Ch. 4: Atomic Structure4.1 Defining the Atom

Democritus, Dalton atomic theories4.2 Structure of the Nuclear Atom

proton, neutron, electron, plum pudding model, Rutherford atomic model

4.3 Distinguishing Among Atomsatomic number, mass number, isotopes, isotopic notation, atomic mass,introduction to the periodic table

4.1 Defining the Atom

The lab technician shown here is using a magnifying lens to examine a bacterial culture in a petri dish. When scientists cannot see the details of what they study, they try to obtain experimental data that help fill in the picture.

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An atom is the smallest particle of an element that retains its identity in a chemical reaction.

Philosophers and scientists have proposed many ideas on the structure of atoms.

Early Models of the AtomDemocritus’s Atomic Philosophy 400 BC (430?)

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“Everything is made up of a few simple parts called atomos.” Atomos means “uncuttable” in Greek.

He envisioned atomos as small, solid particles of many different sizes and shapes.

Democritus’s ideas were limited because they didn’t explain chemical behavior and they lacked experimental support. His ideas were rejected because Aristotle supported *the “earth, air, water, and fire” concept of matter.

Democritus believed that atoms were solid particles that are indivisible and indestructible.

John Dalton, 1803 (1808?)

“Each element consists of a particular kind of atom. All atoms of a particular element are identical.”

His atoms had a specific mass, size, and chemical behavior.

By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory.

Dalton’s atomic theory is described in four parts (following slides):

Early Models of the Atom

1. All elements are composed of tiny indivisible particles called atoms.

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2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Early Models of the Atom3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

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4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element in a chemical reaction.

Summary of John Dalton’s atomic theory as listed by tutorvista website:

John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.

Main assumptions or postulates of Dalton• All matter consists of indivisible particles called atoms. • Atoms of the same element are similar in shape and mass, but differ from the

atoms of other elements. • Atoms cannot be created or destroyed. • Atoms of different elements may combine with each other in a fixed, simple, whole

number ratio to form compound atoms. • Atoms of same element can combine in more than one ratio to form two or more

compounds. • Atoms are the smallest unit of matter that can take part in a chemical reaction.

http://www.tutorvista.com/content/chemistry/chemistry-i/matter/daltons-atomic-theory.php

Continued…Drawbacks of Dalton's atomic theory of matter• The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into

protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.

• According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.

• Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.

• According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.

• The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.

Merits of Dalton's atomic theory• It has enabled us to explain the laws of

chemical combination. • Dalton was the first person to recognize a

workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).

Continued…tutorvista comments

Sizing up the AtomDespite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes.

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Iron Atoms Seen Through a Scanning Tunneling Microscope

Anton van Leeuwenhuek (1632-1723) discovered the light microscope that can magnify up to 1000x. In 1931, Ernst Ruska and Max Knoll built the first electron microscope. The limit to an electron microcope is 1,000,000x. In 1981, Gerd Benning and Heinrich Rohrer developed the atomic force microscope, the STM (scanning tunneling microscope) being one, that can magnify more than 1,000,000,000x.

4.1 Section Quiz1. The ancient Greek philosopher credited with

suggesting all matter is made of indivisible atoms is a) Plato.b) Aristotle.c) Democritus.d) Socrates.

4.1 Section Quiz

2. Individual atoms are observable witha) the naked eye.b) a magnifying glass.c) a light microscope.d) a scanning tunneling microscope.

4.2 Structure of the Nuclear Atom

Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.

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Three kinds of subatomic particles are electrons, protons, and neutrons.

Subatomic ParticlesElectrons

In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. Electrons are negatively charged subatomic particles.

Thomson performed experiments that involved passing electric current through gases at low pressure.

The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

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Subatomic ParticlesCathode Ray Tube

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A cathode ray is deflected by a magnet.

A cathode ray is deflected by electrically charged plates.

Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements.

Subatomic Particles

In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles.

Such positively charged subatomic particles are called protons.

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In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.

Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

Subatomic Particles

Table 4.1 summarizes the properties of electrons, protons, and neutrons.

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The Atomic Nucleus

How can you describe the structure of the nuclear atom?

Plum pudding model-- J.J. Thomson and others supposed the atom was filled with positively charged material and the electrons were evenly distributed throughout.

Rutherford atomic model--This model of the atom turned out to be short-lived, however, due to the work of Ernest Rutherford (1871–1937).

