1 Chapter 3 Atoms and Elements 3.4 The Atom Copyright © 2009 by Pearson Education, Inc

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Chapter 3 Atoms and Elements

3.4 The Atom

Copyright © 2009 by Pearson Education, Inc.

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Dalton’s Atomic Theory

In Dalton’s atomic theory, atoms

• are tiny particles of matter.

• of an element are similar and different from other elements.

• of two or more different elements combine to form compounds.

• are rearranged to form new combinations in a chemical reaction.

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Subatomic Particles

Atoms contain subatomic particles.

• Protons have a positive (+) charge.

• Electrons have a negative (-) charge.

• Neutrons are neutral.

• Like charges repel and unlike charges attract.

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Rutherford’s Gold Foil Experiment

In Rutherford’s gold foil experiment, positively charged particles • were aimed at atoms of gold.• mostly went straight through the atoms. • were deflected only occasionally.

Conclusion: There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.

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Rutherford’s Gold Foil Experiment


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Structure of the Atom

An atom consists• of a nucleus that

contains protons and neutrons.

• of electrons in a large, empty space around the nucleus.


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Atomic Mass Scale

On the atomic mass scale,

• 1 atomic mass unit (amu) has a mass equal to 1/12 of the mass of the carbon-12 atom.

• a proton has a mass of about 1 (1.007) amu.

• a neutron has a mass of about 1 (1.008) amu.

• an electron has a very small mass, 0.000 549 amu.

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Particles in the Atom

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Learning Check

Identify each statement as describing a

1) proton, 2) neutron, or 3) electron.

A. found outside the nucleus

B. has a positive charge

C. is neutral

D. found in the nucleus

Learning Check

Is each of the following statements true or false?Protons are heavier than electronsProtons are attracted to neutronsElectrons are small that they have no

electrical chargeThe nucleus contains all the protons and

neutrons of an atom

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3.5Atomic Number and Mass Number


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The atomic number

• is specific for each element.

• is the same for all atoms of an element.

• is equal to the number of protons in an atom.

• appears above the symbol of an element.

Atomic Number

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Na

Atomic Number

Symbol

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Examples of atomic number and number of protons:

• Hydrogen has atomic number 1; every H atom has one proton.

• Carbon has atomic number 6; every C atom has six protons.

• Copper has atomic number 29; every Cu atom has 29 protons.

• Gold has atomic number 79; every Au atom has 79 protons.

Atomic Number and Protons

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State the number of protons in each.

A. A nitrogen atom 1) 5 protons 2) 7 protons 3) 14 protons

B. A sulfur atom 1) 32 protons 2) 16 protons 3) 6 protons

C. A barium atom 1) 137 protons 2) 81 protons 3) 56 protons

Learning Check

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An atom

• of an element is electrically neutral; the net charge of an atom is zero.

• has an equal number of protons and electrons.

number of protons = number of electrons

Aluminum has 13 protons and 13 electrons. The net

(overall) charge is zero.

13 protons (13+) + 13 electrons (13 -) = 0

Electrons in An Atom

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Mass Number

The mass number

• represents the number of particles in the nucleus.

• is equal to the number of protons + the number of neutrons.

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Atomic Models

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Number of protons = Atomic number

Number of protons + neutrons = Mass number

Number of neutrons

= mass number – atomic number (protons)

Note: Mass number is given for specific isotopes only.

Study Tip: Protons and Neutrons

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An atom of zinc has a mass number of 65.

A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom?

1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that has 37 neutrons?

1) 37 2) 65 3) 67

Learning Check

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An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is

1) 14 2) 16 3) 34

C. The element is

1) Si 2) Ca 3) Se

Learning Check

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3.6Isotopes and Atomic Mass

24Mg 25Mg 26Mg 12 12 12


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Isotopes

• are atoms of the same element that have different mass numbers.

• have the same number of protons, but different numbers of neutrons.

Isotopes

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An atomic symbol

• represents a particular atom of an element.

