chapter 4: the structure of the atom chemistry matter and change

Chapter 4: The Structure of the Atom Chapter 4: The Structure of the Atom

CHEMISTRY Matter and Change

Section 4.1 Early Ideas About Matter

Section 4.2 Defining the Atom

Section 4.3 How Atoms Differ

Section 4.4 Unstable Nuclei and Radioactive Decay

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Table Of ContentsCHAPTER

4

• Compare and contrast the atomic models of Democritus, Aristotle, and Dalton.

theory: an explanation supported by many experiments; is still subject to new experimental data, can be modified, and is considered successful if it can be used to make predictions that are true

• Understand how Dalton's theory explains the conservation of mass.

SECTION4.1

Early Ideas About Matter

Dalton's atomic theory

The ancient Greeks tried to explain matter, but the scientific study of the atom began with John Dalton in the early 1800's.

SECTION4.1


• Many ancient scholars believed matter was composed of such things as earth, water, air, and fire.

• Many believed matter could be endlessly divided into smaller and smaller pieces.

SECTION4.1


Greek Philosophers

Greek Philosophers (cont.)

• Democritus (460–370 B.C.) was the first person to propose the idea that matter was not infinitely divisible, but made up of individual particles called atomos, from which the English word atom is derived.

• Aristotle (484–322 B.C.) disagreed with Democritus because he did not believe empty space could exist.

• Aristotle’s views went unchallenged for 2,000 years until science developed methods to test the validity of his ideas.

SECTION4.1


SECTION4.1



• John Dalton revived the idea of the atom in the early 1800s based on numerous chemical reactions.

• Dalton’s atomic theory easily explained conservation of mass in a reaction as the result of the combination, separation, or rearrangement of atoms.

SECTION4.1



SECTION4.1



Who was the first person to propose the idea that matter was not infinitely divisible?

A. Aristotle

B. Plato

C. Dalton

D. Democritus

SECTION4.1

Section Check

Dalton’s theory also conveniently explained what?

A. the electron

B. the nucleus

C. law of conservation of mass

D. law of Democritus

SECTION4.1

Section Check

• Define atom.

model: a visual, verbal, and/or mathematical explanation of data collected from many experiments

• Distinguish between the subatomic particles in terms of relative charge and mass.

• Describe the structure of the atom, including the locations of the subatomic particles.

SECTION4.2

Defining the Atom

atom

cathode ray

electron

nucleus

proton

neutron

An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus.

SECTION4.2

Defining the Atom

The Atom

• The smallest particle of an element that retains the properties of the element is called an atom.

• An instrument called the scanning tunneling microscope (STM) allows individual atoms to be seen.

SECTION4.2

Defining the Atom

The Electron

• When an electric charge is applied, a ray of radiation travels from the cathode to the anode, called a cathode ray.

• Cathode rays are a stream of particles carrying a negative charge.

• The particles carrying a negative charge are known as electrons.

SECTION4.2

Defining the Atom

• This figure shows a typical cathode ray tube.

SECTION4.2

Defining the Atom

The Electron (Cont.)

• J.J. Thomson measured the effects of both magnetic and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle, then compared it to known values.

• The mass of the charged particle was much less than a hydrogen atom, then the lightest known atom.

• Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron

SECTION4.2

Defining the Atom


• In the early 1910s, Robert Millikan used the oil-drop apparatus shown below to determine the charge of an electron.

SECTION4.2

Defining the Atom


• Charges change in discrete amounts—1.602 10–19 coulombs, the charge of one electron (now equated to a single unit, 1–).

• With the electron’s charge and charge-to-mass ratio known, Millikan calculated the mass of a single electron.

the mass of a hydrogen

atom

SECTION4.2

Defining the Atom


• Matter is neutral. You know that matter is neutral from everyday experiences. You do not receive an electric shock (except under certain conditions) when you touch an object.

• If electrons are negative, then how is matter, which is made up of electrons, neutral?

• J.J. Thomson proposed a model of the atom to answer this question.

SECTION4.2

Defining the Atom


• J.J. Thomson's plum pudding model of the atom states that the atom is a uniform, positively charged sphere containing electrons.

SECTION4.2

Defining the Atom


The Nucleus

• In 1911, Ernest Rutherford studied how positively charged alpha particles interacted with solid matter.

• By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• Although most of the alpha particles went through the gold foil, a few of them bounced back, some at large angles.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• Rutherford concluded that atoms are mostly empty space.

• Almost all of the atom's positive charge and almost all of its mass is contained in a dense region in the center of the atom called the nucleus.

