Chapter 2 Atoms, Molecules, and Ions. Copyright © Cengage Learning. All rights reserved 2 Greeks...

Chapter 2

Atoms, Molecules, and Ions

Copyright © Cengage Learning. All rights reserved 2

• Greeks were the first to attempt to explain why chemical changes occur.

• Alchemy dominated for 2000 years.– Several elements discovered.– Mineral acids prepared.

• Robert Boyle was the first “chemist”.– Performed quantitative experiments.– Developed first experimental definition of an

element.

Early History of Chemistry

Three Important Laws

• Law of conservation of mass (Lavoisier):– Mass is neither created nor destroyed in a

chemical reaction.

• Law of definite proportion (Proust):– A given compound always contains exactly

the same proportion of elements by mass.


Three Important Laws (continued)

• Law of multiple proportions (Dalton):– When two elements form a series of

compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.


• Understanding the structure of the atom will help to understand the properties of the elements.

• Keep in mind that these, as all theories, are subject to constant refinement. The picture of the atom isn’t final.

Composition of the Atom

Development of the Atomic Theory

• Dalton’s Atomic Theory - the first experimentally based theory of atomic structure of the atom.– John Dalton– early 1800’s

• Much of Dalton’s Theory is still regarded as correct today. *See starred items.*

Postulates of Dalton’s Atomic Theory

1. All matter consists of tiny particles called atoms.*

2. An atom cannot be created, divided, destroyed, or converted to any other type of atom.

3. Atoms of a particular element have identical properties.

4. Atoms of different elements have different properties.*

5. Atoms of different elements combine in simple whole-number ratios to produce compounds (stable aggregates of atoms.)*

6. Chemical change involves joining, separating, or rearranging atoms.*

* These postulates are still regarded as true.

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.


CONCEPT CHECK!CONCEPT CHECK!

2

Dalton’s Atomic Theory

8 X2Y16 X 8 Y+

Gay-Lussac and Avogadro (1809—1811)

• Gay—Lussac– Measured (under same conditions of T and P) the

volumes of gases that reacted with each other. • Avogadro’s Hypothesis

– At the same T and P, equal volumes of different gases contain the same number of particles.• Volume of a gas is determined by the number, not

the size, of molecules.


Representing Gay—Lussac’s Results


We will learn that atoms consist of three primary particles.

electronsprotons

neutrons

• Nucleus - small, dense, positively charged region in the center of the atom. Contains:

- protons - positively charged particles

- neutrons - uncharged particles

• Surrounding the nucleus is a diffuse region of negative charge populated by:

- electrons - negatively charged particles

J. J. Thomson (1898—1903)

• Postulated the existence of negatively charged particles, that we now call electrons, using cathode-ray tubes.

• Determined the charge-to-mass ratio of an electron.

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


Subatomic Particles: Thompson ExperimentElectrons, Protons and Neutrons

• Electrons were the first subatomic particles to be discovered using the cathode ray tube.

Indicated that the particles were negatively charged.

Cathode Ray Tube

Joseph John Thomson (1856-1940). Photo courtesy of The Cavendish Laboratory.

Robert Millikan (1909)

• Performed experiments involving charged oil drops.

• Determined the magnitude of the charge on a single electron.

• Calculated the mass of the electron– (9.11 × 10-31 kg).


e- charge = -1.60 x 10-19 C

Thomson’s charge/mass of e- = -1.76 x 108 C/g

e- mass = 9.10 x 10-28 g

Measured mass of e-

(1923 Nobel Prize in Physics)

Millikan Oil Drop Experiment


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Henri Becquerel (1896)

• Discovered radioactivity by observing the spontaneous emission of radiation by uranium.

• Three types of radioactive emission exist:– Gamma rays (ϒ) – high energy light– Beta particles (β) – a high speed electron– Alpha particles (α) – a particle with a 2+ charge


(Uranium compound)

Ernest Rutherford (1911)

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

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

distance relative to the nucleus.


1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

atomic radius ~ 100 pm = 1 x 10-10 m

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

Rutherford’s Model of the Atom

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

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

atom.– Extremely dense; accounts for almost all of

the atom’s mass.


Nuclear Atom Viewed in Cross Section

Copyright © Cengage Learning. All rights reserved

• The proton was discovered in 1918 by Ernest Rutherford. He noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. This he named proton, from protos, the Greek for "first".

Discovery of the Proton

Chadwick’s Experiment (1932)(1935 Noble Prize in Physics)

H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

+ 9Be 1n + 12C + energy

neutron (n) is neutral (charge = 0)

n mass ~ p mass = 1.67 x 10-24 g

• 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.


Atom constitution summary

Selected Properties of the Subatomic Particles

Name Charge Mass(amu) Mass (grams)

Electrons (e) -1 5.4 x 10-4 9.1095 x 10-28

Protons (p) +1 1.00 1.6725 X 10-24

Neutrons (n) 0 1.00 1.6750 x 10-24

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Atomic number, Mass number and Isotopes

CAZ X

Mass

Number

mass number (A) - sum of the number of protons and neutrons

Atomic Number

Charge of particle

Symbol of the atom

atomic number (Z) - the number of protons in the atom

Calculate the number of protons, neutrons and electrons in each of the following:

B115

Fe5526

Isotopes

• 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.


