1. Chapter 4
Atomic Structure

2. Democritus Theory Greek Philosopher in the 4th century B.C
- believed that all matter consisted of extremely small particles
- suggested these particles are made of invisible units called atoms
- term atom is derived from a Greek word meaning “unable to divide”
- believed there were different types of atoms, liquids: round,smooth solids: rough, prickly
- unable to provide evidence that an atom existed, therefore many people were very skeptical

3. Dalton’s Atomic Theory
John Dalton
- interested in predicting the weather SO.. studied the behavior of gases in the air, concluded that a gas consists of individual particles
Evidence
- masses of elements as they combined to form compounds always produced the same ratio no matter what the size of the sample
ex. carbon dioxide
- 1 carbon 2 oxygen: 1:2 ratio

4. Dalton’s Atomic Theory
- used a Greek concept of the atom and the 3 laws to give the atomic theory a scientific basis

5. Dalton’s Atomic Theory Cont.
5 Principles
1. All matter is made of indivisible and indestructible atoms
2. All atoms of a given element are identical in their physical and chemical properties
3. Atoms of different elements differ in their physical and chemical properties
4. Atoms of different elements combine in simple whole-numbers ratios to form compounds

6. Dalton’s Atomic Theory Cont.
5. Chemical reactions consist of the combination, separation, or rearrangement of atoms
- theory explained most of the chemical data of the day and was readily accepted
- evidence since has shown the first two principles are not valid; overlooked that most atoms will combined with other of their own kind
- NOT discarded only modified

7. Thomson’s Experiment J.J Thomson 1st experiment
- pumped most the air out of a glass tube, placed a metal plate at each end
- applied a voltage to two metal plates one became positively charged:anode
other became negatively charged: cathode

8. Thomson’s Experiment Cont.
2nd experiment
- placed a charged metal plate on either side of the glass tube

9. Thomson’s Experiment Cont.
Observations
1st experiment
- glowing ray emerged between the cathode and anode
2nd experiment
- charged plates caused the beam to deflect/bend
- repelled from the negative
- attracted to the positive
Conclusion
- beam of light (stream of charged particles)
- negative

10. Thomson’s Experiment Cont.
Uses
- TV screens
- computer monitors
- radar displays
*Later became known as cathode rays due to their origin now known as an electron beam
1st to provide evidence that atoms are made of smaller particles
revised Dalton’s model

11. Thomson’s Model
* If there is a negative charge there must also be a positive charge
Why?
- believes the atom is neutral
Plum Pudding
- negative particles are evenly scattered throughout an atom with a positively charged mass of matter
- similar to that of chocolate chip ice cream
- later proved to be incorrect

12. Rutherford’s Theory Ernest Rutherford
- a former student of Thomson came up with a more accurate picture of the atom in 1909
-oversaw the now famous gold foil experiment
Gold Foil Experiment
Hypothesis
- alpha particles are thousands of times more massive, hence they would not be impeded as it passed through the “atomic pudding”
- beam of positively charged particles, alpha particles from a radioactive source was directed through a sheet of very thin gold foil

13. Gold Foil Experiment

14. Gold Foil Experiment Cont.
Observations
- nearly all passed through undeflected and produced spots of light
- some were widely deflected, and a few bounced straight back
What massive object did they hit?
atomic nucleus, an extremely dense positively charged center of the atom

15. Gold Foil Experiment Cont.
Why did the others pass through then?

16. Gold Foil Experiment Cont.
Conclusion
- atom is mostly empty space
- most of its mass concentrated in the central region, atomic nucleus

17. Gold Foil Experiment Cont.
- the nucleus and surrounding electrons occupy only a tiny fraction of the atomic volume
- diameter of an atom is generally about 10,000 times greater than the diameter of its nucleus
* If the nucleus were the size of the period at the end of this sentence, the outer edges of the atom would be located some 3.3 meters away *

18. Gold Foil Experiment Cont.
Rutherford’s Model
- all of the atoms positive charge is concentrated in the nucleus, which only takes up a very small amount of the atom
Can we then say we are mainly empty space?

