1. Atomic Theory 1803 1913 1926 Nucleus 2s orbital 1904 2p orbitals
1911

2. Ancient Greek Theories of Matter
Around 400 BC, the respected philosopher of the time, Aristotle, proposed that all matter was composed of 4 elements: earth, fire, air and water. This theory was NOT based on scientific evidence.

3. Ancient Greek Theories of Matter
In contrast, a less popular Greek philosopher, Democritus, thought that matter was made of invisible atoms. He believed that atoms could not be divided, and were different sizes and shapes, depending on the material.
Without scientific research to test the theories, this theory was ignored for approximately years.

4. Dalton’s Theory (The Billiard Ball)
Circa 1800
All elements are composed of atoms. Atoms are indivisible and indestructible particles.
Atoms of the same element are exactly alike.
Atoms of different elements are different.
Compounds are formed by the joining of atoms of two or more elements.

5. Thomson Model
Thomson studied the passage of an electric current through a gas in a cathode ray tube.
As the current passed through the gas, it gave off rays of negatively charged particles.
From this important experiment, Thomson discovered the existence of the electron.
Cathode ray tube

6. Thomson Model
He proposed a model of the atom that is sometimes called the Plum Pudding model.
Thomson thought atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

7. Rutherford Model
Rutherford designed a famous experiment called the gold foil experiment. His experiment revealed the existence of the positively charged nucleus of the atom.

8. Rutherford Gold Foil Experiment

9. Rutherford Model
Based on the experimental results, Rutherford proposed that an atom contains a small, positively charged nucleus in the center which is surrounded by negatively charged electrons scattered randomly. +

10. Bohr Model
According to the Bohr model, electrons move around the nucleus in definite orbits, like the planets orbit the sun. Each orbit has a different energy level and a fixed distance from the nucleus.

11. Bohr Model Bohr model of the Li atom with 3 electrons.
The electrons orbit around the nucleus. The electrons are NOT randomly arranged.

12. Electron Cloud Model of the Atom
Shows how probable it is for an electron to be found in a given location of the atom.

13. Quantum Mechanical Model
Most accurate model
Best explains what is observed through experiments

14. Important Contributions to Development of Atomic Structure
Democritus: proposed the existence of atoms circa 400 BC
Dalton: beginning of modern atomic theory; Dalton’s work supported the Law of Conservation of Mass
Thomson: discovered the electron, proving that atoms have subatomic particles
Max Planck: used the idea of quanta (discrete units of energy) to explain hot glowing matter circa 1900
Millikan: oil drop experiment determined the charge of an electron and allowed calculation of the mass of an electron circa 1910
Rutherford: gold foil experiment determined that atoms contain a dense, positively charged nucleus in the center circa 1915
Neils Bohr: theory of atomic structure relating electron arrangement in atoms to chemical properties and regularities of the periodic table; Bohr model suggests that electrons are found in successive orbitals around the nucleus
Chadwick: discovered the neutron, a neutral atomic particle with a mass close to a proton

15. Timeline of Atomic Theory

16. The Atom
The smallest particle of an element that maintains the properties of that element
Composed of subatomic particles: protons, neutrons, and electrons
Protons have a +1 charge
Electrons have a -1 charge
Neutrons are neutral no charge

17. Structure of the Atom
The basic structure of an atom is represented as:

18. Structure of The Atom Spherical and electrically neutral
Contain a positively charged, dense nucleus at the center composed of protons and neutrons. The nucleus is very small (less than times the size of the whole atom) but contains 99.9% of the mass.
Contain a negatively charged electron cloud which accounts for most of the volume of the atom. Most of the electron cloud is empty space, but the electrons move rapidly through this empty space at high speeds.
The electrons are drawn to the positively charged nucleus.