Two Isotopes of Sodium


• Isotopes - atoms of the same element having different masses.

– contain same number of protons– contain different numbers of neutrons

Isotopes of Hydrogen

Hydrogen(Hydrogen - 1)

Deuterium(Hydrogen - 2)

Tritium(Hydrogen - 3)

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

How many protons, neutrons, and electrons are in C14

6 ?

How many protons, neutrons, and electrons are in C11

6 ?

A certain isotope X contains 23 protons and 28 neutrons.

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

Mass Number = 51Vanadium


EXERCISE!EXERCISE!

A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces

H2 H2O NH3 CH4

A diatomic molecule contains only two atoms

H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

Chemical Bonds

• Covalent Bonds– Bonds form between atoms by sharing

electrons.– Resulting collection of atoms is called a

molecule.


Covalent Bonding




Chemical Bonds

• Ionic Bonds– Bonds form due to force of attraction

between oppositely charged ions.– Ion – atom or group of atoms that has a net

positive or negative charge.– Cation – positive ion; lost electron(s).– Anion – negative ion; gained electron(s).


Although many substances are molecular, others are composed of ions.

Chemical Formulas; Molecular and Ionic Substances

• Ionic substances

An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons.Sodium chloride is a substance made up of ions.

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

How many protons, neutrons and electrons are in the following ions?

K3919

-23216S

22412 Mg

A monatomic ion contains only one atom

A polyatomic ion contains more than one atom

Na+, Cl-, Ca2+, O2-, Al3+, N3-

OH-, CN-, NH4+, NO3

-

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

How many protons and electrons are in ?Al2713

3+

How many protons and electrons are in ?Se7834

2-

Molecular vs Ionic Compounds




The Periodic Table• In 1869, Dmitri Mendeleev discovered that if the known elements

were arranged in order of atomic number, they could be placed in horizontal rows such that the elements in the vertical columns had similar properties.

A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements, is called a periodic table.

A period consists of the elements in one horizontal role of the periodic table.

The Periodic Table• Periods and Groups

A group consists of the elements in any one column of the periodic table.The groups are usually numbered.The eight groups are called main group (or representative) elements.

The “B” groups are called transition elements.


The two rows of elements at the bottom of the table are called inner transition elements.Elements in any one group have similar properties.

The elements in group IA, often known as the alkali metals, are soft metals that react easily with water.


The group VIIA elements, known as the halogens, are also reactive elements.

A metal is a substance or mixture that has a characteristic luster and is generally a good conductor of heat and electricity.

The Periodic Table• Metals, Nonmetals, and Metalloids

A nonmetal is an element that does not exhibit the characteristics of the metal.A metalloid, or semi-metal, is an element having both metallic and nonmetallic properties.

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

Chemistry In ActionNatural abundance of elements in Earth’s crust

Natural abundance of elements in human body

Chemical Substances; Formulas and Names

A monatomic ion is an ion formed from a single atom.

Most of the main group metals form cations with the charge equal to their group number.


• Rules for predicting charges on monatomic ions

The charge on a monatomic anion for a nonmetal equals the group number minus 8.Most transition elements form more than one ion, each with a different charge.

Monatomic cations are named after the element – if there is only one such ion. For example, Al3+ is called the aluminum ion.


• Rules for naming monatomic ions

If there is more than one cation of an element, a Roman numeral in parentheses denoting the charge on the ion is used. This often occurs with transition elements. i.e. Fe2+ = Iron(II); Fe3+ = Iron(III)The names of the monatomic anions use the stem name of the element followed by the suffix – ide. For example, Br- is called the bromide ion.

Naming Compounds

• Binary Compounds– Composed of two elements– Ionic and covalent compounds included

• Binary Ionic Compounds– Metal—nonmetal

• Binary Covalent Compounds– Nonmetal—nonmetal


Binary Ionic Compounds (Type I)

1. The cation is always named first and the anion second.

2. A monatomic cation takes its name from the name of the parent element.

3. A monatomic anion is named by taking the root of the element name and adding –ide.


Binary Ionic Compounds (Type I)

• Examples:KCl Potassium chloride

MgBr2 Magnesium bromide

CaO Calcium oxide


Naming Binary Compounds

• NaF -

• LiCl -

• MgO -

Naming Binary Compounds

• NaF - Sodium Fluoride

• LiCl - Lithium Chloride

• MgO - Magnesium Oxide

Formula of Ionic Compounds

Al2O3

2 x +3 = +6 3 x -2 = -6

Al3+ O2-

CaBr2

1 x +2 = +2 2 x -1 = -2

Ca2+ Br-

Na2CO3

1 x +2 = +2 1 x -2 = -2

Na+ CO32-

Formula of Ionic Compounds

MgO

1 x +2 = +2 1 x -2 = -2

Mg2+ O2-

Use lowest common denominator

1 x +2 = +2 1 x -2 = -2

Mg2+ O2-

NOT

Mg2O2

Binary Ionic Compounds (Type II)

• Metals in these compounds form more than one type of positive ion.

