1. Chapter 4 Atomic Structure
Section 4.1 Studying Atoms
Section 4.2 The Structure of an Atom
Section 4.3 Modern Atomic Theory

2. Section 4.1 Studying Atoms
**Studying atoms is difficult b/c they are so small in size.
Ancient Greek Models of Atoms
Democritus (philosopher)- believed that all matter consisted of extremely small particles that could not be divided
atomos(Greek)-”uncut” or “indivisible”
Different types of atoms with specific sets of properties.

3. Section 4.1 Studying Atoms
Ancient Greek Models of Atoms
Aristotle: there was no limit to the # of times matter could be divided
All substances were built up from only four elements—earth, air, fire, and water.

4. Section 4.1 Studying Atoms
Dalton’s Atomic Theory
John Dalton -England (1766); teacher: interested in predicting weather
Studied behavior of gases in the air (way gas exerts pressure); a gas consists of individual particles

5. Section 4.1 Studying Atoms
Evidence for Atoms (Dalton)
Measured masses of elements that combine when compounds form
The ratio of the masses of the elements in the compound is always the same (compounds have a fixed composition)

6. Section 4.1 Studying Atoms
Dalton’s Theory
Developed theory to explain why elements in a compound always join in the same way
Key Concept: Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

7. Section 4.1 Studying Atoms
Dalton’s Theory
All elements are composed of atoms.
All atoms of the same elements have the same mass; atoms of different elements have different masses.
Compounds contain atoms of more than one element.
In a particular compound, atoms of different elements always combine the same way.

8. Section 4.1 Studying Atoms
Thompson’s Model of the Atom
When some materials are rubbed, they gain the ability to attract or repel other materials.
Based on their behavior: either have positive or a negative charge (like charges repel; opposites attract)
Charged particles (some) can flow from one location to another. (electric current)
J.J. Thompson- used electric current to learn more about atoms

9. Thomson’s Experiments
Figure 5
Thomson’s Experiments

10. Section 4.1 Studying Atoms
Evidence for Subatomic Particles
Thompson concluded that particles in the beam had a negative charge b/c they were attracted to the positive plate.
Key Concept: Thompson’s experiments provided the first evidence that atoms are made of even smaller particles.

11. Section 4.1 Studying Atoms
Thompson’s Model
An atom is neutral (no neg. or pos. charge); how is that possible?
If atoms have negative charge, there must be a positive charge!
Thompson’s model-negative charges evenly scattered throughout w/ a positively charged mass of matter (plum pudding model)

12. Section 4.1 Studying Atoms
Rutherford’s Atomic Theory
Rutherford’s Hypothesis (Ernest Rutherford)-discovered that uranium emits fast-moving particles that have a positive charge (alpha particles)
What happens to alpha particles when they pass through a thin sheet of gold?

13. Section 4.1 Studying Atoms
Rutherford’s Hypothesis
Hypothesized that mass and charge at any location in the gold would be too small to change the path of an alpha particle.
I.e. Most particles would travel in a straight line/path; others would be deflected slightly

14. Section 4.1 Studying Atoms
The Gold Foil Experiment
Narrow beam of alpha particles aimed at gold
Screen around the gold produced flashes of light when struck by alpha particles (path of alpha particles after passing through gold was determined)
More particles were deflected than expected.

15. The Gold Foil Experiment
Figure 5
The Gold Foil Experiment

16. Section 4.1 Studying Atoms
Discovery of the Nucleus
Alpha particles came close to another charged object; greater deflection
Many alpha particles passed through the gold
**Atoms are made up mostly of empty space.
(Concluded that positive charge of an atom is not evenly spread throughout the atom)
The positive charge is concentrated in a very small, central area (nucleus-a dense positively charged mass in the atom’s center)
Key Concept: According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

17. Section 4.2 The Structure of an Atom
Properties of Subatomic Particles
Rutherford-evidence for the existence of 2 subatomic particles; predicted a third
Key Concept: Protons, electrons, and neutrons are subatomic particles.

