1. Chemistry 141
Lecture 4
Wednesday, September 13, 2017

2. Objectives for today
Explain the evidence for the existence of atoms and for their structure
Develop a system for identifying and categorizing atoms of different elements
Determine the number of each of the subatomic particles in a given atom
Determine atomic weights from atomic masses of individual isotopes
Organize the elements based on their structure and properties

3. The Law of Definite Proportions
A given compound always contains exactly the same proportion of elements by mass. (Proust, )

4. The Law of Multiple Proportions
When two elements form a series of compounds, the ratios of the masses of one element that combine with 1 unit of the other element can always be reduced to small whole numbers (Dalton, )
Example:
In X, the amount of C combining with 1 g of H is 6 g
In Y, the amount of C combining with 1 g of H is 12 g
The ratio of C:H in Y to C:H in X is 2

5. Implies the existence of atoms!!
The Fundamental Laws
The Law of Conservation of Mass –Matter cannot be created or destroyed
(Lavoisier, “The father of modern chemistry”, 1789)
The Law of Definite Proportion – A compound always contains the same proportions of elements by mass (Proust, )
The Law of Multiple Proportions – When two elements form a series of compounds, the ratios of the masses of one element that combine with 1 gram of the other element can always be reduced to small whole numbers (Dalton, )
Implies the existence of atoms!!

6. Dalton’s Atomic Theory
1. Matter consists of indivisible and
indestructible atoms
2a. All of the atoms of a given chemical
element are identical in mass and
in all other properties
2b. BUT different chemical elements have
different kinds of atoms and, in
particular, such atoms have different
masses
3. Atoms retain their identity in chemical reactions
4. A compound forms through the combination of atoms of unlike elements in small, whole-number ratios which are always the same for a given compound
Image from:
Chemical Achievers copyright ©2000 The Chemical Heritage Foundation

7. Discovery of the electron
Particles emanated from the cathode (negative electrode)
Particles affected by electric field, move from (-) to (+)
 they must be negatively charged
Observed with many cathode materials
 they must be common to all elements e-
anode
(+)
cathode
(–)

8. Measuring the mass-to-charge ratio
When an electric field is applied across electron rays, they bend toward
the positive electrode
measure deflection
Can be related to e/me (mass-to-charge ratio)

9. Determination of the electron’s charge
Robert Millikan (1909 – 1913)
Very small, charged oil droplets
 charge comes in ‘units’ the same way atoms do
F = neE
(electric field)
F = mg
(gravity)

10. What about the positive charge?
“The plum pudding model”

11. Discovery of the nucleus
Ernest Rutherford (1911) – Beam of α-particles shot at a thin gold foil
Expected
Observed

12. Properties of subatomic particles
Nucleus contains small, massive, positively charged particles (protons)
Most atoms (except H) have an atomic mass greater than the number of protons
Neutrons were proposed to explain this difference (Chadwick, 1932)
Particle
Mass
(amu)
(g)
Charge
electron volts (eV)
Coulombs (C)
Proton
 10-24 +1
 10-19
Neutron
 10-24
Electron
1/1822.9
10-28 -1
 10-19
1 amu = × 10–24 g = 1/12 mass of 12C

13. Radioactivity
A TRAGEDY OF INNOCENCE: Technicians leaning, without protection, over open dishes of radium salts. They are unknowingly getting dangerous doses of radiation which, after a continued exposure over many years, will cause irreparable damage and sometimes death.
Some elements spontaneously emit radiation or particles (Henri Becquerel, Marie Curie )
Three types:
γ-rays: high-energy electromagnetic radiation (light)
β-particles: high-speed electrons
α-particles: particles with +2 charge, relative mass of 4
Existence of additional sub-atomic particles

14. How big is an atom? For graphite… 5 atoms = 1 nm 1 atom = 0.2 nm = 2 Å
(1 nm = 10-9 m = 10-7 cm )
(1 Å = 0.1 nm = m)
Image from:

15. The nuclear model of the atom

16. What happens to most of the α particles that strike the gold foil in Rutherford’s experiment? Why do they behave that way?
α particles are scattered equally across a range of deflection angles due to the high density of the foil nuclei.
Most α particles pass through the foil without being deflected, because most of the volume of the atoms that comprise the foil is empty space.
Most α particles are scattered at acute angles as they pass close to the foil nuclei.
Most α particles are deflected in a backward direction from the foil due to the high density of the foil atom nuclei.
Answer: b

18. What happens to most of the α particles that strike the gold foil in Rutherford’s experiment? Why do they behave that way?
α particles are scattered equally across a range of deflection angles due to the high density of the foil nuclei.
Most α particles pass through the foil without being deflected, because most of the volume of the atoms that comprise the foil is empty space.
Most α particles are scattered at acute angles as they pass close to the foil nuclei.
Most α particles are deflected in a backward direction from the foil due to the high density of the foil atom nuclei.
Answer: b

19. How do atoms of different elements differ?

20. Atomic symbol

21. Mass Spectrometry Used to measure exact relative masses of atoms
Atoms with same # protons but different masses are isotopes

22. Table of Isotopes

23. AN EXAMPLE: fill in the following chart
Symbol 52Cr
Protons
Neutrons
Electrons
Mass #

24. Average atomic weight
What is the average atomic mass of carbon?

25. Table of Isotopes

26. Periodicity

27. Mendeleev’s Original Periodic Table
(Developed in 1869)
Images from:

28. Modern Periodic Table

29. Is it a metal or nonmetal? To what group does it belong?
Chlorine is a halogen (Table 2.3). Locate this element in the periodic table.
Is it a metal or nonmetal?
To what group does it belong?
Answer: b

30. A particular atom of chromium has a mass of 52
A particular atom of chromium has a mass of amu, whereas the atomic weight of chromium is given as amu. Explain the difference in the two masses.
The amu value and the amu value represent two different isotopes of chromium.
The atomic weight of chromium does not depend on the atom with a mass of amu.
The atomic weight of amu is for a different isotope than the amu mass.
The atomic weight of chromium (51.99 amu) is an average atomic mass of all the naturally occurring isotopes of chromium.
Answer: d

31. Which statement about the mass and volume of an atom is incorrect?
The mass is determined mostly by the protons and neutrons.
The volume is mostly empty space.
The mass is concentrated in the nucleus.
The volume is determined by the arrangement of the protons.
Answer: d