chemical foundations: elements, atoms, and ions chapter 4

Chemical Foundations: Elements, Atoms, and

Ions

Chapter 4

Chapter 4

Table of Contents

Copyright © Cengage Learning. All rights reserved 2

4.1 The Elements

4.2 Symbols for the Elements

4.3 Dalton’s Atomic Theory

4.4 Formulas of Compounds

4.5 The Structure of the Atom

4.6 Introduction to the Modern Concept of Atomic Structure

4.7 Isotopes

4.8 Introduction to the Periodic Table

4.9 Natural States of the Elements

4.10 Ions

4.11 Compounds That Contain Ions

Section 4.5

The Structure of the Atom

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• Substances that cannot be broken down by simple chemical means

• 115 known: 88 found in nature, others are man made.

• Just as you had to learn the 26 letters of the alphabet before you learned to read and write, you need to learn the names and symbols of the chemical elements before you can read and write chemistry.

Section 4.5


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Section 4.5


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• Could mean a single atom of that element (Ar or H).

• Could mean molecules of an element (H2), which is hydrogen found in its natural state.

• Could mean atoms of elements are present in some form (sodium found in the human body).

• Look at each particular case to determine its proper use.

How the Term Element is Used

Section 4.2

Symbols for the Elements

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• Each element has a unique one- or two-letter symbol.• First letter is always capitalized and the second is not.• The symbol usually consists of the first one or two

letters of the element’s name. Examples:

Oxygen O Krypton Kr

• Sometimes the symbol is taken from the element’s original Latin or Greek name.

Examples: Gold Au aurum Lead Pb plumbum

Section 4.2

Symbols for the Elements

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Names and Symbols of the Most Common Elements

Section 4.3

Dalton’s Atomic Theory

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1. Most natural materials are mixtures of pure substances.

2. Pure substances are either elements or combinations of elements called compounds.

3. A given compound always contains the same proportions (by mass) of the elements.

Section 4.3


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• A given compound always has the same composition, regardless of where it comes from. Water always contains 8 g of oxygen for every 1 g of

hydrogen. Carbon dioxide always contains 2.7 g of oxygen for

every 1 g of carbon.

Law of Constant Composition

Section 4.3


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1. Elements are made of tiny particles called atoms.

2. All atoms of a given element are identical.

3. The atoms of a given element are different from those of any other element.

4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms.

Dalton’s Atomic Theory (1808)

Section 4.3


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Dalton’s Atomic Theory (continued)

5. Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

ONLY NUCLEAR REACTIONS CAN TRANSFORM ONE KIND OF ELEMENT TO ANOTHER

nCBparticleHe 10

126

94

242 )(

Section 4.3


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Concept Check

Which of the following statements regarding Dalton’s atomic theory are still believed to be true?

I. Elements are made of tiny particles called atoms.

II. All atoms of a given element are identical.

III. A given compound always has the same relative

numbers and types of atoms.

IV. Atoms are indestructible.

Section 4.4

Formulas of Compounds

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• Compound – distinct substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of those elements.

• Chemical Formulas – expresses the types of atoms and the number of each type in each unit (molecule) of a given compound.

Chemical Formulas Describe Compounds

Section 4.4


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1. Each atom present is represented by its element symbol.

2. The number of each type of atom is indicated by a subscript written to the right of the element symbol.

3. When only one atom of a given type is present, the subscript 1 is not written.

Rules for Writing Formulas

Section 4.4


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Exercise

The pesticide known as DDT paralyzes insects by binding to their nerve cells, leading to uncontrolled firing of the nerves. Before most uses of DDT were banned in the U.S., many insects had developed a resistance to it. Write out the formula for DDT. It contains 14 carbon atoms, 9 hydrogen atoms, and 5 atoms of chlorine.

C14H9Cl5

Section 4.5


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• Postulated the existence of electrons using cathode-ray tubes.

• The atom must also contain positive particles that balance exactly the negative charge carried by particles that we now call electrons.

J. J. Thomson (1898—1903)

Section 4.5


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Cathode-Ray Tube

J.J. Thomson, measured mass/charge of e-

Section 4.5


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William Thomson (Plum Pudding Model)

• Reasoned that the atom might be thought of as a uniform “pudding” of positive charge with enough negative electrons scattered within to counterbalance that positive charge.

Section 4.5


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• Explained the nuclear atom.• Atom has a dense center of positive charge

called the nucleus.• Electrons travel around the nucleus at a

relatively large distance.• A proton has the same magnitude of charge as

the electron, but its charge is positive.

