1. Chapter 2: The Atom
Atom: The smallest unit of an element that retains the chemical properties of the element

2. 2.2 Matter and Structure Atoms consist of three primary particles.
Electrons, Protons & Neutrons
Nucleus - small, dense, positively charged region in the center of the atom. Contains:
protons - positively charged particles, with mass of 1 amu.
neutrons - uncharged particles, with mass of 1 amu.
2.2 Matter and Structure 8

3. Surrounding the nucleus is a diffuse region of negative charge populated by:
electrons - negatively charged particles, very little mass
2.2 Matter and Structure
Table 2.1 Selected Properties of the Subatomic Particles
Name Charge Mass(amu) Mass (grams)
Electrons (e) x x 10-28
Protons (p) X 10-24
Neutrons (n) x 10-24

4. Mass
Number
Charge of particle 9
2.2 Matter and Structure
Symbol of
the atom
Atomic Number
atomic number (Z) - the number of protons in the atom
mass number (A) - sum of the number of protons and neutrons

5. Calculate the Composition of an Atom
Calculate the number of protons, neutrons and electrons in each of the following:
2.2 Matter and Structure

6. Isotopes - atoms of the same element having different masses
contain same number of protons
contain different numbers of neutrons
Isotopes of Hydrogen 10
2.2 Matter and Structure
Hydrogen
(Hydrogen - 1)
Deuterium
(Hydrogen - 2)
Tritium
(Hydrogen - 3)

7. Isotopes of the same element have identical chemical properties
Some isotopes are radioactive
Find chlorine on the periodic table.
What is the atomic number? 17
What is the mass given?
35.45
This is not the mass number of an isotope.
2.2 Matter and Structure

8. 2.2 Matter and Structure What is this number?
It is called the atomic mass - the weighed average of the masses of the isotopes that make up chlorine.
Chlorine consists of chlorine-35 and chlorine-37 in a 3:1 ratio.
The weighed average is an average corrected by the relative amounts of each isotope present in nature.
2.2 Matter and Structure

9. 2.2 Matter and Structure Ions are Charged Atoms
Ions - electrically charged particles that result from a gain or loss of one or more electrons by the parent atom.
Cation - positively charged
result from the loss of electrons
23Na  23Na+ + 1e-
Anion - negatively charged
results from the gain of electrons
19F + 1 e-  19F-
2.2 Matter and Structure 10

10. Subatomic particles in ions
How many protons neutrons and electrons are in the following ion?
2.2 Matter and Structure

11. Development of the Atomic Theory11
Dalton’s Atomic Theory - the first experimentally based theory of atomic structure of the atom.
John Dalton
early 1800’s
Much of Dalton’s Theory is still regarded as correct today. See starred items.

12. Postulates of Dalton’s Atomic Theory
1. All matter consists of tiny particles called atoms.*
2. An atom cannot be created, divided, destroyed, or converted to any other type of atom.
3. Atoms of a particular element have identical properties.
2.3 Atomic Theory

13. 4. Atoms of different elements have different properties.*
5. Atoms of different elements combine in simple whole-number ratios to produce compounds (stable aggregates of atoms.)*
6. Chemical change involves joining, separating, or rearranging atoms.*
* These postulates are still regarded as true.
2.3 Atomic Theory

14. 2.3 Atomic Theory Subatomic Particles: Electrons, Protons and Neutrons
Electrons were the first subatomic particles to be discovered using the cathode ray tube.
2.3 Atomic Theory
Indicated that the particles were negatively charged.

15. 2.3 Atomic Theory Protons were the next particle to be discovered
Protons have the same size charge but opposite in sign.
Proton is1837 times as heavy as electron.
Neutrons
Postulated to exist in 1920’s but not demonstrated to exist until 1932
Almost the same mass as the proton.
2.3 Atomic Theory

16. 2.3 Atomic Theory The Nucleus 12
The initial ideas of the atom did not have a “nucleus.”
“Plum Pudding Model”
Earnest Rutherford’s “Gold Foil Experiment” lead to the understanding of the nucleus.
2.3 Atomic Theory

17. 2.3 Atomic Theory
“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue and it came back and hit you.”
-Earnest Rutherford.

18. 2.3 Atomic Theory Light and Atomic Structure 13
Spectroscopy - absorption or emission of light by atoms (energy changes)
Used to understand the electronic structure.
To understand the electronic structure, we must first understand light (or electromagnetic radiation.)
travels in waves from a source
speed of 3.0 x 108 m/s
2.3 Atomic Theory

19. Electromagnetic Spectrum
high energy
short wavelength
low energy
long wavelength

20. The Bohr Atom 14 Initial understanding of the atom by Niels Bohr
Electrons exist in fixed energy levels surrounding the nucleus.
Quantization of energy
Promotion of electron occurs as it absorbs energy
Excited State
Energy is released as the electron travels back to lower levels.
Relaxation

21. Orbit - what Bohr called the fixed energy levels.
Ground state - the lowest possible energy state.

22. The orbits are also identified using letters as “quantum numbers”
When the electron relaxes (c) the energy released is observed as a single wavelength of light.