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Lesson 1: Atoms—Basic Units of Matter

Lesson 2: Discovering Parts of the Atom

Lesson 3:Elements, Isotopes, and Ions—How Atoms Differ

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matter

atom

nucleus

proton

neutron

electron

4.1 Atoms—Basic Units of Matter

What is the current atomic model?

• Matter is everything that has mass and takes up space, such as gases, solids, and liquids.


• Matter is not sound, heat, or light—these are forms of energy.

• An atom is a very small particle that makes up all matter.

Parts of the Atom

• The nucleus is the region located in the center of the atom.

• A particle with a positive charge is a proton.

• A particle with a negative charge is an electron.

• A neutron has no charge.


• Atomic-force microscopes show the surfaces of atoms.

Parts of the Atom (cont.)


The Size of Atoms• Protons, neutrons, and electrons are all smaller than

the atom.


Historical Evidence of Atoms• Democritus (460–370 B.C.) was the first to propose that

atoms were indivisible solid spheres with no holes.


The Law of Conservation of Mass• A chemical reaction rearranges atoms of one substance

into another substance with different properties.

• The total mass of the starting materials is always equal to the total mass of the product.


The Law of Definite Proportions• Any pure compound always contains the same

elements in the same proportion.

– Water from your kitchen is the same as water in a glacier on Mars.

– H2O: two hydrogen atoms and one oxygen atom


Dalton’s Atomic Model

1. All matter is made up of atoms.2. Atoms are neither created nor destroyed in chemical reactions.

3. Atoms of different elements combine in whole-number ratios.

4. Each element is made of a different kind of atom.

5. The atoms of different elements have different masses and properties.


Dalton’s Atomic Model (cont.)


Lesson 1 Review

Which is NOT a particle in an atom?

A positron

B neutron

C electron

D proton


0% 0%0%0%

1. A

2. B

3. C

4. D

Lesson 1 Review

Which law states that the total mass of the starting materials equals the total mass of the product in a chemical reaction?

A Dalton’s atomic model

B the law of conservation of mass

C the law of definite proportions

D Democritus’ law


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1. A

2. B

3. C

4. D

Lesson 1 Review

____ make up the nucleus of an atom.

A Protons and electrons

B Neutrons and electrons

C Protons and neutrons

D Neutrons and photons


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1. A

2. B

3. C

4. D

End of Lesson 1

spectral lines

energy level

electron cloud

4.2 Discovering Parts of the Atom

How were electrons discovered?• Scientists have put together a detailed model of

atoms and their parts.


• Cathode-ray tubes were used in early experiments on atoms.

How were electrons discovered? (cont.)

• A cathode ray is a stream of particles that can be seen when an electric current passes through a vacuum tube.

• The ray travels from a negatively charged disk at one end to a positively charged disk at the other end.


Thomson’s Experiments• Thomson discovered the cathode rays did not travel in a

straight line, but bent towards the positively charged plate.


Thomson’s Experiments (cont.)

• Opposite charges attract each other.

• Thomson concluded the cathode ray must have a negative charge and named the particles electrons.


Thomson’s Atomic Model• Thomson proposed that an atom was a positively

charged sphere.

• Electrons mixed in to balance the charge.


Discovering the Nucleus• In Rutherford’s gold foil experiment, particles were shot

through a thin sheet of gold into a detector behind the foil.


Discovering the Nucleus (cont.)

• Rutherford predicted the path of the particles would bend only slightly because the particles would not come upon a charge large enough to strongly repel them.



• Most of the particles did pass straight though.• Some particles were strongly

bounced to the side.

• One in about 8000 bounced completely backwards.



• If the positive charge was spread evenly, all the particles would have passed through the foil with only a small direction change.

• Only something with a larger mass and positive charge could cause some of the particles to bounce backwards.


Rutherford’s Atomic Model


Rutherford’s Atomic Model (cont.)

• The positively charged nucleus is in the center of an atom.

• Electrons with a negative charge travel around empty space surrounding the nucleus.


Completing Rutherford’s Model• Rutherford also discovered the proton, a particle with

a positive charge.

• Rutherford knew the mass of a proton, but could not account for the total mass of an atom.

• Rutherford’s theory was later confirmed when the existence of the neutron—a neutral atomic particle with a mass similar to a proton but without a charge—was proved.