• Charge on the metal ion must be specified.• Roman numeral indicates the charge of the

metal cation.• Transition metal cations usually require a Roman

numeral.• Elements that form only one cation do not need

to be identified by a roman numeral.


Binary Ionic Compounds (Type II)

• Examples:CuBr Copper(I) bromide

FeS Iron(II) sulfide

PbO2 Lead(IV) oxide


• Transition metal ionic compounds– indicate charge on metal with Roman numerals

FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride

FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride

Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide

Polyatomic Ions

• Must be memorized (see Table 2.5 on pg. 65 in text).

• Examples of compounds containing polyatomic ions:

NaOH Sodium hydroxideMg(NO3)2 Magnesium nitrate

(NH4)2SO4 Ammonium sulfate


MinimumPolyatomic Ions You Should Know

• NH4+ - Ammonium

• OH- - Hydroxide• CN- - Cyanide

• SO42- - Sulfate

• ClO4- - Perchlorate

• O22- - Peroxide

• PO43- - Phosphate

• CO32- - Carbonate

• HCO3- - Hydrogen

carbonate

A polyatomic ion is an ion consisting of two or more atoms chemically bonded together and carrying a net electric charge.


• Polyatomic ions

nitrite NO

nitrate NO

2

3

sulfite SO

sulfate SO2

3

24

Prefix thio-

• The prefix thio- means that an oxygen atom in the root ion name has been replaced by a sulfur atom.

Example:

• SO42- --> S2O3

2-

sulfate ion thiosulfate ion

Chemical Nomenclature• Ionic Compounds

– often a metal + nonmetal– anion (nonmetal), add “ide” to element name

BaCl2 barium chloride

K2O potassium oxide

Mg(OH)2 magnesium hydroxide

KNO3 potassium nitrate

More Practice

• Na2SO4 Na2SO3

Sodium Sulfate Sodium Sulfite

• AgCN Cd(OH)2

Silver Cyanide Cadmium Hydroxide

• Ca(OCl)2 KClO4

Calcium Hypochlorite Potassium Perchlorate

Formation of Ionic Compounds




• Molecular compounds• nonmetals or nonmetals + metalloids• common names

• H2O, NH3, CH4, C60

• element further left in periodic table is 1st

• element closest to bottom of group is 1st

• if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom

• last element ends in ide

Binary Covalent Compounds (Type III)

• Formed between two nonmetals.1. The first element in the formula is named first,

using the full element name.2. The second element is named as if it were an

anion.3. Prefixes are used to denote the numbers of

atoms present.4. The prefix mono- is never used for naming the

first element.


Prefixes Used to Indicate

Number in Chemical Names


Binary Covalent Compounds (Type III)

• Examples:CO2 Carbon dioxide

SF6 Sulfur hexafluoride

N2O4 Dinitrogen tetroxide


HI hydrogen iodide

NF3 nitrogen trifluoride

SO2 sulfur dioxide

N2Cl4 dinitrogen tetrachloride

NO2 nitrogen dioxide

N2O dinitrogen monoxide

Molecular Compounds

TOXIC!

Laughing Gas

Acids

• Acids can be recognized by the hydrogen that appears first in the formula—HCl.

• Molecule with one or more H+ ions attached to an anion.


An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water.

HCl•Pure substance, hydrogen chloride•Dissolved in water (H+ Cl-), hydrochloric acid

An oxoacid is an acid that contains hydrogen, oxygen, and another element.

HNO3 nitric acid

H2CO3 carbonic acid

H2SO4 sulfuric acidHNO3

Acids

• If the anion does not contain oxygen, the acid is named with the prefix hydro– and the suffix –ic.

• Examples:HCl Hydrochloric acidHCN Hydrocyanic acidH2S Hydrosulfuric acid


Oxoacid

• If the anion does contain oxygen:– The suffix –ic is added to the root name if the

anion name ends in –ate.• Examples:

HNO3 Nitric acid

H2SO4 Sulfuric acid

HC2H3O2 Acetic acid


Oxoacid

• If the anion does contain oxygen:– The suffix –ous is added to the root name if

the anion name ends in –ite.• Examples:

HNO2 Nitrous acid

H2SO3 Sulfurous acid

HClO2 Chlorous acid


Acid anions (provide H+ ions)

• Uses of mono– and di-

• Rule: to be used if oxyanion is bonded to one or more hydrogen ions.

Example:

• HPO42- = monohydrogen phosphate ion

• H2PO4- = dihydrogen phosphate ion

Flowchart for Naming Acids


A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water.

NaOH sodium hydroxide

KOH potassium hydroxide

Ba(OH)2 barium hydroxide

Which of the following compounds is named incorrectly?

a) KNO3 potassium nitrate

b) TiO2 titanium(II) oxide

c) Sn(OH)4 tin(IV) hydroxide

d) PBr5 phosphorus pentabromide

e) CaCrO4 calcium chromate


EXERCISE!EXERCISE!

Worked Example 2.1

Chapter 2 Atoms, Molecules, and Ions. Copyright © Cengage Learning. All rights reserved 2 Greeks...

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