Chapter 2: Atoms and Elements

The Building Blocks of Chemistry

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Atoms and Paper Clips

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Courtesy Ken Karp

Compounds are result of combination of elements(in a fix ratio)

What elements composed of ?

2.1 Origins of Atomic Theory

Matter is composed of Atoms

Atomistic Philosophers-Leucippus & Democritus (ca., 400 BC)

Atomos - indivisible (Greek) Democritus

http://dbhs.wvusd.k12.ca.us/AtomicStructure/AtomicStructure.html

Piles of paper clips, individual paper clips, and fragments of a paper clip.

© 2003 John Wiley and Sons Publishers

Courtesy Ken Karp

Figure 2.1: In a parallel to the demonstration with the paper clips, repeatedly subdividing a piece of gold produces smaller and smaller groups of atoms.

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What are atoms composed of ?

The Structure of Atoms

• Cathode-Ray Tube (Thomson, 1856–1940):

• Cathode rays

consist of tiny

negatively

charged particles,

now called

electrons.

The Structure of Atoms

• Deflection of electron depends on three factors:

– Strength of electric or magnetic field

– Size of negative charge on electron

– Mass of the electron

• Thomson calculated the electron’s charge to mass

ratio as 1.758820 x 108 Coulombs per gram.

The Structure of Atoms

• Oil Drop Experiment (Millikan, 1868–1953): Applied a voltage to oppose the downward fall of charged drops and suspend them.

• Voltage on plates place 1.602176 x 10-19 C of charge on each oil drop.

• Millikan calculated the electron’s mass as 9.109382 x 10-28 grams.

• Discovery of Nucleus (Rutherford, 1871 – 1937):

• Rutherford irradiatedgold foil with a beamof alpha () particlesto search for positivecharged particles.

The Structure of Atoms

Discovery of Nucleus (Rutherford, 1871–1937):Rutherford irradiatedgold foil with a beamof alpha () particlesto search for positivecharged particles.

Atom must be mostly empty space except for a central positive mass concentration.

The Structure of Atoms

Figure 2.10: Relative distances and masses in the hydrogen atom.

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The Structure of Atoms

• Structure of the Atom:

Figure 2.9: The structure of the hydrogen atom.

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Figure 2.7: The nucleus and electron shells of a lithium atom, a typical small atom.

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The atomic number is equal to the number of protons in an atom.

On the periodic chart, the atomic number appears above the symbol of an element.

Atomic Number

11

Na

Atomic Number

Symbol

18

Fig 3.2 The periodic table

Atomic Numbers and Protons for Lithium and Carbon Atoms

State the number of protons in each. A. Nitrogen

1) 5 protons 2) 7 protons 3) 14 protons

B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons

C. Barium 1) 137 protons 2) 81 protons 3) 56 protons

Learning Check

State the number of protons in each. A. Nitrogen 2) Atomic number 7; 7 protons

B. Sulfur 2) Atomic number 16; 16 protons

C. Barium 3) Atomic number 56; 56 protons

Solution

An atom is electrically neutral; the net charge is zero.

In an atom, the number of protons is equal to the number of electrons.

# protons = # electrons Therefore, the atomic number is also equal to the

number of electrons in a neutral atom.

Electrons in An Atom

What Else is There in an Atom?

• Mp = 1.67 X 10 –24 g

• Me = 0.0009 X 10 –24 g, 1amu = 1.661X10 –24 g

• M(atom) > Mp+Me Neutrons?

• Mn = 1.67X10 –24 g

The mass number gives the total number of protons and neutrons in the nucleus. Mass number = # protons + # neutrons

Mass Number

The atomic symbol represents a particular atom of an element.

The atomic symbol shows the mass number in the upper left corner and the atomic number in the lower left corner.

For 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

When we know the atomic symbol of an atom, we can determine the number of protons (p+), neutrons, (n), and electrons (e-).16 31 65

O P Zn 8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Information from Atomic Symbols

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

12C 13C 14C 6 6 6

Protons 6 6 6

Neutrons 6 7 8

Electrons 6 6 6

Solution

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

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

16OA. 8 p+, 8 n, 8 e- 8

B. 17p+, 20n, 17e- 37Cl 17

C. 47p+, 60 n, 47 e- 107Ag 47

Solution

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

An atom of zinc has a mass number of 65.A. How many protons are in this zinc atom?

1) 30 B. How many neutrons are in the zinc atom?

2) 35

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

3) 67

Solution

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

An atom has 14 protons and 20 neutrons.A. It has atomic number

1) 14

B. It has a mass number of 3) 34

C. The element is 1) Si

Solution

Atoms and Elements

Isotopes and Atomic Mass

24Mg 25Mg 26Mg 12 12 12

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

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

Most elements have two or more isotopes.

Isotopes

Atomic mass is the weighted average mass of all the isotopes of that element.

The atomic mass of each element is compared to 12C, whith a mass of 12 amu.

The atomic mass of each element is listed below the symbol of the element on the periodic table.

Atomic Mass

Atomic Mass of Magnesium

The individual atomic masses of all the isotopes contribute to the atomic mass of Mg.

As a weighted average, the atomic mass is not a whole number.

Figure 2.11: Relative abundance of deuterium.