Ernest Rutherford (1911)

Section 4.5


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Rutherford’s alpha particle scattering experiment.5.5

Section 4.5


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Deflection and scattering of alpha particles by positive gold nuclei.

Rutherford’s alpha particle scattering experiment.

Section 4.5


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Rutherford and Chadwick (1932)

• Most nuclei also contain a neutral particle called the neutron.

• A neutron is slightly more massive than a proton but has no charge.

EnergynCB 10

126

94

42

α particle is a helium nucleus: 24

2He

Section 4.5


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• We know atoms are composed of three main pieces - protons, neutrons and electrons

• The nucleus contains protons and neutrons

• The nucleus is only about 10-13 cm in diameter

• The electrons move outside the nucleus with an average distance of about 10-8 cm

– therefore the radius of the atom is about 100,000 times larger than the radius of the nucleus

Section 4.6

Introduction to the Modern Concept of Atomic Structure

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The atom contains:

• Electrons – found outside the nucleus; negatively charged

• Protons – found in the nucleus; positive charge equal in magnitude to the electron’s negative charge

• Neutrons – found in the nucleus; no charge; virtually same mass as a proton

Section 4.6


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• The nucleus is: Small compared with the overall size of the

atom. Extremely dense; accounts for almost all of

the atom’s mass.atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

“If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”

Section 4.6


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Section 4.6


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• The chemistry of an atom arises from its electrons.

• Electrons are the parts of atoms that “intermingle” when atoms combine to form molecules.

• It is the number of electrons that really determines chemical behavior.

Why do different atoms have different chemical properties?

Section 4.7

Isotopes

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• Atoms with the same number of protons but different numbers of neutrons.

• Show almost identical chemical properties; chemistry of atom is due to its electrons.

• In nature most elements contain mixtures of isotopes.

Isotopes

XAZ

H11 H (D)2

1 H (T)31 U235

92 U23892

Mass Number

Atomic NumberElement Symbol

Section 4.7

Isotopes

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Two Isotopes of Sodium

A – Z = n (number of neutrons)

Section 4.7

Isotopes

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• X = the symbol of the element• Z = the atomic number (# of protons)• A = the mass number (# of protons and

neutrons)

Isotopes

XAZ


Section 4.7

Isotopes

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Isotopes – An Example

• C = the symbol for carbon

• 6 = the atomic number (6 protons)

• 14 = the mass number (6 protons and 8 neutrons)

• C = the symbol for carbon

• 6 = the atomic number (6 protons)

• 12 = the mass number (6 protons and 6 neutrons)

146C 12

6C


Section 4.7

Isotopes

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Exercise

A certain isotope X contains 23 protons and 28 neutrons.

• What is the mass number of this isotope?• Identify the element.

Mass Number = 51

Vanadium

Introduction to Periodic Table

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Section 4.8


• The periodic table shows all of the known elements in order of increasing atomic number.

The Periodic Table


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Section 4.8



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Section 4.8


Mendeleev

• order elements by atomic mass• saw a repeating pattern of properties • Periodic Law – When the elements are arranged in

order of increasing relative mass, certain sets of properties recur periodically

• used pattern to predict properties of undiscovered elements

• where atomic mass order did not fit other properties, he re-ordered by other properties– Te & I


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Section 4.8


Periodic Pattern

H

nm H2O a/b

1 H2

Lim Li2O

b

7 LiH

Nam Na2O

b

23 NaH

Bem/nm BeO

a/b

9 BeH2

m MgO

b

24 MgH2

Mg

nm B2O3

a

11 ( BH3)n

B

m Al2O3

a/b

27 (AlH3)

Al

nm CO2

a

12 CH4

C

nm/m SiO2

a

28 SiH4

Si

nm N2O5

a

14 NH3

N

nm P4O10

a

31 PH3

P

nm O2

16 H2O

O

nm SO3

a

32 H2S

Snm Cl2O7

a

35.5 HCl

Cl

nm OF2

19 HF

F


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Section 4.8


Mendeleev's Predictions for Ekasilicon (Germanium)

Property Silicon’s Props

Tin’s Props

Predicted Value

Measured Value

Atomic Mass

28 118 72 72.6

Color Grey White metal

Grey Grey- White

Density 2.32 7.28 5.5 5.4

Reaction w/ Acid &

Base

Resists Acid, Reacts Base

Reacts Acid,

Resists Base

Resists Both

Resists Both

Oxide SiO2 SnO2 Eks1O2 GeO2


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Section 4.8


• Metals vs. Nonmetals• Groups or Families – elements in the same

vertical columns; have similar chemical properties

• Periods – horizontal rows of elements

The Periodic Table


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Section 4.8


• Most elements are metals and occur on the left side.• The nonmetals appear on the right side.• Metalloids are elements that have some metallic and

some nonmetallic properties.