• gives the mass number in the upper left corner and the atomic number in the lower left corner.

Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol:

mass number 23 Na

atomic number 11

Atomic Symbol

The atomic symbol for a specific atom of an element

gives the

• number of protons (p+),

• number of neutrons (n),

• and number of electrons (e-).

Information from Atomic Symbols

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Examples of number of subatomic particles for atoms

Atomic symbol

16 31 65

O P Zn 8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n 8 e- 15 e- 30 e-

Information from Atomic Symbols

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Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following:

12C 13C 14C 6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

Learning Check

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Write the atomic symbols for atoms with the following subatomic particles:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Learning Check

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Learning Check

1. Which of the pairs are isotopes of the same element?

2. In which of the pairs do both atoms have 8 neutrons?

A. 15X 15X 8 7

B. 12X 14X 6 6

C. 15X 16X 7 8

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Isotopes of Magnesium

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Isotopes of Magnesium

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Isotopes of Sulfur

A sample of naturallyoccurring sulfur containsseveral isotopes with thefollowing abundances

Isotope % abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02

32S, 33S, 34S, 36S16 16 16 16


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Atomic Mass

The atomic mass of an element

• is listed below the symbol of each element on the periodic table.

• gives the mass of an “average” atom of each element compared to 12C.

• is not the same as the mass number.

Na22.99

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Isotopes of Some Elements and Their Atomic Mass

Most elements have two or more isotopes that contribute to the atomic mass of that element.

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Atomic Mass for Cl

The atomic mass of chlorine is

• due to all the Cl isotopes.

• not a whole number.

• the average of two isotopes: 35Cl and 37Cl.

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Using the periodic table, specify the atomic mass of

each element.

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check

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Calculating Atomic Mass

The calculation for atomic mass requires the

• percent(%) abundance of each isotope.

• atomic mass of each isotope of that element.

• sum of the weighted averages.

mass of isotope(1)x (%) + mass of isotope(2) x (%) +

100 100

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35Cl has atomic mass 34.97 amu (75.76%) and 37C

has atomic mass 36.97 amu (24.24%).

• Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average.

34.97 x 75.76 = 26.49 amu 10036.97 x 24.24 = 8.961 amu

100

• Sum is atomic mass of Cl 35.45 amu

Calculating Atomic Mass for Cl

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Calculating Atomic Mass Mg

Isotope Mass Abundance

24Mg = 23.99 amu x 78.70/100 = 18.88 amu25Mg = 24.99 amu x 10.13/100 = 2.531 amu26Mg = 25.98 amu x 11.17/100 = 2.902 amu

Atomic mass (average mass) Mg = 24.31 amu

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Atomic Mass of Magnesium

The atomic mass of Mg• is due to all the Mg

isotopes.• is a weighted average.• is not a whole number.

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Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.

What is the atomic mass of gallium?

Learning Check

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3.7 Electron Energy Levels


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Energy Levels

Energy levels

• are assigned numbers n = 1, 2, 3, 4, and so on.

• increase in energy as the value of n increases.

• are like the rungs of a ladder with the lower energy levels nearer the nucleus.

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Energy Levels

Energy levels have a maximum number of electrons equal to 2n2.

Energy level Maximum number of electrons

n = 1 2(1)2 = 2(1) = 2

n = 2 2(2)2 = 2(4) = 8

n = 3 2(3)2 = 2(9) = 18

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Orbitals

An orbital

• is a three-dimensional space around a nucleus, where an electron is most likely to be found.

• has a shape that represents electron density (not a path the electron follows).

• can hold up to 2 electrons.

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Orbitals

A p orbital

• has a two-lobed shape.

• is one of three p orbitals in each energy level from n = 2.

An s orbital

•has a spherical shape around the nucleus.

•is found in each energy level.

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Orbitals

Electron Level Arrangement

In the electron level arrangement for the first 18 elements • electrons are placed in energy levels (1, 2, 3, etc.),

beginning with the lowest energy level• there is a maximum number in each energy level.

