1. Studying Atoms
Lecture 4.1

2. Objectives: Describe ancient Greek models of matter
List the main points of Dalton’s atomic theory and describe his evidence for the existence of atoms
Explain how Thomson and Rutherford used data from experiments to produce their atomic models

3. Do you know what makes up the parts of an atom?
Daily Question
Do you know what makes up the parts of an atom?

4. Overview
Most natural materials are mixtures of pure substances
“Pure” substances are either elements or compounds

5. Ancient Greek Models
Compounds always contain the same elements in the same proportions
Law of Constant Composition
Democritus ( B.C.): was the first to use the word “atomos”( which meant atoms); stated that matter is composed of atoms; atoms are indestructible and indivisible

6. Ancient Greek Models
Aristotle ( B. C.): said that empty space could not exist; matter is made of earth, fire, air, and water

7. Dalton’s Theory
Dalton proposed that all matter is made up of individual particles called atoms, which cannot be divided.

8. Dalton’s Atomic Theory (1808)
1) Elements are made of tiny particles called atoms
2) Atoms of any element are identical
3) Atoms of an element are different from atoms of any other element
4) Atoms of elements combine to form compounds
Always the same numbers and types of atoms

9. Dalton’s Atomic Theory (1808)
5) Atoms are indivisible
Cannot be created or destroyed
Atoms rearrange during chemical reactions

10. Importance of Dalton’s Theory
It explained many important observations
Conservation of mass
Constant composition

11. Atoms and Conservation of Mass+ A B
1 mass unit
3 mass units
4 mass units A B C +
4 mass units
5 mass units
2 mass units
7 mass units

12. Thomson Thomson took Dalton’s model, and expanded on it.
His experiments provided the first evidence that atoms are made of even smaller particles.

13. Importance of Thomson’s Experiments
Confirmed existence of electrons
Implied existence of protons

14. Schematic of a cathode ray tube.

15. Cathode Ray Experiments
Proved the existence of electrons
Sir Joseph John Thomson
Mass of the electron (9.109 x g)
Approximately 1/1800 the mass of the simplest hydrogen atom
The electron is the smallest possible negative charge
All other negative charges are whole-number multiples of the electron

16. Plum Pudding model of Thompson pg 114

17. Models of the Atom
THOMSON
“Plum Pudding” - - -
DALTON
Indivisible -

18. The Nucleus
A dense, positively charged mass located at the center of the atom
Central portion of the atom
Contains most of the atom’s mass, but . . .
Occupies only a tiny fraction of its volume
The plural form of nucleus is nuclei

19. 3min

20. Earnest Rutherford (1899)
Discovered that uranium emits fast-moving particles with positive charges
He wanted to test Thompson’s Plum pudding model
He predicted that most of the particles would go straight through
According to Rutherford’s model, all of the atom’s positive charge is concentrated in its nucleus

21. Rutherford’s Gold Foil experiment.

22. Results of foil experiment… If the Plum Pudding model had been correct.

23. Actual results

24. Rutherford’s Model of an Atom

25. Niels Bohr
( )
Worked with Rutherford and agreed with Rutherford’s model
But his model focused more on the electrons which surrounded the nucleus

26. Bohr’s Model
Electrons move with constant speed in fixed orbits around the nucleus
Like planets around a sun
Each electron in an atom has a specific amount of energy
Electron can gain or lose energy

27. Bohr Model Picture

28. Evidence of Energy Levels
Movement of electrons between energy levels explains the light you see from fireworks
Light is form of energy
No two elements have same set of energy levels , so different elements emit different colors of light

29. Firework Colorants Color Compound Red
strontium salts, lithium salts lithium carbonate, Li2CO3 = red strontium carbonate, SrCO3 = bright red
Orange
calcium salts calcium chloride, CaCl2 calcium sulfate, CaSO4·xH2O, where x = 0,2,3,5
Gold
incandescence of iron (with carbon), charcoal, or lampblack
Yellow
sodium compounds sodium nitrate, NaNO3 cryolite, Na3AlF6
Electric White
white-hot metal, such as magnesium or aluminum barium oxide, BaO
Green
barium compounds + chlorine producer barium chloride, BaCl+ = bright green
Blue
copper compounds + chlorine producer copper acetoarsenite (Paris Green), Cu3As2O3Cu(C2H3O2)2 = blue copper (I) chloride, CuCl = turquoise blue
Purple
mixture of strontium (red) and copper (blue) compounds
Silver
burning aluminum, titanium, or magnesium powder or flakes
Firework Colorants

30. Electron Cloud
Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus
Electrons move in a less predictable way

31. Orbital
Is a region of space around the nucleus where an electron is likely to be found
The electron cloud represents all of the orbitals in an atom

33. Figure 15 pg 117 Energy Level # of orbitals Max. # of electrons 1 2 48 3 9 18 16 32

34. Electron Configuration
arrangement of electrons in the orbitals
The most stable electrons in orbitals with lowest possible energies
Electron moving up an higher energy orbital= configuration is in excited state

35. Subatomic particles Particle Symbol Location Charge Rel. mass
Actual mass
electron e-
Space around nucleus 1-
1/1840
9.11 x 10-28
Proton p
In nucleus 1+ 1
1.673 x 10-24
Neutron n
In nucleas
1.675 x 10-24

36. 4.2 Structure of the Atom

37. The Nucleus is: Positively charged Very Dense
the center portion of the atom
Contains most of the atom’s mass, but . . .
Occupies only a tiny fraction of its volume

38. Composition of the Nucleus:
Proton
found in the nucleus of an atom
Positive charge
Mass of x grams or 1amu
Neutron
found in the nucleus of an atom.
No electrical charge (neutral)
Mass approximately equal to the proton

39. Composition of the Nucleus:
Electron
moves around the nucleus
Negative charge
Very small compared to proton and neutron 1/1800
relatively far away from the nucleus
Most of the atom is empty space
Atom
Electron
Nucleus

40. Protons, Neutrons, and Electrons
Relative Mass
Charge
proton 1
1/1800 +1 –1
neutron
electron

41. Atomic number The atomic number (“Z”) = number of protons in an atom
The atomic number identifies element
elements have different numbers of protons

42. Mass number Mass number (“A”) =protons + neutrons.
Mass number identifies the particular isotope
In an atom, electrons equal the number of protons makes atoms electrically neutral

43. Nuclear Symbol
Mass Number 65 Cu 29
Atomic Number (Z)

44. Cu 65 29 Protons Remember!! Atoms are neutral!! Protons + Neutrons
29 Electrons 65 Cu
36 Neutrons 29
Protons
Remember!! Atoms are neutral!!

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46. Why do atoms have different properties?
Chemical properties are related to the number and arrangement of electrons
Electrons “intermingle” when molecules form

47. Isotopes
Isotopes are atoms of the same element with different mass numbers
Same atomic number
Different mass numbers because they have different numbers of neutrons

48. 2. The Hydrogen Atom
An element is identified by the number of protons in the nucleus
Hydrogen exists in three forms
Protium
One p+, one e–
Most abundant form
Deuterium
One p+, one n0, one e–
Tritium
One p+, two n0, one e–

49. Two isotopes of sodium.

50. How to calculate the number of neutrons in an atom?
Number of neutrons= mass number – atomic number

51. Practice Write the nuclear symbol for bromine-80
How many protons, electrons, and neutrons are in this atom?
Write the hyphen notation for the element that has 6 protons and 7 neutrons
How many electrons does this atom have?
Write the nuclear symbol for this atom