Chapter 3

Atoms: The Building Blocks of Matter

Objectives:

Students should be able to:• Summarize the essential points of Dalton’s

atomic theory.• Describe the particle theory of matter.• Use the Bohr model to differentiate among

the three basic particles in an atom• Compare the Bohr atomic model to the

electron cloud.

Atomic Models

• The Bohr Model The nucleus is orbited

by electrons. The electrons are in different energy

levels.

Early theories of matter

Philosophers (ideas) 1.Democritus (460-370 B.C.)

a. First to propose the idea that matter was not divided into smaller parts

b. Atomos: Atoms: tiny particles could not be divided

2. Aristotle (384-322 B.C.) a. Rejected Democritus

b. believed in 4 elements air, earth, water and wind

(everything came from these 4 elements)

3. Alchemist

a. Changed metal into gold

b. Tried to prolong life

c. Through their mistakes they learned chemistry

d. First to do actual experiments

• Dalton’s Model - Performed a number of experiments that

eventually led the idea of atoms - Theory: 1. All elements are composed of atoms.

Atoms are indivisible and indestructible particles.

2. Atoms of the same element are exactly alike.

3. Atoms of different elements are different.

4. Compounds are formed by the joining of atoms of two or more elements.

This theory became the foundation of modern chemistry.

J.J. Thomson

- Discovered the electron - Plum-pudding model of the atom Stated atoms were made from a

positively charged substance with negatively charged electrons scattered about

His Experiment 1. Passed an electric current through a gas.

2. As the current passed through the gas, it gave off rays of negatively charged

particles

3. Passed a magnet over the gas. The beam bent away or moved towards the magnet.

Conclusions: 1. The negative charges came from within the

atom. 2. A particle of matter smaller than the atom

had to exist. 3. The atom was divisible. 4. Called the negatively particles “corpuscles” (now called electrons) 5. Since the gas was known to be neutral,

there had to be positive charged particles in the gas.

• His experiment:

Rutherford

- Discovered the nucleus and it’s charge, positive

- His experiment: Gold-Foil experiment

Results: 1. Most of the positively charged particles

passed straight through 2. A few of the alpha particles were slightly

deflected 3. A very small number of alpha particles

hit and bounced back off the foil

Conclusions:1. Most of matter is empty space

2. Scattered throughout- not everywhere- are small areas of positive charge

3. Scattered throughout are areas of very dense matter

4. He called the center the nucleus; it is positive and tiny compared to the rest of the atom

• Rutherford’ model

Chadwick: discovered the neutron

Completing the atom

1. Atomic number (Z) - tells the number of protons - in a neutral atom: protons = electrons

2. Mass number (A) protons + neutrons

3. A

z

x ex.

40

20

Ca

nucleotide

4. Protons and neutrons make up 99.7% of the atoms mass

5. Chemical name Hydrogen atomic number 1 chemical symbol H average atomic mass 1.008

Examples:

How many protons, neutrons and electrons in the following:

1. 2. 3. 4.

Properties of subatomic particles

Isotopes 1. atoms with the same number of protons but different number of

neutrons

2. Identified by the mass

3. Example: Chlorine-35 Chlorine-37 mass number

Examples of neutral atoms and isotopes Atomic # Mass # p n e

27 Al3+ 13 27 13 14 10

13

70 Zn 30 70 30 40 3030

• Relative Atomic Masses

1.Atomic mass unit (u) : Is exactly 1/12 the mass of a carbon-12

atom

2. Atomic mass: expression of atomic mass unit

3. Atomic mass unit– weighted average mass of the isotopes of that element

Average atomic mass formula(mass)(% of element) + (mass)(% of element) = (total mass)

Examples: 1. A carbon isotope has an abundance of 98.90%

with a amu of 12.0 g and the other isotope has an abundance of 1.10% with a amu 13.003 g. What is the average atomic mass of the two isotopes?

2. An element, E, has an atomic mass of 18.40 and consists of two isotopes:

E-17 with a mass of 16.95 and E-20 with a mass of 19.35. How much E-20 does this element contain?

(amu)(%) + (amu)(%) = total mass (100)

Unstable Nuclei and Radioactive DecayRadioactivity

1. Substances spontaneously emitting radiation

2. radiation – ray and particle emitted by radioactive materials

3. Elements (atoms) can change into atoms of another element

Radioactive elements1. Nuclei are unstable

2. Gain stability by losing electrons

3. Radioactive decay – process of becoming stable by losing energy

Type of radiation

1. alpha radiation (α)

a. Deflected towards (-) charged plate

b. alpha particles have 2p+ and 2n0

c. α particles equivalent to He-4

d. Nuclear equation (Shows how decay occurs)

226 4 + 222 Ra α Rn 88 2 86

2. Beta radiation

a. Deflected towards the positive plate

b. Beta particles ( )

c. 14 0 14 C β + N 6 -1 7

Half-Life (t1/2)

1. Time required for one-half of a radioisotope’s nuclei to decay into it products

2. Examples:Iron-59 half-life is 44.5 days. How much of a 2.000 g sample will

remain after 133.5 days.

t1/2 = 44.5 days

half-life