Weakness in the Rutherford Model

• How are electrons arranged?

• Why do different elements have different chemical properties?

• Why are some elements more reactive than others?


Bohr and the Hydrogen Atom• Rutherford thought the electrons moved around the

nucleus like a ball swinging on a rope at any distance.

• Bohr thought electrons traveled in circles with a certain diameter.


Bohr and the Hydrogen Atom (cont.)

• Bohr studied hydrogen because it has only one electron.

• When atoms are excited, they absorb and release energy as light.


The Spectrum of Hydrogen• Light passing through a prism is broken into a continuous spectrum of light

—red, orange, yellow, green, blue, and violet blend into each other.


The Spectrum of Hydrogen (cont.)

• Ultraviolet rays have shorter wavelengths and higher energies than visible light.

• Infrared light has longer wavelengths and lower energies than visible light.


The Spectrum of Hydrogen (cont.)

• When excited, hydrogen and neon give off unique narrow bands of light on the spectrum that are called spectral lines.


Spectral Lines and Energy Levels• Each color in a spectral line is a different wavelength

and different energy.

• Electrons can have only certain amounts of energy.

• Electrons can only move at a certain distance from the nucleus that corresponds to that amount of energy.


Spectral Lines and Energy Levels (cont.)

• The region in space that an electron can move about the nucleus is called the energy level.


Electrons in the Bohr Atom• The electrons can move only in an orbit that is a set

distance from the nucleus.

• Each energy level can hold a certain number of electrons.

Bohr’s Model


Electrons in the Bohr Atom (cont.)

• Electrons fill the energy levels in order.– The lowest level is filled first.

– The second level has no electrons until the first level is full.

– The first level holds 2 electrons, the second level holds 8 electrons.

– The last energy level may or may not be filled.


Electrons in the Bohr Atom (cont.)

• This diagram shows how electrons are placed in the elements with atomic numbers 1–10.


Bohr’s Model and Chemical Properties• Elements have different chemical properties because they

have different numbers of electrons in their outer energy level.


Bohr’s Model and Chemical Properties• Unreactive elements have the exact number of

electrons needed to fill their outer energy level.

• Elements with incomplete outer energy levels are likely to form compounds.

(cont.)


Limitations of Bohr’s Model

• Energy levels were like circular orbits.

• Bohr’s theory works for the simple hydrogen atom, but not for more complex elements.


Atomic Model

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The Electron Cloud Model• The electron cloud is the region surrounding an atomic

nucleus where an electron is most likely to be found.

• Electrons are more likely to be near the nucleus because they are attracted to the positive charge of the protons.

Electron Cloud Model


Lesson 2 Review

An excited hydrogen atom emits narrow bands of light called ____.

A energy lines

B wave lines

C spectral lines

D wavelengths


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1. A

2. B

3. C

4. D

Lesson 2 Review

Elements that do not react with other elements must have ____.

A completely filled energy levels

B excited electrons

C empty energy levels

D the same number of protons and neutrons


0% 0%0%0%

1. A

2. B

3. C

4. D

Lesson 2 Review

In the gold foil experiment, why did some particles pass straight through the foil?

A because they were repelled by the protons in the foil

B because they were attracted by the protons in the foil

C because atoms have no effect on charged particles

D because atoms are mostly empty spaces


0% 0%0%0%

1. A

2. B

3. C

4. D

End of Lesson 2

element

atomic number

mass number

isotope

average atomic mass

ion

4.3 Elements, Isotopes, and Ions—How Atoms Differ

Different Elements—Different Numbers of Protons

• An element is a pure substance made from atoms that all have the same number of protons.

• Atoms of a particular element always have the same number of protons.


Different Elements—Different Numbers of Protons (cont.)

• The number of protons in an atom of an element is the element’s atomic number.


How can you tell which elements form chemical bonds?

Atomic Number and the Periodic Table

• The periodic table shows elements that increase in atomic number horizontally.

• Elements in the same column have similar properties.

• Most of the elements on the periodic table are metals.


Atomic Number and the Periodic Table (cont.)

Isotopes—Different Numbers of Neutrons

• Atoms of the same element always have the same number of protons, but they may have different numbers of neutrons.


Isotopes



Protons, Neutrons, and Mass Number

• An atom’s mass number is the sum of the number of protons and the number of neutrons.

• To calculate the number of neutrons, subtract the atomic number from the mass number.


