Chapter 2

Atoms, Molecules, and Ions

Chapter 2: Topics

• Early history of chemistry

• Fundamental chemical laws

• Dalton’s atomic theory

• Early experiments to characterize the atom

• The modern view of atomic structure

• Molecules and ions

• An introduction to the Periodic Table

• Naming simple compounds

• Greeks

• Democritus and others - atomos

• Alchemy

• 1660 - Robert Boyle- experimental definition of element.

• Lavoisier- Father of modern chemistry.

2.1 The early history of chemistry

GreeksGreeks Matter is composed of Matter is composed of fire, earth, fire, earth,

water and airwater and air The Greek philosopher The Greek philosopher DemocritusDemocritus

(460 B.C. – 370 B.C.) was among the (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms first to suggest the existence of atoms (from the Greek word “atomos”)(from the Greek word “atomos”) He believed that atoms were He believed that atoms were

indivisible and indestructibleindivisible and indestructible His ideas did agree with later His ideas did agree with later

scientific theory, but did not explain scientific theory, but did not explain chemical behavior, and was chemical behavior, and was not not based on the scientific methodbased on the scientific method – but – but just philosophyjust philosophy

Alchemy

• Turning Cheep metals into gold

• Alchemists discovered several elements and prepared mineral acids

17th Century17th Century

• Robert BoyleRobert Boyle: : First “chemist” to perform First “chemist” to perform quantitative experimentsquantitative experiments

• He published his first book: “The Skeptical He published his first book: “The Skeptical Chemist” in 1661.Chemist” in 1661.

• He talked about elementsHe talked about elements

18th Century18th Century

• George Stahl: Phlogiston flows out of a burning material.

• Joseph Priestley: Discovers oxygen gas, “dephlogisticated air, i.e., low in phlogistone”

2.2 Fundamentals chemical Laws

• Law of Conservation of Mass• Law of Definite Proportion• Law of Multiple

Law of Conservation of MassLaw of Conservation of Mass

It was discovered by Antoine Lavoisier

It was the basis for development of chemistry in the 19th century

Mass is neither created nor destroyed

Combustion involves oxygen, not phlogiston

Law of Definite Proportion(Proust’s Law)

Law of Definite Proportion(Proust’s Law)

A given compound always contains exactly the same proportion of elements by mass.

Water is composed of 11.1% H and 88.9% O (w/w)

Law of Multiple ProportionsLaw of Multiple Proportions

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.

The ratio of the masses of oxygen that combine with 1g of H in H2O and H2O2 will be a small whole number (“2”).

Example

• Water, H2O has 8 g of oxygen per 1g of hydrogen.

• Hydrogen peroxide, H2O2, has 16 g of oxygen per 1g of hydrogen.

• 16/8 = 2/1

• Small whole number ratios.

• This fact could be explained in terms of atoms

2.3 Dalton’s Atomic Theory (1808) Elements are made up of small particles

called atoms Atoms of each element are identical.

Atoms of different elements are different.

Compounds are formed when atoms combine. Each compound has a always same type and relative number of atoms

Chemical reactions are rearrangement of atoms but atoms are never changed into atoms of other element. , or created or destroyed.

• Provided basics to determining absolute formulas of compounds

• Gay-Lussac- under the same conditions of temperature and pressure, compounds always react in whole number ratios by volume.– 2volumes of H react with one volume of O to form

2volumes of gaseous water and

Gay-Lussac hypothesis (1809)

Avogadro’s Hypothesis (1811)Avogadro’s Hypothesis (1811)

• 5 liters of oxygen

• 5 liters of nitrogen

• Same number of particles!

At the same temperature and pressure, equal At the same temperature and pressure, equal volumes of different gases contain the same volumes of different gases contain the same number of particlesnumber of particles..

•If Avogadro's hypothesis is correct, Gay-Lussac’s can be interpreted as

follows: • 2 molecules of H react with 1 molecule

of O 2 molecules of H2O

2.4 Early experiments to characterize the atom2.4 Early experiments to characterize the atom

Based on Dalton, Gay-Lussac, Avogadro and others, work started to identify the nature of the atom

What is an atom made of? How do atoms of various elements differ?

The electron

J. J. Thomson - postulated the existence of electrons using cathode ray tubes.

Ernest Rutherford - explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.

Thomson’s Experiment

Voltage source

+-

When high voltage is applied to the tube a ray emanates from the cathode is called cathode ray.

Thomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive end.positive end.