• Electrons are held within the atom by their attraction to the positively charged nucleus.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• Rutherford refined the model to include positively charged particles in the nucleus called protons.

• James Chadwick received the Nobel Prize in 1935 for discovering the existence of neutrons, neutral particles in the nucleus which accounts for the remainder of an atom’s mass.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron.

• Atoms are spherically shaped.

• Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• Scientists have determined that protons and neutrons are composed of subatomic particles called quarks.

SECTION4.2

Defining the Atom

The Nucleus (cont.)

• Scientists do not yet understand if or how quarks affect chemical behavior.

• Chemical behavior can be explained by considering only an atom's electrons.

SECTION4.2

Defining the Atom

Atoms are mostly ____.

A. positive

B. negative

C. solid spheres

D. empty space

SECTION4.2

Section Check

What are the two fundamental subatomic particles found in the nucleus?

A. proton and electron

B. proton and neutron

C. neutron and electron

D. neutron and positron

SECTION4.2

Section Check

• Explain the role of atomic number in determining the identity of an atom.

• Define an isotope.

• Explain why atomic masses are not whole numbers.

• Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

SECTION4.3

How Atoms Differ

atomic number

isotopes

mass number

The number of protons and the mass number define the type of atom.

periodic table: a chart that organizes all known elements into a grid of horizontal rows (periods) and vertical columns (groups or families) arranged by increasing atomic number

atomic mass unit (amu)

atomic mass

SECTION4.3

How Atoms Differ

Atomic Number

• Each element contains a unique positive charge in their nucleus.

• The number of protons in the nucleus of an atom identifies the element and is known as the element’s atomic number.

SECTION4.3

How Atoms Differ

Isotopes and Mass Number

• All atoms of a particular element have the same number of protons and electrons but the number of neutrons in the nucleus can differ.

• Atoms with the same number of protons but different numbers of neutrons are called isotopes.

SECTION4.3

How Atoms Differ

• In nature, most elements are found as mixtures of isotopes. Usually, the relative abundance of each isotope is constant.

–Ex. In a banana, 93.26% is potassium-39, 6.73% is potassium-41 and 0.01% is potassium-40. In another banana or in a different source of potassium, the percentage composition of the potassium isotopes will still be the same.

• Isotopes containing more neutrons have a greater mass.

• Isotopes of an atom have the same chemical behavior.

SECTION4.3

How Atoms Differ

Isotopes and Mass Number (Cont.)

• The mass number is the sum of the protons and neutrons in the nucleus.

SECTION4.3

How Atoms Differ

Isotopes and Mass Number (Cont.)

Mass of Atoms

• One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.

• One amu is nearly, but not exactly, equal to one proton and one neutron.

SECTION4.3

How Atoms Differ

Mass of Atoms (cont.)

• The atomic mass of an element is the weighted average mass of the isotopes of that element.

SECTION4.3

How Atoms Differ

An unknown element has 19 protons, 19 electrons, and 3 isotopes with 20, 21 and 22 neutrons. What is the element’s atomic number?

A. 38

B. 40

C. 19

D. unable to determine

SECTION4.3

Section Check

Elements with the same number of protons and differing numbers of neutrons are known as what?

A. isotopes

B. radioactive

C. abundant

D. ions

SECTION4.3

Section Check

• Explain the relationship between unstable nuclei and radioactive decay.

element: a pure substance that cannot be broken down into simpler substances by physical or chemical means

• Characterize alpha, beta, and gamma radiation in terms of mass and charge.

SECTION4.4

Unstable Nuclei and Radioactive Decay

radioactivity

radiation

nuclear reaction

radioactive decay

alpha radiation

Unstable atoms emit radiation to gain stability.

alpha particle

nuclear equation

beta radiation

beta particle

gamma rays

SECTION4.4


Radioactivity

• Nuclear reactions can change one element into another element.

• In the late 1890s, scientists noticed some substances spontaneously emitted radiation, a process they called radioactivity.

• The rays and particles emitted are called radiation.

• A reaction that involves a change in an atom's nucleus is called a nuclear reaction.

SECTION4.4


Radioactive Decay

• Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.

• Unstable radioactive elements undergo radioactive decay thus forming stable nonradioactive elements.

SECTION4.4


Radioactive Decay (cont.)

• Alpha radiation is made up of positively charged particles called alpha particles.

• Each alpha particle contains two protons and two neutrons and has a 2+ charge.

SECTION4.4



• The figure shown below is a nuclear equation showing the radioactive decay of radium-226 to radon-222.

• An alpha particle is equivalent to a helium-4 nucleus and is represented by 4He or .

