chapter 3 atoms and elements. classification of matter

Chapter 3

Atoms and Elements

Classification of Matter

Elements are

• Pure substances that cannot be separated into simpler substances by ordinary laboratory processes

• The building blocks of matter

gold (Au) carbon (C) aluminum (Al)

Elements

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**Memorize the symbols of the first 36 elements!!

Periodic Table

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Groups and Periods

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Names of Some Representative Elements

• Several groups of representative elements are known by common names.

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Identify the element described by the following:

A. Group 7A(17), Period 4 1) Br 2) Cl 3) Mn

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

C. Group 5A(15), Period 2 1) phosphorus 2) arsenic 3) nitrogen

A. Group 7A, Period 4 1) Br

B. Group 2A, Period 3

2) magnesium

C. Group 5A(15), Period 2

3) nitrogen

Solution

Metals, Nonmetals, and Metalloids

The heavy zigzag line separates metals and nonmetals.• Metals are located to the

left.• Nonmetals are located to

the right. • Metalloids are located

along the heavy zigzag line between the metals and nonmetals.

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Comparing a Metal, Metalloid, and Nonmetal

Dalton’s Atomic Theory

• Matter is composed of tiny particles called atoms.

• Atoms of each element are similar, and different from atoms of other elements.

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

• A chemical reaction involves changes in the arrangement or combination of atoms.

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Rutherford’s Gold-Foil Experiment

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings•Positively charged particles were aimed at atoms of gold

•Most went straight through the atoms

•Only a few were deflected

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

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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Particles in the Atom

The atomic number • Is specific for each element (DEFINES an 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

11

Na

Atomic Number

Symbol

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

State the number of protons in each:

A. A nitrogen atom 2) atomic number 7; 7 protons

B. A sulfur atom 2) atomic number 16; 16 protons

C. A barium atom, 3) atomic number 56; 56 protons

Solution

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

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

number of protons = number of electrons

• For example, an atom of aluminum has 13 protons and 13 electrons. The net charge is zero.

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

Electrons in an Atom

Mass Number

The mass number • Represents the number of particles in the nucleus• Is equal to the

Number of protons + Number of neutrons

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

An atom of zinc has a mass number of 65.

A. How many protons are in this zinc atom? 1) 30 (atomic number 30)

B. How many neutrons are in the zinc atom?

2) 35 (65 – 30 = 35)

C. What is the mass number of a zinc atom that has 37 neutrons? 3) 67 (30 + 37 = 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

An atom has 14 protons and 20 neutrons.

A. Its atomic number is 1) 14

B. It has a mass number of 3) 34 (14 + 20 = 34)

C. The element is 1) Si (Atomic number 14)

Solution

• From the nuclear symbol, we can determine the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom. 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-

Nuclear Symbols

Atomic number

Mass number

Isotopes

• Are atoms of the same element that have different mass numbers

• Have the same number of protons, but different numbers of neutrons

Isotopes

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 ______ ______ ______

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 6 p+ 6 p+ 6 p+

neutrons 6 n 7 n 8 n

electrons 6 e- 6 e- 6 e-

Solution

Write the nuclear 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- ___________

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

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

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

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

Solution

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

Atomic Mass for Cl

• The atomic mass of chlorine is the weighted average of two isotopes 35Cl and 37Cl.

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35Cl has atomic mass 34.97 (75.78%) and 37C has

atomic mass 36.97 (24.22%).

• The atomic mass and percent of each isotope are used to calculate the contribution of each isotope to the weighted average. 34.97 x 75.78 = 26.50 amu 10035.97 x 24.22 = 8.953 amu

100 35.453 amu

• The sum is the weighted average or atomic mass Cl = 35.45 amu

Calculating Atomic Mass for Cl

Examples of Isotopes and Atomic Masses

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

Gallium is an element found in small lasers used in compact disc players. In a sample of gallium, there is 60.11% of 69Ga (atomic mass 68.93) atoms and 39.89% of 71Ga (atomic mass 70.93) atoms.

What is the atomic mass of gallium?

69Ga

68.93 amu x 60.11 = 41.43 amu (from 69Ga)

10071Ga

70.93 amu x 39.89 = 28.29 amu (from 71Ga)

100

Atomic mass Ga = 69.72 amu

Solution

31

Ga

69.72