19. Relative Masses and Charges

20. The Atom
Identity of an atom is determined by the number of protons in the atom
Periodic table arranges elements based on the atomic number (Z)
Atomic number equals the number of protons in the atom

21. The Atom atomic number = number of protons = number of electrons
Atoms are electrically neutral (zero charge).
proton = +1, electron = -1 (equal but opposite)
atomic number = number of protons = number of electrons

22. Learning Check Element # Protons # Electrons Lithium 3 Zinc 30 Bromine
Complete the following:
Element
# Protons
# Electrons
Lithium 3
Zinc 30
Bromine 54 16

23. The Atom How many neutrons are in an atom?
We can calculate the number of neutrons as follows:
mass number – number of protons = number of neutrons OR
mass number – atomic number = number of neutrons

24. The Atom Mass number is always a whole number.
Sometimes it is given in a problem.
We can estimate the mass number by rounding the atomic mass to a whole number.
We can calculate the mass number from the number of protons and neutrons:
mass number = number of protons + number of neutrons

25. Isotopes of Atoms Atomic number (Z) = number of protons.
Atoms are electrically neutral, so
number of protons = number of electrons.
Are all atoms of an element identical?
No!

26. Isotopes of Atoms
The number of neutrons in an atom of an element can vary.
Atoms of a given element having different numbers of neutrons are known as isotopes.
Isotopes are written as
OR X-mass number C-12

27. Isotope An atom of an element with a different number of neutrons.
Carbon has 3 isotopes: C-12, C-13, C-14
Protons
Neutrons

28. Isotopes: Summary
Isotopes have different mass numbers due to their different numbers of neutrons
Isotopes have the same chemical behavior, since chemical behavior is determined by the electrons of the atom

29. Learning Check What is the atomic number of these isotopes?
How many neutrons and protons does each have?
Write the isotope symbols for Copper with a mass number of 65.

30. Isotopes Isotopes in nature have different percent abundances
For instance, Cl-35 tends to be found with 75.78% abundance and Cl-37 has a percent abundance of %. This means that ¾ of the Cl in nature is Cl-35.
Since most of the Cl found in nature is Cl-35, the atomic mass of Cl is closer to 35 than to 37.
Atomic mass of Cl = g/mole

31. Isotopes
Using the percent abundance of the isotopes, we can calculate the weighted average atomic mass of the element (this is why atomic mass is usually not a whole number!)
Name
Symbol
Mass of atom
% abundance in nature
Hydrogen 1H
Deuterium 2H
0.0115
Tritium 3H
Not natural

32. Isotopes
Calculate the average atomic mass of H using the % abundances of its naturally occurring isotopes.
( )( ) = mass contribution of 1H
( )( ) = mass contribution of 2H
= average atomic mass of H

33. Learning Check
Calculate the weighted average atomic mass of the element with the given data.
Predict: Will the actual weighted average atomic mass be closer to 6 or to 7?
Isotope
Mass (amu)
Percent Abundance
X-6
6.015
7.59%
X-7
7.016
92.41%

34. Learning Check Calculate the contribution of each isotope:
(6.015 amu)(0.0759) = amu
(7.016 amu)(0.9241) = amu
Atomic mass = amu amu = amu
pay attention to significant figures!
Identity of the atom is Li

35. Nuclear Changes In a chemical reaction, the nucleus NEVER changes.
In a nuclear reaction, the nucleus DOES change… and the ELEMENT’S identity

36. Nuclear Decay 3 forms of nuclear decay: a decay b decay
g decay emission of high energy photon
(g ray)

37. Radioactive Nuclei are Unstable
Why does this happen?
Nuclei of radioactive elements are very unstable.
Through radioactive decay, radioactive elements become more stable.
LOTS of energy is given off

38. Nuclear Reactions and the Atom
Electrons are involved in chemical reactions. Gaining or losing electrons does not change the element.
The nucleus does not change in a chemical reaction.
The nucleus only changes in a nuclear reaction. When the nucleus changes, the element’s identity is usually changed.