chem chapt 4

Chemistry Chapter 4Chemistry Chapter 4Structure of the AtomStructure of the Atom

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

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

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

Unstable atoms emit radiation to gain stability.

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

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

Early Ideas about MatterEarly Ideas about MatterObjectives:Objectives:

Greek PhilosophersMany 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.

• 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.

DemocritusDemocritus

AristotleAristotle• 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.

Greek PhilosophersGreek Philosophers

John DaltonJohn Dalton(English school teacher -1766-1844)(English school teacher -1766-1844)

Responsible for the beginning of the Responsible for the beginning of the development of modern atomic theory. Dalton development of modern atomic theory. Dalton revised Democritus’s theories by performing and revised Democritus’s theories by performing and studying many chemical reactions. studying many chemical reactions.

Through careful observations and Through careful observations and measurements he was able to determine mass measurements he was able to determine mass ratios of the elements involved in those ratios of the elements involved in those reactions. reactions.

• 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.

John DaltonJohn Dalton

Dalton’s Atomic TheoryDalton’s Atomic Theory

Even though some of the theory was incorrect, It provided the basis we have today for atomic theory.

As is common with science, his theory has since been revised and additional information learned.

Dalton’s Atomic TheoryDalton’s Atomic Theory

Which parts of Dalton’s Atomic Which parts of Dalton’s Atomic Theory was incorrect?Theory was incorrect?

Atoms are indivisible.

Atoms of a given element are identical in size, mass and chemical properties.

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

A. Aristotle

B. Plato

C. Dalton

D. Democritus

Dalton’s theory also conveniently explained what?

A. the electron

B. the nucleus

C. law of conservation of mass

D. law of Democritus

Question?Question?

• Define atom.

• 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.

Defining the AtomDefining the AtomObjectivesObjectives

• 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.

• Gerd Binnig and Heinrich Rohrer won the Nobel Prize in 1986 for the invention of the STM

The AtomThe Atom

STMSTM

Scanning tunneling microscopy (STM) is the highest resolution imaging and nanofabrication technique available. It relies on quantum tunneling of electrons from a sharp metal tip to a conducting surface. The 71 Angstrom diameter "quantum corral" shown above was created and imaged with an ultra-high vacuum cryogenic STM. Each sharp peak in the circle is an iron atom resting on atomically flat copper

STMSTM

• 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.

Cathode Ray TubeCathode Ray Tube

Cathode Ray TubeCathode Ray Tube

Cathode Ray TubeCathode Ray TubeThomson received the Nobel Prize in 1906 for identifying Thomson received the Nobel Prize in 1906 for identifying

the first subatomic particle—the electron. Research the first subatomic particle—the electron. Research with the Cathode Ray Tube determined the following:with the Cathode Ray Tube determined the following:

1.1. Cathode rays were a stream of charged particlesCathode rays were a stream of charged particles

2.2. The particles carried a negative charge.The particles carried a negative charge.

3.3. Changing the metal that made up the electrodes did not Changing the metal that made up the electrodes did not change the ray, therefore all types of matter had these change the ray, therefore all types of matter had these particles. particles.

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

Milikan’s Oil Drop Milikan’s Oil Drop ExperimentExperiment

• 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

The ElectronThe Electron

• Matter is neutral.

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

Early Atomic ModelsEarly Atomic Models

• 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.

The NucleusThe Nucleus

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


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


• 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.


Niels Bohr ModelNiels Bohr Model

• 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.


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

• Atoms are spherically shaped.


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


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

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


Question?Question?

Atoms are mostly ____.

A. positive

B. negative

C. solid spheres

D. empty space

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

Question?Question?

How Atoms DifferHow Atoms DifferObjectives:Objectives:

• 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.

Atomic NumberAtomic Number

• Moseley (1887-1915) discovered 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.

Isotopes and Mass Isotopes and Mass Number 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.

Isotopes and Mass Isotopes and Mass NumberNumber

• The relative abundance of each isotope is usually constant.

• Isotopes containing more neutrons have a greater mass.

• Isotopes have the same chemical behavior.

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

Isotopes and Mass Isotopes and Mass NumberNumber

Mass of AtomsMass 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.

Mass of AtomsMass of Atoms• The atomic mass of an element is the

weighted average mass of the isotopes of that element.

Question?Question?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

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

A. isotopes

B. radioactive

C. abundant

D. ions

Question ?Question ?

Unstable Nuclei and Unstable Nuclei and Radioactive DecayRadioactive DecayObjective:Objective:

• Explain the relationship between unstable nuclei and radioactive decay.

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

RadioactivityRadioactivity• 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.

• Nuclear reactions can change one element into another element.

Radioactive DecayRadioactive 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.

• Nuclear stability is determined by the ratio of neutrons to protons.

Radioactive DecayRadioactive Decay

• Atoms that contain too many or too 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.

Alpha ParticleAlpha Particle

• 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.

• Found because they are deflected towards a negavtively charged plate.

Alpha ParticleAlpha Particle

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

• The mass is conserved in nuclear equations.

Beta ParticleBeta Particle• Beta radiation is radiation that has a

negative charge and emits beta particles.

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

• Found because they are deflected toward the positive charged plate

Beta ParticleBeta Particle

Gamma RaysGamma Rays• Gamma rays are high-energy radiation with no

mass and are neutral and cannot, by themselves, result in the formation of a new atom.

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


A reaction that changes one element into another is called what?

A. chemical reaction

B. beta radiation

C. nuclear reaction

D. physical reaction

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.


Study GuideStudy GuideKey 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.


• 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.

Study Guide 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.


• Chemical reactions involve changes in the electrons surrounding 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.

Whose work led to the modern atomic theory?

A. Dalton

B. Rutherford

C. Einstein

D. Aristotle

Question?Question?

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

A. neutron

B. proton

C. dust

D. electron

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

A. protons

B. neutrons

C. electrons

D. both A and C

Question?Question?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

Question?Question?

What happens when an element emits radioactive particles?

A. It gains energy.

B. It gains neutrons.

C. It loses stability.

D. It loses energy.

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

A. proton

B. atom

C. electron

D. neutron

Question?Question?

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

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

A. protons

B. electrons

C. alpha particles

D. isotopes

Question?Question?

What is the densest region of an atom?

A. electron cloud

B. nucleus

C. isotopes

D. atomic mass

Question?Question?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.

The EndThe End

IB 4IB 4

IB 5IB 5

IB 6IB 6

IB 7IB 7

IB 8IB 8

IB 9IB 9

IB 10IB 10

IB 11IB 11

IB 12IB 12

IB 13IB 13

IB 14IB 14

IB 15IB 15

IB 16IB 16

IB 17IB 17

CIMCIMTable 4.3 Properties of Subatomic Particles

Figure 4.12 Rutherford's Experiment

Figure 4.14 Features of an Atom

Figure 4.21 Types of Radiation

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