18. Section 4.2 The Structure of an Atom
Properties of Subatomic Particles: Protons
Proton-a positively charged subatomic particle
Found in the nucleus of an atom
Each proton has a 1+ charge

19. Section 4.2 The Structure of an Atom
Properties of Subatomic Particles: Electrons
Electron-a negatively charged subatomic particle
Found in the space outside the nucleus
Each electron has a 1- charge

20. Section 4.2 The Structure of an Atom
Properties of Subatomic Particles: Neutrons
Neutron-a neutral subatomic particle
Found in the nucleus of an atom
Has a mass almost exactly equal to that of a proton

21. Figure 12

22. Section 4.2 The Structure of an Atom
Comparing Subatomic Particles
Key Concept: Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.
Protons and neutrons have the same mass
An electron’s charge is equal in size to, but is opposite of a proton’s charge

23. Section 4.2 The Structure of an Atom
Comparing Subatomic Particles
Neutrons have no charge (neutral)
Protons and neutrons are inside the nucleus of an atom.
Electrons are outside of the nucleus.

25. Section 4.2 The Structure of an Atom
Atomic Number and Mass Number
Dalton: atoms of one element are different from the atoms of another element.
Discovery of subatomic particles=ability to describe the differences between atoms of elements

26. Section 4.2 The Structure of an Atom
Atomic Number
# of protons in an atom of an element
# of protons in the atom of an element is always the same
# of protons in the atoms of different elements is different
Atoms are neutral: each positive charge is balanced by a negative charge (ie. The atomic number of an element is equal to the # of electrons in an atom.)
Ie. 2 protons=2+, so atom has 2 electrons=2-: so =0 NO CHARGE!

27. Section 4.2 The Structure of an Atom
Mass Number
Def.-the sum of the protons and neutrons in the nucleus of an atom
Ex. Al (Aluminum): 13 protons and 14 neutrons; mass number is 27
Find the # of neutrons: need mass # and atomic #
#neutrons=mass# - atomic #

28. Section 4.2 The Structure of an Atom
Isotopes
Def.-atoms of the same element that have different numbers of neutrons and different mass numbers
Key Concept: Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

29. Section 4.2 The Structure of an Atom
Isotopes
Ex. Oxygen (O)-has 3 isotopes: O-16, O-17, O-18
All have 8 protons; all react w/ hydrogen (water) or w/ iron (rust)

30. Section 4.2 The Structure of an Atom
Isotopes
Hard to notice any difference in physical or chemical properties of most elements and their isotopes except….Hydrogen
H-1 (no neutrons); H-2 (1 neutron); H-3 (2 neutrons)
Add neutron=doubles hydrogen’s mass so…water w/ H-2=heavy water

31. Section 4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Niels Bohr ( ) Danish physicist; had worked w/ Rutherford
Model focused on electrons; move like planets orbiting the sun

32. Section 4.3 Modern Atomic Theory
Energy Levels
Def.-the possible energies that electrons in an atom can have
Bohr’s model: electrons move w/ constant speed in fixed orbits around the nucleus
Each electron has a specific amt. of energy
If atom gains or loses energy, the electron’s energy can change

33. Section 4.3 Modern Atomic Theory
Energy Levels
An electron cannot exist between energy levels
Key Concept: An electron in an atom can move from one energy level to another when the atom gains or loses energy.

34. Section 4.3 Modern Atomic Theory
Evidence of Energy Levels
Scientists can measure the energy gained when electrons absorb energy (move to higher energy level)
Can also measure the energy released when the electron returns to a lower energy level

35. Section 4.3 Modern Atomic Theory
Electron Cloud Model
Bohr: electrons moved like planets in the solar system (WRONG)
Today: scientists know that electrons move in a less predictable way
Electron cloud-a visual model of the most likely locations for electrons in an atom

36. Section 4.3 Modern Atomic Theory
Electron Cloud Model
Cloud is denser-higher probability of finding an electron at that location
Key Concept: Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus.

38. Section 4.3 Modern Atomic Theory
Atomic Orbitals
Electron cloud=all orbitals in an atom
Orbital-a region of space around the nucleus where an electron is likely to be found
Key Concept: An electron cloud is a good approximation of how electrons behave in their orbitals.

39. Section 4.3 Modern Atomic Theory
Atomic Orbitals
Lowest energy level (where electron has least energy) has only 1 orbital
Each orbital can contains a maximum of 2 electrons.

40. Number of Orbitals and Electrons in Energy Levels
Figure 15
Number of Orbitals and Electrons in Energy Levels

41. Section 4.3 Modern Atomic Theory
Electron Configurations
Def.-the arrangement of electrons in the orbitals of an atom
Key Concept: The most stable electron configuration is one in which the electrons are in orbitals with the lowest possible energies

42. Section 4.3 Modern Atomic Theory
Electron Configurations
When electrons in an atom have the lowest possible energies—atom is in its ground state.
An electron can move to orbital w/ a higher energy (excited state—less stable)