1. Section 1 Development of Atomic Theory
Chapter 4 Atoms
Section 1
Development of Atomic Theory

2. BR. Who came up with the first theory of atoms?
Objectives-
1. Give an example of how new scientific data can cause an existing scientific explanation to be supported, rejected or revised
2. Evaluate selected theories based on supporting scientific evidence.

3. Objectives (cont.)
3. Cite evidence that scientific investigations are conducted for many reasons.
4. Identify scientific evidence that has caused modifications in previously accepted theories.
GLE’s:

4. Democritus Over 2000 years ago Democritus
Universe made of indivisible units called atoms.
Atomos- unable to be cut or divided
Did not have evidence to support theory

5. Dalton 1808 John Dalton revised atomic theory
Atoms could not be divided
All atoms of a given element were exactly alike; and atoms of different elements could join to form compounds.
Based on experimental evidence

6. Dalton (cont.)
Law of definite proportions- a chemical compound always contains the same elements in exactly the same proportions by weight or mass. (supported Dalton’s theory)
Foundation of modern atomic theory
Could not explain all experimental evidence

7. Thomson 1897 experiment suggested that atoms were not indivisible
He was experimenting with electricity
studying cathode rays not atoms
Cathode ray tube experiment suggested
that cathode rays were made of negatively charged particles that came from inside the atoms

8. Thomson (cont.) Revealed that atoms could be divided
Discovered electrons, negative particles
New model- electrons spread through-out
the atom ( plum pudding model)
Mass and positive charge evenly distributed
Electrons scattered through out

9. Rutherford Found Thomson’s model needing revising
Proposed that most of the mass of the atom was in the center
Conducted Gold-foil experiment where most particles passed straight through
Some particles were deflected
Some particles came straight back
Not what he expected

10. Rutherford (cont.) Discovered the nucleus Nucleus was very small
Electrons orbit the nucleus (like sun and planets)
Led to new model of the atom

11. Section 2- Structure of Atom
BR. Rutherford’s Gold foil experiment led to the discovery of what?
Objectives:
1. Identify the 3 subatomic particles by location, charge, and relative mass
2. Describe the results of loss or gain of electrons on charges of atoms

12. Objectives (cont.) 3. Identify valence electrons in first 20 elements.
GLE’S:

13. Atom
Three subatomic particles compared by mass, charge, and location in the atom.
Copy chart on page 119
Nucleus- small, dense, center of atom
Atoms are Neutral.

14. Atom (cont.) Nucleus is made of 1. protons- + charged particle
2. neutron- neutral or no charge particle
Protons and neutrons are almost equal in size and mass.

15. Electrons
Move in a dense cloud (fan blades moving) outside the nucleus
Very tiny electrons = 1 neutron or proton
Negatively charged
Exact location cannot be determined. Speed and direction cannot be determined
Located by shading; shaded region is orbital: darker shading better chance to find

16. Protons Each element has a unique number of protons
Elements are identified by the number of protons that they have in the nucleus of an atom
Atoms are neutral because they have the same number of p and e. They cancel each other out.

17. Ions Atoms that have lost or gained electrons.
Lose electrons become positive.
Gain electrons become negative
If atoms lose or gain electrons they are not atoms, but IONS.

18. Atoms Atoms are held together by an electric force
+ and – charges attract each other by an electric force
This attraction is what holds the atom together just like the attractive force between solids and liquids.

19. Atoms and Elements Atoms of different elements have unique structures.
Because atoms have different structures, they have different properties.
Atoms of the same element can vary in structure also.
Atoms of each element have the same number of protons, but different numbers of neutrons.

20. Atomic number Atomic number equals the number of protons.
Since atoms are neutral, it also equals the number of electrons.
Neutral atom = -
Atomic numbers go from 1 to 116

21. Mass Number
Equals the total number of subatomic particles in the NUCLEUS of the atom.
Nucleus contains p and n.
Mass number is equal to p + n.
Neutrons vary so mass number can vary for the same element.
See figure 3 on page 121

22. Isotopes
Isotope has same atomic number but different number of neutrons.
Isotopes have same atomic number but different mass numbers.
Isotopes have the same number of protons but different number of neutrons

23. Isotopes (cont.) Some isotopes are more common than others.
Radioisotopes- unstable isotopes that emit
radiation and decay into other isotopes.
They continue to decay until they reach a stable isotope.
They decay at a fixed rate. (fraction of a second to millions of years)

24. Isotopes (cont.)
Since isotopes have the same number of protons and electrons, they have the same chemical properties.
Isotopes have different masses.
Isotopes of an element vary in mass because their numbers of neutrons differ.

