1. Christopher G. Hamaker, Illinois State University, Normal IL
The Atom
Christopher G. Hamaker, Illinois State University, Normal IL
© 2005, Prentice Hall

2. Dalton Model of the Atom
John Dalton proposed that all matter is made up of tiny particles.
These particles are molecules or atoms.
Molecules can be broken down into atoms by chemical processes.
Atoms cannot be broken down by chemical or physical processes.

3. Dalton’s Model
According to the law of definite composition, the mass ratio of carbon to oxygen in carbon dioxide was always the same. Carbon dioxide was composed 1 carbon atom and 2 oxygen atoms.
Similarly, 2 atoms of hydrogen and 1 atom of oxygen combine to give water.
Dalton proposed that 2 hydrogen atoms could substitute for each oxygen atom in carbon dioxide to make methane with 1 carbon atom and 4 hydrogen atoms. Indeed, methane is CH4!

4. Dalton’s Theory A Summary of Dalton’s Atomic Theory:
An element is composed of tiny, indivisible, indestructible particles called atoms.
All atoms of an element are identical and have the same properties.
Atoms of different elements combine to form compounds.
Compounds contain atoms in small whole number ratios.
Atoms can combine in more than one ratio to form different compounds.

5. Dalton’s Atomic Theory
The first two parts of Dalton’s theory were later proven incorrect.
Proposals 3, 4, and 5 are still accepted today.
Dalton’s theory was an important step in the further development of atomic theory.

6. Subatomic Particles
About 50 years after Dalton’s proposal, evidence was seen that atoms were divisible.
Two subatomic particles were discovered.
Negatively charged electrons, e–.
Positively charge protons, p+.
An electron has a relative charge of -1 and a proton has a relative charge of +1.

7. Thomson Model of the Atom
J.J. Thomson proposed a subatomic model of the atom in 1903.
Thomson proposed that the electrons were distributed evenly throughout a homogeneous sphere of positive charge.
This was called the “Plum Pudding” Model of the atom.

8. Mass of Subatomic Particles
Originally, Thomson could only calculate the mass-to-charge ratio of a proton and an electron.
Robert Millikan determined the charge of an electron in 1911.
Thomson calculated the masses of a proton and electron:
An electron has a mass of 9.11 × g
A proton has a mass of 1.67 × g

9. Rutherford’s Gold Foil Experiment
Rutherford fired alpha particles at thin gold foils. If the “plum pudding” model of the atom was correct, most a-particles should pass through undeflected.
However, some of the alpha particles were deflected backwards.

10. Explanation of Scattering
Most of the alpha particles passed through the foil because an atom is largely empty space.
At the center of an atom is the atomic nucleus which contains the atom’s protons.
The a-particles that bounced backwards did so after striking the dense nucleus.

11. Rutherford’s Model of the Atom
Rutherford proposed a new model of the atom:
The negatively charges electrons are distributed around a positively charged nucleus.
An atom has a diameter of about 1 × 10-8 cm and the nucleus has a diameter of about1 × cm.
If an atom were the size of the Astrodome, the nucleus would be a marble.

12. Subatomic Particles Revisited
Based on the heaviness of the nucleus, Rutherford predicted that it must contain neutral particles in addition to protons.
Neutrons, n0, were discovered about 30 years later. A neutron is about the size of a proton without any charge.

13. Atomic Notation
Each element has a characteristic number of protons in the nucleus. This is the atomic number, Z.
The total number of protons and neutrons in the nucleus of an atom is the mass number, A.
We use atomic notation to display the number of protons and neutrons in the nucleus of an atom:

14. Using Atomic Notation An example: The element is sodium (symbol Na).
The atomic number is 11 – sodium has 11 protons.
The mass number is 23 – the atom of sodium has 23 protons + neutrons.
The number is neutrons is: A – Z = 23 – 11 = neutrons.

15. Isotopes
All atoms of the same element have the same number of protons.
Most elements occur naturally with varying numbers of neutrons.
Atoms of the same element that have a different number of neutrons in the nucleus are called isotopes.
Isotopes have the same atomic number but different mass numbers.

16. Isotopes Continued
We often refer to an isotope by stating the name of the element followed by the mass number.
Cobalt-60 is
Carbon-14 is
How many protons and neutrons does an atom of mercury-202 have?
The atomic number of Hg is 80, so it has 80 protons
Hg-202 has 202 – 80 = 122 neutrons Co 60 37 C 14 6

17. Simple & Weighted Averages
A simple average assumes the same number of each object.
A weighted average takes into account the fact that we do not have equal numbers of all the objects.
A weighted average is calculated by multiplying the percentage of the object (as a decimal number) by its mass for each object and adding the numbers together.

18. Average Atomic Mass
Since not all isotopes of an atom are present in equal proportions, we must use the weighted average.
Copper has two isotopes:
63Cu with a mass of amu and 69.09% abundance
65Cu with a mass of amu and 30.91% abundance
The average atomic mass of copper is:
( amu)(0.6909) + ( amu)(0.3091)
= amu

19. Periodic Table
We can use the periodic table to obtain the atomic number and atomic mass of an element.
The periodic table shows the atomic number, symbol, and atomic mass for each element.

20. Conclusions Atoms are composed of protons, neutrons, and electrons.
The protons and neutrons are located in the nucleus and the electrons are outside the nucleus.
Atoms are mostly empty space.
The number of protons is referred to as the atomic number for the atom.

21. Conclusions Continued
All atoms of the same element have the same number of protons.
Isotopes are atoms with the same number of protons but differing numbers of neutrons.
The mass number for an isotope is the total number of protons plus neutrons.
The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes.

22. Complete the following table:
Isotope
Symbol
Atomic Number
# of protons
# of Neutrons
# of electrons
C -12 6
Ca2+ 20
Br-80 36
Te-128 7 10

23. 1. Copper has two isotopes:
Cu-63 Cu-65
Relative abundances % 30.8%
Atomic masses (amu)
Calculate the average atomic mass of copper.
2. Element X has two natural isotopes.
Isotope1 Isotope 2
Relative abundances % %
Atomic masses (amu)
Calculate the atomic mass of this element and name it.