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s Experiment

• By adding an electric field

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field, he found that the By adding an electric field, he found that the moving particles were negatively charged moving particles were negatively charged

+

-

Results of Thomson Experiment

• Electrons are produced from electrodes made from various types of metals, all atoms must contain electrons.

• Since atoms are electrically neutral, they must contain positively charged particles.

• Thomson determined charge-to-mass ratio of an electron:

• e/m = -1.76X108C/g

Thomson’s Model

• Atom consisted of a diffuse cloud of positive charge with negative electrons embedded randomly

• Atom was like plum pudding.

• Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Millikan’s Experiment

Oil

Atomizer

Oil droplets

Telescope

-

+

Millikan’s Experiment

X-rays

X-rays give some electrons a charge.

Millikan’s Experiment

From the mass of the drop and the charge on the plates, the mass of an electron is calculated

Radioactivity• Certain elements produce high energy

radiation• Discovered by accident and was a result of

spontaneous emission by uranium• Bequerel (1896) found that a piece of mineral

containing uranium could produce an image on a photographic plate in the absence of light.

• Three types of radiation were known:– alpha- helium nucleus (+2 charge, 7300 times that

of the electron)– beta- high speed electron– gamma- high energy light

The nuclear atomRutherford’s Experiment

• Aimed at testing Thomson’s plum pudding model• Used uranium to produce alpha particles.• Alpha particles are directed at gold foil through

hole in lead block.• Since the mass is evenly distributed in gold

atoms alpha particles should go straight through.• Used gold foil because it could be made atoms

thin.

Lead block

Uranium

Gold Foil

Florescent Screen

What he expected

Because

Because, he thought the mass was evenly distributed in the atom.

What he got

How he explained it

+

• Atom is mostly empty

• Small dense, positive particle at

center.

• Alpha particlesare deflected by

it if they get close enough.

Proof for nuclear atom

+

Nuclear atom model

• According to Rutherford:

The atom consists of a dense center of positive charge (Nucleus) with electrons moving around it at distance that is large relative to the nuclear radius

2.5 The modern view of an atomic structure:An introduction

• The atom is mostly empty space.

• Two regions• Nucleus- protons and

neutrons.• It is characterized by

small size and high density

• Electron cloud- region where you might find an electron.

• The chemistry of atom• Results mainly from

electrons

A cross section of nuclear atom

Mass and charge of nuclear particles

Particle Mass (Kg) Charge

Electron 9.11X10-31 -1

Proton 1.67X10-27 +1

Neutron 1.67X10-27 None

Why atoms of different elements have different properties?

• Atoms of different elements have different number of protons and electrons.

• Number and arrangement of electrons around nucleus differ from one element to another.

Sub-atomic Particles

• Z - atomic number = number of protons determines type of atom.

• A - mass number = number of protons + neutrons.

• Number of protons = number of electrons if atom is neutral.

Symbols

XA

Z

Na23

11

Mass number

Atomic number

Na-23

Isotopes

Atoms of the same element (same atomic

number) with different mass numbers

Atoms with the same number of protons, but

different numbers of neutrons.

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine – 37Cl-35 Cl-37

Two isotopes of sodium

• Isotopes show almost identical chemical properties. Why?

• They possess same number of electrons

2.6 Molecules and ionsIntroduction to chemical bonding

• The forces that hold atoms together are called chemical bonding

• Covalent bonding - sharing electrons.• Collection of atoms by covalent bonding lead

to molecules• Molecules can be represented by formulas • Chemical formula- Symbol relates number

and type of atoms in a molecule. • Diatomic molecule: two atoms of same

element are connected by a covalent bond.

• Molecular formulas– give the actual numbers and types of atoms

in a molecule.– Examples: H2O, CO2, CO, CH4, H2O2, O2, O3,

and C2H4.• Structural formula: bonds are shown as lines

Molecular formula and structural formula

H

H

H H

H

HC C

Representing Structure in MoleculesRepresenting Structure in Molecules

Accurately represents the angles at which molecules are attached.

Ions• Atoms or groups of atoms with a charge.

• Cations- positive ions - get by losing electrons(s).

• Anions- negative ions - get by gaining electron(s).

• Ionic bonding- Force of attraction between oppositely charged ions.

• Ionic solids are called salts.