25. Isotopes of Water
Water has 3 isotopes. Each isotope has 1 proton and 1 electron.
Protium- 1 proton and no neutrons (mass number of 1
Deuterium-1 proton and 1 neutron (mass number of 2)
Tritium- 1 proton and 2 neutrons mass number of 3)

26. Calculating Neutrons Isotopes are written 35 mass number ( p+n)
Cl symbol
atomic number (p)
Mass number – atomic number = neutrons

27. Atomic mass
Atoms are expressed in unified atomic mass units because the mass is so small.
Unified atomic mass = equal to 1/12th of the mass of a carbon-12 atom.
Also called atomic mass unit

28. Atomic Mass Average atomic mass for an element is a weighted average.
More common isotopes have more effect than less common isotopes of the element.

29. Mole Mole- collection of a very large number of particles.
Written 6.02 x particles and is called Avogadro’s number
Avogadro’s number= the number of atoms in
12 grams of carbon-12. (Popcorn kernels covering the US 310 miles tall)

30. Molar mass Molar mass- the mass in grams of 1 mole of a substance
1 mole C12 = 12 g

31. Converting between Moles and Grams
Amount x molar mass of element = Mass(g Moles mole of element
3 moles x g S = g S
S mole S
96.27 g S x 1 mole S = 3 mol S
32.21 g S
Work problem 1 a-d on page 126

32. Compounds have Molar mass
Add all molar masses in compounds and then work the same way.
H2O x (1.01 g H x 2) + 16 g O = g/mol
1 mole H2O H2O
1 mol H20 = g

33. Section 3 Modern Atomic Theory
BR List the 3 subatomic particles and give their location, relative mass, and charge.
Objectives:
Describe the results of the loss or gain of electrons on the charges of atoms.
Identify valence electrons in first 20 elements.
Draw Bohr models of 1st 20 elements
GLE’S:

34. Modern Model of Atom
Electrons are found only in certain energy levels. NOT between levels
Location of electrons can not be predicted precisely
Bohr- electrons can be in only certain energy levels.
Bohr energy level related to electron’s path around the nucleus

35. Modern Model of Atom (cont.)
Electrons must gain energy to move to a higher energy level.
Electrons must lose energy to move to a lower energy level.
1925 Bohr’s model revised.

36. Modern Atomic Theory (cont.)
Old out – not like sun and planets
New in –
Electrons behave more like waves on a vibrating string than like particles.

37. Energy levels and Electrons
Many energy levels for electron to occupy
The number of energy levels that are filled in an atom depends on the number of electrons.
Valence electrons- those electrons in outer most energy level
Valence electrons determine the chemical properties of the atom.

38. Energy levels Maximum electrons in energy level 1st = 2 2nd= 8 3rd= 18
4th= 32
Must fill 1st and 2nd energy level before going to the 3rd energy level.

39. Energy levels (cont.) There are 4 types of orbitals.
Orbitals are s, p, d, f.
Orbitals determine the number of electrons that each level can hold.

40. Electron Jumping
Electrons jump between energy levels when an atom gains or loses energy.
Lowest energy level called ground state.
Excited state-gains energy it moves to another level

41. How Electrons Move
Electrons gain energy by absorbing a photon and move to a higher energy level
Photon- particle of light; each have different energies
The electron may fall back to previous energy level when it releases a photon.
Photons determine which level the electron will
jump to.

42. Light- Photons determine which level the electron will jump to.
Atoms absorb or emit light at certain wavelengths.
Energy of photon is related to the wavelength of the light.
High energy photons= short wavelengths
Low energy photons= long wavelengths

43. Atomic Fingerprint
Because of each element’s unique atomic structure, the wavelengths emitted depend on the particular element.
Each element emits its own characteristic color. Neon= red blue=copper
sodium= yellow strontium= red
orange = calcium green= barium