Chapter 2: The AtomAtom: The smallest unit of an element that

retains the chemical properties of the element

Atoms consist of three primary particles.

Electrons, Protons & Neutrons

2.2

Mat

ter

and

Str

uctu

re

• 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.

8

2.2

Mat

ter

and

Str

uctu

re• Surrounding the nucleus is a diffuse

region of negative charge populated by:

- electrons - negatively charged particles, very little mass

Table 2.1 Selected Properties of the Subatomic Particles

Name Charge Mass(amu) Mass (grams)

Electrons (e) -1 5.4 x 10-4 9.1095 x 10-28

Protons (p) +1 1.00 1.6725 X 10-24

Neutrons (n) 0 1.00 1.6750 x 10-24

2.2

Mat

ter

and

Str

uctu

re

CAZ X

Mass

Number

mass number (A) - sum of the number of protons and neutrons

Atomic Number

Charge of particle

Symbol of the atom

atomic number (Z) - the number of protons in the atom

9

2.2

Mat

ter

and

Str

uctu

re

Calculate the number of protons, neutrons and electrons in each of the following:

B115

Fe5526

2.2

Mat

ter

and

Str

uctu

re• Isotopes - atoms of the same element

having different masses

– contain same number of protons– contain different numbers of neutrons

Isotopes of Hydrogen

H H H 31

21

11

Hydrogen(Hydrogen - 1)

Deuterium(Hydrogen - 2)

Tritium(Hydrogen - 3)

10

2.2

Mat

ter

and

Str

uctu

re • 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

Mat

ter

and

Str

uctu

re • 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

Mat

ter

and

Str

uctu

re

• 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-

10

Ions are Charged Atoms

How many protons neutrons and electrons are in the following ion?

2.2

Mat

ter

and

Str

uctu

re

K3919

-23216S

Development of the Atomic Theory

• 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.

11

2.3

Ato

mic

The

ory 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.

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

Ato

mic

The

ory

2.3

Ato

mic

The

ory

Subatomic Particles: Electrons, Protons and Neutrons

• Electrons were the first subatomic particles to be discovered using the cathode ray tube.

Indicated that the particles were negatively charged.

2.3

Ato

mic

The

ory • 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

Ato

mic

The

ory

The Nucleus12

• 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

Ato

mic

The

ory

“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.

2.3

Ato

mic

The

ory

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

Electromagnetic Spectrum

high energyshort wavelength

low energylong wavelength

The Bohr Atom

Electrons exist in fixed energy levels surrounding the nucleus.

Quantization of energy

• Initial understanding of the atom by Niels Bohr

Promotion of electron occurs as it absorbs energy

Excited State

Energy is released as the electron travels back to lower levels.

Relaxation

14

• Orbit - what Bohr called the fixed energy levels.

• Ground state - the lowest possible energy state.

• The orbits are also identified using letters as “quantum numbers”

– n = 1, 2, 3, …

• When the electron relaxes (c) the energy released is observed as a single wavelength of light.