1. Chapter 4
Atomic Structure

2. Theories about matter were based on the ideas of Greek philosophers:
Democritus (400 B.C. ) – coins the term “atom” saying matter can be subdivided only as small as an elemental particle.
from Greek “a” means not and “tomos” means cutting
atom means “indivisible”

3. Aristotle thought that all matter consisted of 4 basic elements:
earth, air, fire, water
*Their views were not backed by any evidence.

4. Dalton’s Atomic Theory (1808)
All matter is composed of small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.

5. Atoms of different elements combine in simple-whole number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.

6. scanning electron microscope OR (STM – scanning tunneling microscope)
In 1981 we see atoms!
scanning electron microscope OR
(STM – scanning tunneling microscope)
See page (Ni atoms)

7. What instrument is used to observe individual atoms?
A: scanning electron microscope (or STM)
B: particle accelerator
C: graduated cylinder

8. Please make your selection
Democritus, who lived in Greece during the fourth century B.C., suggested that matter is made up of tiny particles that cannot be divided. He called these particles .
Electrons
Ions
atoms

9. Please make your selection
Suppose you could grind a sample of the element copper into smaller and smaller
particles. The smallest particle that could no longer be divided, yet still has the chemical properties of copper, is ___________.
A copper atom
An ion
A proton

10. Please make your selection
Dalton’s atomic theory included the idea that the atoms of different elements can chemically combine in _______ ratios.  
Decimal
Fraction
Partially whole numbers
Whole numbers

11. Please make your selection
The modern process of discovery about atoms began with the theories of an English schoolteacher named .
Aristotle
Albert Einstein
John Dalton
Democritus

12. Please make your selection
Which of the following is not a part of Dalton’s atomic theory?
a. All elements are composed of atoms.
b. Atoms of the same element are alike.
c. Atoms are always in motion.
d. Atoms that combine do so in simple whole-number ratios.

13. Please make your selection
Dalton theorized that atoms are indivisible and that all atoms of an element are identical. Scientists now know that
a. Dalton’s theories are completely correct.
b. atoms of an element can have different numbers of protons.
c. atoms are all divisible.
d. all atoms of an element are not identical but they all have the same mass.

14. Please make your selection
Which of these statements is included in Dalton’s atomic theory?
a. Chemical reactions occur when atoms are separated, joined, or rearranged.
b. Some but not all elements are composed of atoms.
c. Atoms of the same element can have different numbers of protons.
d. Atoms are divisible.

15. Please make your selection
The identity of an element can be determined on the basis of which of the following?
a. the number of protons in an atom of the element
b. the number of neutrons in an atom of the element
c. the mass number of the element
d. the atomic mass of the element

16. What instrument is used to observe individual atoms?
A: scanning electron microscope (or STM)
B: particle accelerator
C: graduated cylinder

17. Section 2: The Structure of the Atom
consists of: Protons Neutrons Electrons
symbols p no e-
location: in nucleus in nucleus outside nucleus
gain/loss: not gained can be can be gained
or lost different number or lost
(isotopes) (forms ions)

18. mass number: 1 amu 1 amu 0 amu
consists of: Protons Neutrons Electrons
mass number: 1 amu amu amu
*important # p *holds nucleus *involved in
info: determines together chemical
type of element reactions
(atomic number) 

19. Please make your selection
What is the atomic number and the mass number of an atom with 11 protons and 12 neutrons?
a. atomic number = 11 and mass number = 12
b. atomic number = 12 and mass number = 11
c. atomic number = 11 and mass number = 23
d. atomic number = 23 and mass number = 12

20. Nuclear Force:
Very strong, short range force that holds the particles of the nucleus together
Question: What is it about the composition of the nucleus that requires the concept of nuclear forces?
A: Like charges do repel each other, so protons would not be expected to be close to other protons. Nuclear forces prevent the protons from repelling each other.

21. Question: What are the attraction relationships in the nucleus (application of nuclear forces)? Answer: proton-proton, neutron-proton, neutron-neutron

22. Section 4.2 Structure of the Nuclear Atom
One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles (p+ n0 e-)

23. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron (pg. 106)

24. Mass of the Electron
Mass of the electron is
9.11 x g
The oil drop apparatus
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

25. Conclusions from the Study of the Electron:
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

26. Conclusions from the Study of the Electron:
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

27. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1
9.11 x 10-28
Electron cloud
Proton (p+) +1
1.67 x 10-24
Nucleus
Neutron
(no)

28. Ernest Rutherford’s Gold Foil Experiment - 1911
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
Particles that hit on the detecting screen (film) are recorded

29. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

30. The Rutherford Atomic Model
Based on his experimental evidence:
The atom is mostly empty space
All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
The nucleus is composed of protons and neutrons (they make the nucleus!)
The electrons distributed around the nucleus, and occupy most of the volume
His model was called a “nuclear model”

31. Rutherford’s atom

32. Section 4.3

33. Isotopes For each element there are different kinds
They are chemically alike & react the same in chemical reactions
Differ in mass because there are different numbers of neutrons in the nucleus.

34. Isotope hyphen notation:
Element name-dash mass number Example: carbon-14 where14 is the mass number. (NOTE: mass number = #p+ and #n0) ________ protons ________electrons ________ neutrons

35. How many protons, neutrons, and electrons are there in cobalt-60
How many protons, neutrons, and electrons are there in cobalt-60? _______ protons _______ electrons _______ neutrons

36. isotope symbols
Contain the symbol of the element, the mass number and the atomic number.
Mass
number X
Superscript →
Atomic
number
Subscript →

37. Br Symbols 80 35 Find each of these: number of protons
number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

38. Symbols
If an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
number of protons
complete symbol

39. Symbols
If an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
complete symbol

40. *Another word for isotope is nuclide.
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

41. Hydrogen isotopes have names
Hydrogen isotopes have names. All other isotopes are named with the mass number.
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

42. Measuring Atomic Mass
Instead of grams, the unit we use is the Atomic Mass Unit (amu)
It is defined as one-twelfth the mass of a carbon-12 atom.
Carbon-12 chosen because of its isotope purity.
Each isotope has its own atomic mass, thus we determine the average from percent abundance.

43. Composition of the nucleus
Atomic Masses pg. 116
Atomic mass is the average of all the naturally occurring isotopes of that element.
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%
Carbon =

44. To calculate the average: p. 118
Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.
If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

45. Example problem:
Element X has two natural isotopes. The isotope with a mass of amu (10X) has a relative abundance of 19.91%. The isotope with a mass of amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element. See page 119

46. Example problem:
The relative abundances and atomic masses are 0.337% (mass = amu), 0.063% (mass = amu), and % (mass = amu), respectively. Calculate the average atomic mass of argon.