Chapter 2

The composition and Structure of the Atom

Denniston Topping Caret

4th Edition

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.1 Matter and Structure

• Understanding the structure of the atom will help to understand the properties of the elements.

• Keep in mind that these, as all theories, are subject to constant refinement. The picture of the atom isn’t final.

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2.2 Composition of the Atom• Atom - the basic structural unit of an element.

– The smallest unit of an element that retains the chemical properties of that element.

• Radioactive decay - certain kinds of atoms can “split” into smaller particles and release large amounts of energy.

• 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

- neutrons - uncharged particles

• Surrounding the nucleus is a diffuse region of negative charge populated by:

- electrons - negatively charged particles

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

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

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• Isotopes - atoms of the same element having different masses.

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

Isotopes of Hydrogen

Hydrogen(Hydrogen - 1)

Deuterium(Hydrogen - 2)

Tritium(Hydrogen - 3)

• 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.

• 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 weighted average is an average corrected by the relative amounts of each isotope present in nature.

• Calculate the atomic mass of naturally occurring chlorine if 75.77% of chlorine atoms are chlorine-35 and 24.23% of chlorine atoms are chlorine-37.

Step 1: Convert the percentage to a decimal fraction.

0.7577 chlorine-35

0.2423 chlorine-37Step 2: Multiply the decimal fraction by the mass of that isotope to obtain the isotope contribution to the atomic mass.For chlorine-35:

0.7577 x 35.00 amu = 26.52 amuFor chlorine-37

0.2423 x 37.00 amu = 8.965 amuStep 3: sum to get the weighted averageatomic mass of chlorine =

26.52 amu + 8.965 amu = 35.49 amu

• 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-

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How many protons, neutrons and electrons are in the following ions?

K3919

-23216S 224

12 Mg

2.3 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.*

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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.

Subatomic Particles: Electrons, Protons and Neutrons

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

• Protons were the next particle to be discovered.– Protons have the same size charge but opposite in

sign.

– Proton is 1837 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.

The Nucleus6

• 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.

• Most of the atom is empty space.

• Most of the mass is located in a small, dense region.

2.4 The Relationship between Light and Atomic Structure

• Spectroscopy - absorption or emission of light by atoms.

– Used to understand the electronic structure.

• To understand the electronic structure, we must first understand light. Electromagnetic radiation– travels in waves from a source

– speed of 3.0 x 108 m/s

Electromagnetic Spectrum

high energyshort wavelength

low energylong wavelength

• emission spectrum - light emitted when a substance is excited by an energy source.

•The emission spectrum of hydrogen lead to the modern understanding of the electronic structure of the atom.

2.5 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

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• Orbit - what Bohr called the fixed energy levels.

• Ground state - the lowest possible energy state.

• The orbits are also identified using “quantum numbers”: n = 1, 2, 3, …

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

n=1

n=2

n=3

2.6 Modern Atomic Theory• Bohr’s model of the atom when applied to atoms

with more than one electron failed to explain their line spectra.

• One major change from Bohr’s model is that electrons do not move in orbits.

• Atomic orbitals - regions in space with a high probability of finding an electron.

• Electrons move rapidly within the orbital giving a high electron density. (More in Ch 3)

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