Formation of Cations

Formation of Anions

Examples of ionsExamples of ions

• Cation: A positive ion

Mg2+, NH4+

• Anion: A negative ion

Cl, SO42

Polyatomic ions

2.7 Introduction to the Periodic Table2.7 Introduction to the Periodic Table

• Elements are classified by:• -properties• - atomic number

• Groups (vertical)• 1A = alkali metals• 2A = alkaline earth metals• 7A = halogens• 8A = noble gases

• Periods (horizontal)

Periodic TablePeriodic TableGroups /Families

Periods

Metals• Conductors• Lose electrons• Malleable and ductile

Nonmetals• Brittle• Gain electrons• Covalent bonds

Semi-metals or Metalloids

Alkali Metals

Alkaline Earth Metals

Halogens

Transition metals

Noble Gases

Inner Transition Metals

Lanthanides

Lanthanides

Select an element

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2.8 Naming Simple Compounds

• Binary compounds are composed of two electrons

• Both ionic and covalent compounds will be considered

Binary Ionic Compounds(Type I)Binary Ionic Compounds(Type I)Binary Ionic Compounds(Type I)Binary Ionic Compounds(Type I)

11. . Cation first, then anionCation first, then anion

22. . Monatomic cation takes its name from Monatomic cation takes its name from the name of the elementthe name of the element

CaCa2+2+ = = calciumcalcium ionion

33. . Monatomic anion takes its name from Monatomic anion takes its name from the the rootroot of the elementof the element + + --ideide

ClCl = = chlorchlorideide

CaClCaCl22 = = calcium chlorcalcium chlorideide

Name the following compoundsName the following compounds

CsFCsF

Calcium fluorideCalcium fluoride

AlClAlCl33

Aluminum chlorideAluminum chloride

LiHLiH

Lithium hydrideLithium hydride

Calcium hydroxideCalcium hydroxide

Ca(OH)Ca(OH)22

Some Common CationsSome Common Cations

Some Common AnionsSome Common Anions

-- metal forms more than one metal forms more than one cationcation

-- use use Roman numeral Roman numeral in namein name

PbClPbCl22

PbPb2+2+ is cationis cation

PbClPbCl22 = = lead lead ((IIII) ) chloridechloride

PbClPbCl44 = lead (IV) chloride = lead (IV) chloride

Binary Ionic Compounds Binary Ionic Compounds ((Type IIType II):):

Example

• FeCl3

Iron(III) chloride

Ferric chloride

• FeCl2

Iron(II) chloride

Ferrous chloride

Example

• Write the name of the compound Fe2O3 Oxidation state of Fe = +3

Iron(III) oxide

• Group 1A, Group 2A and Al3+ do not take Roman numerals

• Silver, Ag forms more than one oxidation state but Roman numerals are not used.

Common Cations and Anions

Ionic compounds with polyatomic ions

• Polyatomic ions are assigned special names that need to be memorized

• Oxyanions: anions that contain an atom of a given element and different numbers of oxygen atoms

• Polyatomic anions (with many atoms) containing oxygen end in -ate or -ite. (The one with more oxygen is called ate.)

• Examples: NO3- is nitrate, NO2

- is nitrite.• (Exceptions: hydroxide (OH), cyanide (CN),

peroxide (O22).)

Examples:

NaNO3 Sodium nitrate

K2SO4 Potassium sulfate

Al(HCO3)3 Aluminum bicarbonate

or aluminum hydrogen

carbonate

Common Polyatomic Ions

Compounds between two Compounds between two nonmetalsnonmetals

Although they do not contain ions, they are named similarly to Although they do not contain ions, they are named similarly to binary ionic compoundsbinary ionic compounds

-- First element First element in the formula is in the formula is named firstnamed first

using full element nameusing full element name

-- Second element Second element is named as if it were an is named as if it were an anionanion..

-- Use prefixes to denote number of atomsUse prefixes to denote number of atoms

-- Never use Never use monomono-- for naming first element for naming first element

PP22OO55 = = didiphosphorus phosphorus pentpentoxideoxide

Binary Covalent compounds Binary Covalent compounds ((Type IIIType III):):

Acids

• Substances that produce H+ ions when dissolved in water.

• All acids begin with H.

• Two types of acids: – Oxyacids– Non-oxyacids

Naming acids

• If the formula has oxygen in it• write the name of the anion, but change

– ate to -ic acid– ite to -ous acid

• Watch out for sulfuric and sulfurous

• HNO3

• HNO2

• HC2H3O2

Naming acids

• If the acid doesn’t have oxygen

• add the prefix hydro-

• change the suffix -ide to -ic acid

• HCl

• H2S

• HCN