Energy level Number of electrons1 2 (up to He)2 8 (up to Ne)3 8 (up to Ar)4 2 (up to Ca)

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Learning Check

Write the electron level arrangement for each:

1. N

2. Cl

3. K

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Learning Check

Identify the element with each electron level

arrangement:

1. 2, 2

2. 2, 8, 3

3. 2, 7

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3.8 Periodic Trends


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Valence Electrons

The valence electrons

• determine the chemical properties of the elements.

• are the electrons in the highest energy level.

• are related to the group number of the element.

Example: Phosphorus has 5 valence electrons. 5 valence electrons

P in Group 5A(15) 2, 8, 5

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All the elements in a group have the same number ofvalence electrons.

Example: Elements in group 2A (2) have two (2) valence electrons.

Be 2, 2

Mg 2, 8, 2

Ca 2, 8, 8, 2

Sr 2, 8, 18, 8, 2

Groups and Valence Electrons

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Periodic Table and Valence Electrons

Representative Elements Group Numbers

1 2 3 4 5 6 7 8

H He

1 2

Li Be B C N O F Ne

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

Na Mg Al Si P S Cl Ar

2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8

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State the number of valence electrons for each.A. O

1) 4 2) 6 3) 8

B. Al

1) 13 2) 3 3) 1

C. Cl

1) 2 2) 5 3) 7

Learning Check

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State the number of valence electrons for each.

A. calcium

1) 1 2) 2 3) 3

B. group 6A (16)

1) 2 2) 4 3) 6

C. tin

1) 2 2) 4 3) 14

Learning Check

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For the element with the following electron arrangement:

1) State the number of valence electrons.

2) Identify the element.

A. 2, 8, 5

B. 2, 8, 8, 2

C. 2, 7

Learning Check

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Electron-Dot Symbols

An electron-dot symbol• shows the valence electrons

around the symbol of the element.

• for Mg has 2 valence electrons as single dots on the sides of the symbol Mg.

. . ·Mg · or Mg · or ·Mg or ·Mg

·

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Writing Electron-Dot Symbols

Electron-dot symbols for• groups 1A (1) to 4A (14) use single dots.

· · Na · · Mg · · Al · · C ·

·

• groups 5A (15) to 7A (17) use pairs and single dots. · · · ·

· P · : O · · ·

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Groups and Electron-Dot Symbols

In a group, all the electron-dot symbols have thesame number of valence electrons (dots). Example: Atoms of elements in Group 2A (2) each have 2 valence electrons.

· Be ·· Mg ·· Ca ·· Sr ·· Ba ·

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.A. X is the electron-dot symbol for

1) Na 2) K 3) Al

. .B. . X .

. is the electron-dot symbol of

1) B 2) N 3) P

Learning Check

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Atomic Size

Atomic size is described

using the atomic radius;

the distance from the

nucleus to the valence

electrons.

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Atomic Radius Within A Group

Atomic radius increases going down each group of representative elements.

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Atomic Radius Across a Period

Going across a period from left to right,

• an increase in the number of protons increases attraction for valence electrons.

• atomic radius decreases.


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Learning Check

Select the element in each pair with the larger atomic

radius.

A. Li or K

B. K or Br

C. P or Cl

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Ionization Energy

Ionization energy is the energy it takes to remove a valence electron.

Na(g) + Energy (ionization) -> Na+(g) + e-

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Ionization Energy In a Group

Going up a group ofrepresentative elements,

• the distance decreases between nucleus and valence electrons.

• the ionization energy increases.

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Ionization Energy

• Metals have lower ionization energies.

• Nonmetals have higher ionization energies.


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Learning Check

Select the element in each pair with the higher ionization

energy.

A. Li or K

B. K or Br

C. P or Cl

Documents

1 Chapter 3 Atoms and Elements 3.4 The Atom Copyright © 2009 by Pearson Education, Inc