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Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check

Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium 40.1

B. aluminum 27.0

C. lead 207.2

D. barium 137.3

E. iron 55.9

Solution

The periodic table arranges elements according to similar properties.

Periodic Table

• On the periodic table, each vertical column is called a group of elements.

• A group contains elements with similar chemical and physical properties.

• Each group is identified by a group number at the top of the column.

• The representative elements have group numbers of 1A – 8A. The transition elements use the letter “B.”

Groups on the Periodic Table

Groups and Group Numbers

Numbering Groups

• One numbering system for groups assigns the letter A to the representative elements and the letter B to the transition elements.

• Another system assigns the numbers 1-18 to the columns across the periodic table.

Group Numbers

Group numbers

• Several groups of representative elements are classified by name.

Group 1A Alkali MetalsGroup 2A Alkaline Earth MetalsGroup 7A HalogensGroup 8A Noble Gases

Representative Elements

Names of Some Representative Groups

• Group 1A, thealkali metals, includes sodium, lithium, and potassium.

• Group 7A, the halogens, includes chlorine, bromine, and iodine.

Representative Elements

• On the periodic table, each horizontal row is called a period.

• A period is identified by a number such as 1, 2, 3, or 4.

• Periods 2-7 include representative elements and transition elements.

Periods on the Periodic Table

Location of Periods

Period 4

Identify the element described by the following:A. Element in Group 7A, Period 4

1) Br 2) Cl 3) Mn

B. Element in Group 2A, Period 3 1) beryllium 2) magnesium 3) boron

Learning Check

A. Group 7A, Period 4 1) Br

B. Group 2A, Period 3

2) magnesium

Solution

Figure 2.12: Names, symbols, electron structures, and compositions of nuclei of the first 10 elements.

© 2003 John Wiley and Sons Publishers

Which particle has a mass approximately equal to the mass of a proton?

a. Atom

b. Electron

c. Neutron

d. Nucleus

e. quark

Which particle has a mass approximately equal to the mass of a proton?a. Atom

b. Electron

c. Neutron

d. Nucleus

e. quark

Protons possess a ________ charge, and electrons possess a ________ charge.a. negative, negative

b. negative, positive

c. positive, negative

d. positive, zero

e. zero, positive

Protons possess a ________ charge, and electrons possess a ________ charge.a. negative, negative

b. negative, positive

c. positive, negative

d. positive, zero

e. zero, positive

In a neutral atom the number of ________ is equal to the number of ________.

a. protons, electrons

b. protons, neutrons

c. neutrons, electrons

d. protons + electrons, neutrons

e. none of the above

In a neutral atom the number of ________ is equal to the number of ________.

a. protons, electrons

b. protons, neutrons

c. neutrons, electrons

d. protons + electrons, neutrons

e. none of the above

An atom with a mass number of 58 and with 32 neutrons will have ________ protons.a. 16

b. 26

c. 32

d. 58

e. 90

An atom with a mass number of 58 and with 32 neutrons will have ________ protons.

a. 16

b. 26

c. 32

d. 58

e. 90

The symbol of the element with 23 protons is

a. Mg

b. Na

c. V

d. B

e. None of the above.

The symbol of the element with 23 protons isa. Mg

b. Na

c. V

d. B

e. None of the above.

An atom with Z = 26 and A = 58 contains ________ protons, ________ electrons, and

________ neutrons.

a. 26; 26; 58

b. 58; 26; 26

c. 26; 26; 32

d. 32; 26; 32

e. 26; 32; 84

An atom with Z = 26 and A = 58 contains ________ protons, ________ electrons, and ________ neutrons. a. 26; 26; 58

b. 58; 26; 26

c. 26; 26; 32

d. 32; 26; 32

e. 26; 32; 84

An atom containing 29 protons, 29 electrons, and 34 neutrons has a mass number ofa. 5

b. 29

c. 34

d. 58

e. 63

An atom containing 29 protons, 29 electrons, and 34 neutrons has a mass number ofa. 5

b. 29

c. 34

d. 58

e. 63

The value of Z for an atom containing 29 protons, 29 electrons, and 34 neutrons, is

a. 5

b. 29

c. 34

d. 58

e. 63

The value of Z for an atom containing 29 protons, 29 electrons, and 34 neutrons, isa. 5

b. 29

c. 34

d. 58

e. 63

An atom containing 47 protons, 47 electrons, and 60 neutrons has a mass number of

a. 13.

b. 47.

c. 60.

d. 107.

e. 154.

An atom containing 47 protons, 47 electrons, and 60 neutrons has a mass number ofa. 13.

b. 47.

c. 60.

d. 107.

e. 154.

The atomic number of an atom containing 47 protons, 47 electrons, and 60 neutrons isa. 13

b. 47

c. 60

d. 107

e. 154

The atomic number of an atom containing 47 protons, 47 electrons, and 60 neutrons is

a. 13

b. 47

c. 60

d. 107

e. 154

Figure 2.15: Two paper clips of the same kind but different sizes can be used as models for atomic isotopes.

© 2003 John Wiley and Sons Publishers

Courtesy Andy Washnik

Figure 2.14: The many varieties of paper clips reflect the varieties of elements and their atoms.

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Courtesy Andy Washnik

Figure 2.6: Weight depends on the size of the gravitational force.

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The astronauts have mass but no weight.

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Courtesy NASA