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The Atomic NucleusRutherford’s Gold-Foil Experiment

In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil.

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Alpha particles scatter from the gold foil.

The Atomic Nucleus

The Rutherford Atomic ModelErnest Rutherford concluded that the atom is mostly empty

space. All the positive charge and almost all of the mass are concentrated in a small region called the nucleus.

The nucleus is the tiny central core of an atom and is composed of protons and neutrons.

In the nuclear atom, the protons and neutrons are located in the nucleus.

The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

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4.2 Section Quiz

1. Which of the following is NOT an example of a subatomic particle?

a) protonb) moleculec) electrond) neutron

4.2 Section Quiz

2. The nucleus of an atom consists ofa) electrons only.b) protons only.c) protons and neutrons.d) electrons and neutrons.

4.2 Section Quiz

3. Most of the volume of the atom is occupied by the

a) electrons.b) neutrons.c) protons and neutrons.d) protons.

4.3 Distinguishing Among Atoms

Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

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The Periodic Table—A PreviewA periodic table is an arrangement of elements in

which the elements are separated into groups based on a set of repeating properties.

A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element.

Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties.

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The Periodic Table—A Preview

• A Period

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Period goes across

The Periodic Table—A Preview

• A Group or Family

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Group, or family, goes down

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Atomic NumberThe atomic number of an element is the number of

protons in the nucleus of an atom of that element. Elements are different because they contain different numbers

of protons.

Practice #1 (on worksheet)

Mass Number

The total number of protons and neutrons in an atom is called the mass number.

The number of neutrons in an atom is the difference between the mass number and atomic number.

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Isotopes

Isotopes are atoms that have the same number of protons but different numbers of neutrons.

Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

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Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

C126

C136

Isotopic notation

• Atomic number = # p– Elements are put in order of atomic number on

the periodic table.

Mass number = p + n (in the nucleus)

EA

Z

Element symbol

Ex: An atom of carbon with 7 neutrons:

An atom of lead with 125 neutrons:

13C6

207Pb82

Mass Number

Au is the chemical symbol for gold.

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How many protons, electrons, and neutronsdoes a gold atom have?

The atomic number is 79. Therefore, there are 79 protons and 79 electrons. The mass number is 197, which is the total number of protons and neutrons. Therefore, 197-79= 118 neutrons.

Practice #2 (on worksheet)

Atomic Mass

An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.

It is useful to to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Carbon-12 has a mass of exactly 12 atomic mass units.

4.3Some elements and their isotopes

Atomic Mass

The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.

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A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

Atomic MassTo calculate the atomic mass of an element, multiply

the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

For example, carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89% and a

mass of 12.000 amu Carbon-13, which has a natural abundance of 1.11% and a mass

of 13.003 amu.

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Silver is found in two isotopes with atomic masses 106.9041 and 108.9047 amu, respectively. The first isotope represents 51.82% and the second 48.18%. Determine the average atomic mass of silver.

(106.9041)(.5182)= 55.398(108.9047)(.4818)= 52.470

55.398 + 52.470 = 107.868 amu

4.3 Section Quiz

1. Isotopes of an element havea) the same mass number.b) different atomic numbers.c) the same number of protons but different numbers of

neutrons.d) the same number of protons but different numbers of

electrons.

4.3 Section Quiz

2. How many neutrons are in sulfur-33?a) 16 neutronsb) 33 neutronsc) 17 neutronsd) 32.06 neutrons

4.3 Section Quiz3. If sulfur contained 90.0% sulfur-32 and 10.0%

sulfur-34, its atomic mass would bea) 32.2 amu.b) 32.4 amu.c) 33.0 amu.d) 35.4 amu.

What you need to know: Chapter 4Atomic people and general timeframe—

– Democritus– Dalton– Thomson, Goldstein, Chadwick– Rutherford– Specifics—Dalton’s Atomic Theory, Thomson’s plum pudding, Rutherford’s gold foil experiment

Subatomic particles– Electron, proton, neutron– Location, relative size– Number of each based on periodic table and mass number

• Isotopes, isotopic notation (neon-21 and formats), mass number• Atomic mass (amu, carbon-12, weighted average)• UFO Activity (Rutherford)• Penny Isotope Activity (weighted averages, isotope definition)• Vocabulary—atom, nucleus, atomic number, mass number, isotope, amu, atomic

mass• Ch. 4 worksheet and textbook problems