19. Objectives Identify three subatomic particles
Understand how subatomic particle was discovered
Compare the properties of the subatomic particles
Distinguish between atomic number and mass number
Calculate the number of protons, electrons and neutrons in an atom

20. Subatomic Particles Subatomic particles - 3 important to chemistry
- protons, neutrons, electrons

21. Protons
Definition
- a positively charge subatomic particle that is found in the nucleus of an atom
About Protons
- proton is nearly 2000 times more massive than the electron, but equal in charge and opposite in sign to the electron
- number of protons in the nucleus is electrically balanced by an equal number of electrons
ex. oxygen atom: contains 8 electrons and protons: neutral atom, no net charge

22. Electron
Definition
- a negatively charged subatomic particle that is found in the space outside the nucleus
- name comes from the Greek word for amber
- Amber: material discovered by early Greeks that was found to exhibit the effects of electrical charging
ex. Ben Franklin: Key/Kite
- lead others to experiment with electric currents through gases in sealed tubes

23. Neutrons
Definition
- is a neutral subatomic particle that is found in the nucleus of the atom
- mass almost exactly equal to that of the proton

24. Comparing Subatomic Particles

25. Atomic Number Definition - number of protons in the atom
ex. Oxygen 8p + 8n = 16
- elements are classified by this number
- continues up to 119
- unique to a given element
- all atoms are electrically neutral, meaning the number of electrons must equal the number of protons
this arrangement of elements by their atomic numbers makes up the periodic table
Usually located at the upper left hand corner

26. Mass Number Definition
- the total number of protons and neutrons in the nucleus of an atom
- mass number – atomic number = neutrons
ex. N: mass number of 14
atomic number of 7
7 neutrons

27. Mass Number Cont.
usually found at the bottom of the atomic symbol, and sometimes found written at the bottom left of an atomic symbol
ex. 16O

28. Mass Number
- although a given type of atom will usually contain a certain number of neutrons in the nucleus, a small percentage will not
ex. most hydrogen atoms contain no neutrons
- a small percentage contain one neutron and a smaller percentage two neutrons
What do we call atoms with a different number of neutrons?
- isotopes

29. Isotopes
Definition
- the number of neutrons in the nucleus of a given element may vary, protons remain the same
ex. H contains 1 protons (1H)
H contains 1 protons and 1 neutrons (2H) deuterium
H contains 1 protons and 2 neutrons
(3H) tritium
ex. 14C: Carbon-14

30. Atomic Mass Definition - mass of an atom in atomic mass units (amu)
- atoms have very little mass
- equal to 1/12th of the mass of carbon
- often an average mass
- weighted mass
AMU or the Dalton (Da)
- equal to x kg

31. Atomic Mass Number Cont.
ex. 99% of all carbon atoms are the isotope containing 6 neutrons, the remaining 1% is the heavier isotope containing 7 neutrons, which raises the average mass of carbon from to

32. Objectives
Describe Bohr’s model of the atom and the evidence for energy levels
Explain how the electron cloud model represents the behavior and locations of electrons in atoms

33. Bohr’s Model of the Atom
- an early conceptual model of the atom
- classic planetary model in which electrons whirl around the small but dense nucleus: like planets orbiting the Sun
- developed by the Danish physicist Niels Bohr in 1911
- each electron has a certain energy that is determined by it’s path around the nucleus
- explains how atoms (lose) emit or (gain) absorb energy
resulting energy: energy level

34. Bohr’s Model: Energy Levels
Definition
- any of the possible energies an electron may have in an atom
Evidence
- measured amount of energy gained or lost
- when energy is lost we often see it as a light
ex. fireworks
this model no longer explained all observations being made by scientists and new models were created
ex. Electron cloud model

35. Electron Cloud Model
visual model of the most likely locations for electrons in an atom
Orbitals
- a region in an atom where there is a high probability of finding an electron
ex. propeller on a helicopter (you know its there you see a blur, can’t pinpoint exact location)
- 4 orbitals
s – 2 houses 2 e-
p – 3 houses 6 e-
d – 5 houses 10 e-
f – 7 houses 14 e-

36. Orbitals - electrons occupy the lowest energy levels first
- electrons in the outermost energy levels of an atoms are called valence electron

37. Electron Configurations
Definition
- arrangement of electrons in the orbitals of an atom
(similar to seating assignments on an airplane)
- when all electrons at their lowest energies this is called ground state