Isotopes• Atoms of the same element that have different numbers

of neutrons are called isotopes.


Isotopes (cont.)

• The average atomic mass of an element is the weighted average mass of the mixture of an element’s isotopes.


Isotopes (cont.)

• Radioactive isotopes are unstable and break down releasing particles, radiation, and energy.


Isotopes of Hydrogen

• Hydrogen has three isotopes.


Isotopes of Hydrogen (cont.)


Ions—Gaining or Losing Electrons

• Atoms can gain or lose electrons.• When an electron is gained or lost, the atom no longer has the same number of electrons as protons.

• The atom no longer has a neutral charge.

• An atom that has gained or lost an electron and is no longer neutral is called an ion.


Ions—Gaining or Losing Electrons (cont.)

Positive Ions—Losing Electrons

• When an atom loses an electron, it has more protons than electrons.

• The atom has a positive charge and is called a positive ion.


Negative Ions—Gaining Electrons

• When an atom gains an electron, it has more electrons than protons.

• The atom has a negative charge and is called a negative ion.

• Positive ions and negative ions attract each other and form compounds.


Lesson 3 Review

The carbon element has several isotopes. How is carbon-14 different from carbon-13?

A Carbon-14 has a positive charge.

B Carbon-13 has a positive charge.

C Carbon-14 has one more neutron than carbon-13.

D Carbon-14 has one less neutron than carbon-13.


0% 0%0%0%

1. A

2. B

3. C

4. D

Lesson 3 Review

What is the number of protons in an element called?

A isotope

B atomic number

C atomic mass

D radiotope


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1. A

2. B

3. C

4. D

Lesson 3 Review

How does a neutral atom become a negative ion of the same element?

A It gains a proton.

B It loses a proton.

C It loses an electron.

D It gains an electron.


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1. A

2. B

3. C

4. D

End of Lesson 3

Chapter Resources Menu

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Chapter Assessment

California Standards Practice

Concepts in Motion

Image Bank

Science Online

Interactive Table

Virtual Lab

BrainPOP

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Atoms of the same element always have the same number of ____.

A electrons

B neutrons

C ions

D protons

Chapter Assessment 1

0% 0%0%0%

1. A

2. B

3. C

4. D

An electron moves into a higher energy level. What must have happened? A The electron

released energy.B The electron

absorbed energy.C The atom gained a

negative charge.D The atom gained a

positive charge.


0% 0%0%0%

1. A

2. B

3. C

4. D

An atom contains 10 electrons, 11 protons, and 11 neutrons. What is the charge on the atom?

A 1−

B 1+

C 2−

D 2+


0% 0%0%0%

1. A

2. B

3. C

4. D

Elements with the same number of protons but differing numbers of neutrons are called ____.

A ions

B electrons

C tritium

D isotopes


0% 0%0%0%

1. A

2. B

3. C

4. D

A spectral line is caused by ____.

A an excited electron releasing energy and falling to a lower energy level

B an excited electron absorbing electricity and jumping to a higher energy level

C an atom gaining an ionic charge

D an atom losing a proton


0% 0%0%0%

1. A

2. B

3. C

4. D

The atomic mass of an atom is equal to ____.

A the number of protons

B the number of neutrons

C the number of protons and neutrons

D the number of electrons and protons

CA Standards Practice 1

SCI 3.a

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1. A

2. B

3. C

4. D

The atomic number of carbon is 6. Carbon-14 contains ____.

A 7 protons and 7 neutrons

B 6 protons and 8 neutrons

C 8 protons and 6 neutrons

D 6 protons and 6 neutrons


SCI 7.b

0% 0%0%0%

1. A

2. B

3. C

4. D

What happens when an atom gains a negative charge?

A The atom gains an electron.

B The atom gains a proton.

C The atom loses an electron.

D The atom loses a proton.


SCI 3.a

0% 0%0%0%

1. A

2. B

3. C

4. D

Which particle has a positive charge?

A nucleus

B proton

C electron

D neutron


SCI 3.a

0% 0%0%0%

1. A

2. B

3. C

4. D

Who thought that atoms were very small, solid spheres?

A Dalton

B Rutherford

C Thompson

D Democritus


SCI 3.a

0% 0%0%0%

1. A

2. B

3. C

4. D

Concepts in Motion 1

Image Bank

Interactive Table

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