• Thus, showing mass is conserved in a nuclear equation.

2

SECTION4.4



• Beta radiation is radiation that has a negative charge and emits beta particles.

• Each beta particle is an electron with a 1– charge.

• During Beta decay, a neutron is converted to a proton and an electron. The electron is emitted and the proton stays in the nucleus.

SECTION4.4



SECTION4.4



• Gamma rays are high-energy radiation with no mass and are neutral.

• They usually accompany alpha and beta radiation.

• Gamma rays account for most of the energy lost during radioactive decay.

SECTION4.4


• Because gamma rays are massless, the emission of gamma rays by themselves cannot result in the formation of a new atom.


SECTION4.4



• Atoms that contain too many or two few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus.

• Few exist in nature—most have already decayed to stable forms.

SECTION4.4


A reaction that changes one element into another in the nucleus of an atom is called what?

A. chemical reaction

B. beta radiation

C. nuclear reaction

D. physical reaction

SECTION4.4

Section Check

Why are radioactive elements rare in nature?

A. They do no occur on Earth.

B. Most have already decayed to a stable form.

C. They take a long time to form.

D. They are too hard to detect.

SECTION4.4

Section Check

Chemistry Online

Study Guide

Chapter Assessment

Standardized Test Practice

Resources

CHAPTER

4 The Structure of the Atom

Key Concepts

• Democritus was the first person to propose the existence of atoms.

• According to Democritus, atoms are solid, homogeneous, and indivisible.

• Aristotle did not believe in the existence of atoms.

• John Dalton’s atomic theory is based on numerous scientific experiments.

SECTION4.1


Study Guide

Key Concepts

• An atom is the smallest particle of an element that maintains the properties of that element.

• Electrons have a 1– charge, protons have a 1+ charge, and neutrons have no charge.

• An atom consists mostly of empty space surrounding the nucleus.

SECTION4.2

Defining the Atom

Study Guide

Key Concepts

• The atomic number of an atom is given by its number of protons. The mass number of an atom is the sum of its neutrons and protons. atomic number = number of protons = number of electrons

mass number = atomic number + number of neutrons

• Atoms of the same element with different numbers of neutrons are called isotopes.

• The atomic mass of an element is a weighted average of the masses of all of its naturally occurring isotopes.

SECTION4.3

How Atoms Differ

Study Guide

Key Concepts

• Chemical reactions involve changes in the electrons surrounding the nucleus of an atom. Nuclear reactions involve changes in the nucleus of an atom.

• There are three types of radiation: alpha (charge of 2+), beta (charge of 1–), and gamma (no charge).

• The neutron-to-proton ratio of an atom’s nucleus determines its stability.

SECTION4.4


Study Guide

Whose work led to the modern atomic theory?

A. Dalton

B. Rutherford

C. Einstein

D. Aristotle

CHAPTER

4

Chapter Assessment

The Structure of the Atom

Which particle is not found in the nucleus of an atom?

A. neutron

B. proton

C. gamma ray

D. electron

CHAPTER

4

Chapter Assessment


Two isotopes of an unknown element have the same number of:

A. protons

B. neutrons

C. electrons

D. both A and C

CHAPTER

4

Chapter Assessment


Lithium has an atomic mass of 6.941 and two isotopes, one with 6 neutrons and one with 7 neutrons. Which isotope is more abundant?

A. 6Li

B. 7Li

C. Both isotopes occur equally.

D. unable to determine

CHAPTER

4

Chapter Assessment


What happens when an element emits radioactive particles?

A. It gains energy.

B. It gains neutrons.

C. It loses stability.

D. It loses energy.

CHAPTER

4

Chapter Assessment


What is the smallest particle of an element that still retains the properties of that element?

A. proton

B. atom

C. electron

D. neutron

CHAPTER

4



How many neutrons, protons, and electrons does 124

54Xe have?

A. 124 neutrons, 54 protons, 54 electrons

B. 70 neutrons, 54 protons, 54 electrons

C. 124 neutrons, 70 protons, 54 electrons

D. 70 neutrons, 70 protons, 54 electrons

CHAPTER

4



The primary factor in determining an atom's stability is its ratio of neutrons to ____.

A. protons

B. electrons

C. alpha particles

D. isotopes

CHAPTER

4



What is the densest region of an atom?

A. electron cloud

B. nucleus

C. isotopes

D. atomic mass

CHAPTER

4



Why are electrons attracted to the cathode in a cathode ray tube?

A. The cathode is more stable.

B. The cathode has a positive charge.

C. The cathode has a negative charge.

D. The cathode has no charge.

CHAPTER

4



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