The Periodic Table


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Section 4.8


= Alkali Metals

= Alkali Earth Metals

= Noble Gases

= Halogens

= Lanthanides

= Actinides

= Transition Metals


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Section 4.8


1. Efficient conduction of heat and electricity

2. Malleability (they can be hammered into thin sheets)

3. Ductility (they can be pulled into wires)

4. A lustrous (shiny) appearance

5. High densities

Physical Properties of Metals

Section 4.9

Natural States of the Elements


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Section 4.8


• Most elements are very reactive.• Elements are not generally found in

uncombined form. Exceptions are:

• Noble metals – gold, platinum and silver• Noble gases – Group 8


Section 4.9


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Section 4.8


Diatomic Molecules

• Nitrogen gas contains N2 molecules.

• Oxygen gas contains O2 molecules.


Section 4.9


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Section 4.8


Molecular Elements


Section 4.9


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Section 4.8


• Different forms of a given element.• Example:

– Solid carbon occurs in three forms.• Diamond• Graphite• Buckminsterfullerene

Allotropes

Carbon Allotropes

Natural States of the Elements Natural States of the Elements

Section 4.9


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Section 4.8


• Atoms can form ions by gaining or losing electrons.– Metals tend to lose one or more electrons to form

positive ions called cations.

– Cations are generally named by using the name of the parent atom.

Section 4.10

Ions


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Section 4.8


Ions

Section 4.10

Ions

• Nonmetals tend to gain one or more electrons to form negative ions called anions.

• Anions are named by using the root of the atom name followed by the suffix –ide.


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Section 4.8


Ions

Section 4.10

Ions

Ion Charges and the Periodic Table

The ion that a particular atom will form can be predicted from the periodic table.

Group or Family Charge

Alkali Metals (1A) 1+

Alkaline Earth Metals (2A) 2+

Halogens (7A) 1–

Noble Gases (8A) 0


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Section 4.8


Section 4.10

Ions

Ion Charges and the Periodic Table


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Section 4.8


Section 4.10

Ions

Exercise

An ion with a 3+ charge contains 23 electrons. Which ion is it?

– a) Fe3+– b) V3+– c) Ca3+ – d) Sc3+

+ve charge ≡ electrons are lost-ve charge ≡ electrons are gained

# of e- = Z – positive chargeor

# of e- = Z + negative charge


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Section 4.8


Section 4.10

Ions

Exercise

A certain ion X1+ contains 54 electrons and 78 neutrons.

What is the mass number of this ion?

133


0XAZZ = # of e- + positive charge

or Z = # of e- - negative charge

+ve charge ≡ electrons are lost-ve charge ≡ electrons are gained


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Section 4.8


• Ions combine to form ionic compounds. • Properties of ionic compounds

High melting points Conduct electricity

• If melted

• If dissolved in water

Section 4.11

Compounds That Contain Ions


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Section 4.8


• Ionic compounds are electrically neutral.• The charges on the anions and cations in the

compound must sum to zero.

Section 4.11



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Section 4.8


• Write the cation element symbol followed by the anion element symbol.

• The number of cations and anions must be correct for their charges to sum to zero.

Formulas for Ionic Compounds

Section 4.11



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Section 4.8


Concept Check

A compound contains an unknown ion X and has the formula XCl2. Ion X contains 20 electrons. What is the identity of X?

a) Ti2+

b) Sc+

c) Ca2+

d) Cr2+

Section 4.11


Z = # of e- + positive chargeor

Z = # of e- - negative charge


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Section 4.8


Concept Check

A member of the alkaline earth metal family whose most stable ion contains 36 electrons forms a compound with bromine. What is the correct formula for this compound?

a) CaBr2

b) KrBr

c) RbBr

d) SrBr2

Section 4.11


• Ionic compounds are electrically neutral.

• The charges on the anions and cations in the compound must sum to zero.


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Section 4.8


Homework

• Reading assignment– Pages 74 through 105

• Homework Problems: pages 107 - 111– Questions and problems 3, 5, 9, 13, 19, 25, 27, 31,

33, 35, 37, 39, 42, 45, 47, 49, 51, 53, 57, 59, 61, 63, 67, 69, 71, 73, 75, 77, 81.

• Due on

Chapter 4 Homework

chemical foundations: elements, atoms, and ions chapter 4

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