speeds in fixed orbits around the nucleus (like planets and sun)
If atoms gain or lose energy the energy of the electrons changed
Electrons had energy levels
Electrons could move up electrons levels or down electrons levels (like stairs)

24. Bohr’s Model Energy e- gain and lose can be measured
We can see some of the energy released by e- as visible light
Fireworks:
Explosions cause e- to move to different energy levels, when e- move back down energy levels they release light
No two elements have the same energy levels so we get different colors of light

26. Bohr model

27. Electron cloud model
Atoms have energy levels, but do not move like planets
Electron cloud model = most likely location of e- in an atom, denser the locations have higher probability of find an electron
Like a fan – when still you can count fan blades, when it is on high they look like a blur, you know they are there but can’t not exactly sure where they are

28. Electron cloud model

29. Atomic orbitals
Orbital = region of space around the nucleus where e- is likely to be
Example: Map of school, you place a dot every 10 minutes of where you would be, after a week you would have your orbital, a place where you would likely to be found
Electron cloud = good approximation of how e- behave in orbitals
Lowest energy level has 1 orbital, higher energy levels have more orbitals
Each orbital can have 2 e- at most

30. Energy levels, orbitals and E-
# of Orbitals
Max. # of E- 1 2 4 8 3 9 18 16 32

31. Electron configurations
E- configuration = arrangement of e- in orbitals of an atom
Most stable e- configuration is the one in which e- are in lowest energy level possible
Ground state = when all electrons in atom have lowest possible energy levels
Excited state = when e- move to higher energy levles