1. “Atomic Structure”
Essential Question: Describe how the model of the atom has changed since the Greek idea of “Atomos”

2. Defining the Atom Democritus
He believed that atoms were indivisible and indestructible “Atomos”

3. Dalton’s Atomic Theory (experiment based!)
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical.
John Dalton
Atoms combine in whole-number ratios to form compounds
In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

4. 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
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5. Thomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
J. J. Thomson

6. Ernest Rutherford’s Gold Foil Experiment - 1911
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7. 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 neutron was discovered
by Chadwick in 1932

8. Bohr Model
Bohr refined Rutherford's model of the atom by finding that electrons were found in distinct energy levels around the nucleus. When electrons move to different energy levels energy is released.

9. Quantum Atomic Model
This is the current view of atoms, it is a mathematical model rather than a physical model. It describes the electrons as being found in regions of probability around the nucleus, an “electron cloud”

10. Changes in Atomic model
Copy these diagrams

11. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1
negligible
Electron cloud
Proton (p+) +1 1
Nucleus
Neutron
(no)
Copy this chart

12. In a neutral atom the # protons is equal to the # electrons
Atomic Number
The “atomic number” of an element is the number of protons in the nucleus
In a neutral atom the # protons is equal to the # electrons

13. Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope:
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

14. Complete Symbols
Mass
number X
Superscript →
Atomic
number
Subscript →

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

16. Isotopes and Atomic Mass Notes
Essential Question: Why is the mass listed on the periodic table an average?

17. Isotopes are….
Atoms of the same element that have a different numbers of neutrons.
Thus, different mass numbers.

18. To show different isotopes the mass # is written after element:
carbon carbon-14
Elements occur in nature as mixtures of isotopes.

19. Isotope Examples

20. Examples of Isotopes: Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium)
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

21. Average Atomic Mass
This is based on the abundance (percentage) of each isotope of that element in nature.
Instead of grams, the unit we use is the Atomic Mass Unit (amu)

22. 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.

23. To calculate the average:
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)

24. (mass ) x (% as a decimal)
Atomic Masses
Atomic mass is the average of all the naturally occurring isotopes of that element.
Isotope
Mass
% in nature
(mass ) x (% as a decimal)
Carbon-12 12
98.89%
12 x =11.87
Carbon-13 13
1.11%
13 x = 0.14
Carbon-14 14
0.01%
14 x = .0014
Carbon = =12.011

25. The Periodic Table: A Preview
Each horizontal row (there are 7 of them) is called a period
Each vertical column is called a group, or family