Atoms, Molecules and IonsChapter 2

ChemistryThe Study of Matter

Matter includes everything (solid, liquid & gas) in the universe made of atoms.

The name atom comes from the Greek átomos, which means uncuttable, or indivisible, or something that cannot be divided further.

The Greek philosopher: Speculation, not experiment, was the method of the Greek thinker. Manual labor (doing hands-on experiments) was beneath them in their “class system” society. They did “thought experiments” by applying logic to the things they observed in nature. They would develop new ideas through discussion and debate (discourse) and through this process they felt they could determine “ultimate truth”.

The AtomEarliest Theory attributed to Democritus (460

– 370 B.C.)

Democritus assumed a void (empty space) in which unchangeable atoms of the four elements, earth, air, water, and fire were in continual random motion. These atoms had some physical size and shape resulting in their many properties.

The “idea” of the atom had been around for at least 150 yrs before Democritus but he developed the first theory connecting the atom to the element concept explaining how they are related and work together, and… gave us the term “atom”.

Aristotle (384 – 323 B.C.)Although the idea of the atom had been mostly accepted, Aristotle didn't like it. He claimed that there was no smallest part of matter but he did agree that there were the basic elements of fire, air, earth, and water. Since there was no experimental way available to test either view, it simply came down to who presented the better argument.

Aristotle had produced a very broad, comprehensive set of theories on every known topic of the day and was looked upon as a higher authority than Democritus had been. Aristotle's views on the physical sciences profoundly shaped medieval thinking, and his influence extended well into the Renaissance (17th century) and beyond. Eventually, his ideas were replaced when experimenting and scientific research were introduced as the proper way to explore nature.

Aristotle’s idea was: matter is continuousTherefore, if you cut matter into pieces over and over, you never reach a “smallest particle” – it just goes on continuously. In other words, no atoms.

WHY was Aristotle one of the last great philosophers?

The Romans conquered the Greeks around 150 B.C. and no more philosophers came along to change the ideas.

Ancient Rome contributed greatly to the development of law, war, art, literature, architecture, technology and language but not much in the way of scientific theories.

Alchemy

Started during the 1st century AD and lasted through the middle ages.

The best-known goals of the alchemists were:(1) the transmutation of common metals into gold or silver; (2) the creation of a "panacea", or the elixir of life, a remedy that supposedly

would cure all diseases and prolong life indefinitely; and (3) the discovery of a universal solvent.

Although these were not the only uses for the discipline, they were the ones most documented and well known.

AlchemyAlchemy was practiced in Mesopotamia (comprising much of today's Iraq), Egypt, Persia (today's Iran), India, China, Japan, Korea and in Classical Greece and Rome.

Alchemy is the technical (non-theoretical) ancestor of today’s Chemistry. It’s ideas were rooted in magic and not in substantiated theories.

Modern chemistry relies on both technology and theory to be able to advance itself.

The Atom Comes BackRobert Boyle (1627 – 1691)

Boyle was a natural philosopher, chemist, physicist, inventor, and gentleman scientist, also noted for his writings in theology. He is best known for the formulation of Boyle's law (P x V = constant).

Boyle wrote a book, The Sceptical Chymist, seen as a cornerstone book in the field of chemistry.

This book presented Boyle's hypothesis that matter consisted of atoms and groups of atoms in motion and that every phenomenon was the result of collisions of particles in motion.

He strongly opposed the concept of only the four classic elements (earth, fire, air, and water). He advocated a rigorous approach to scientific experiment: he believed all theories must be proved experimentally before being regarded as true. For these reasons Robert Boyle has been called the founder of modern chemistry.

Chemical Theories Guide the Way• Antoine-Laurent de Lavoisier (1743 –1794)The undisputed father of modern chemistry; He stated the first version of the law of conservation of mass, recognized and named oxygen (1778) and hydrogen (1783), helped construct the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature.

Left: One of Lavoisier’s laboratories

Conservation of Mass = although matter may change its form or shape (chemical or physical properties), its mass always remains the same.

Lavoisier was connected to the aristocratic French Government. He helped run a private tax collection company; he was the chairman of a bank; and, he had made someenemies among those leaders of the French Revolution.

All of these political and economic activities enabled him to fund his scientific research, however, at the height of the French Revolution (aka: the Reign of Terror) he was accused of some wrong-doing and was tried, convicted, and guillotined in Paris, at the age of 50.

John Dalton (1766 – 1844)

FIRST ATOMIC THEORY 1808(Well, … one of the first anyway.)

Dalton was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and hisresearch into color blindness (sometimes referred to as Daltonism, in his honor).

The most important of all Dalton's investigations are those concerned with the atomic theory in chemistry, with which his name is inseparably associated.

Dalton arrived at his theory by studying the physical properties of the atmosphere and other gases.

(We’ll talk about Dalton’s Law of Partial Pressures later in this course)

5 main points of Dalton's Atomic Theory• Elements are made of tiny particles called atoms.

Above: Various atoms and molecules as depicted in John

Dalton's A New System of Chemical Philosophy (1808).

• All atoms of a given element are identical.

• The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights.

• Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms.

• Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.

2

2.1

8 X2Y16 X 8 Y+

2.1

Early Experiments to Characterize the Atom

• J. J. Thomson: the Electron• Robert Millikan: Mass of of electron• Henri Becquerel: Radioactivity• Ernest Rutherford: the Nucleus

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

(1906 Nobel Prize in Physics)

Cathode Ray Tube

Measured mass of e-

(1923 Nobel Prize in Physics)

• 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.

(Uranium compound)

The modern view of the atom was The modern view of the atom was developed by developed by Ernest RutherfordErnest Rutherford(1871(1871--1937).1937).

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)

α 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

Chadwick’s Experiment (1932)

H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

α + 9Be 1n + 12C + energy

neutron (n) is neutral (charge = 0)

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

mass p = mass n = 1840 x mass e-

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

Mass Number

H11 H (D)2

1 H (T)31

U23592 U238

92

Atomic NumberElement Symbol

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 ?

Molecules and ionsA molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds

H2 H2O NH3 CH4

A diatomic molecule contains only two atomsH2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atomsO3, H2O, NH3, CH4

ELEMENTS THAT EXIST AS DIATOMIC MOLECULES

ELEMENTSELEMENTS THAT EXIST THAT EXIST AS AS DIATOMICDIATOMIC MOLECULESMOLECULES

Remember:

BrINClHOFThese elements

only exist as PAIRS. Note that

when they combine to make compounds, they

are no longer elements so they are no longer in

pairs!

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

Forming Cations & AnionsForming Forming CationsCations & Anions& AnionsA A CATIONCATION forms forms when an atom when an atom loses one or loses one or more electrons.more electrons.

An An ANIONANION forms forms when an atom when an atom gains one or gains one or more electronsmore electrons

F + e- --> F-Mg --> Mg2+ + 2 e-

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 ?Al7834

2-

2.5

A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance

An empirical formula shows the simplest whole-number ratio of the atoms in a substance

H2OH2Omolecular empirical

C6H12O6 CH2O

O3 ON2H4 NH2

ionic compounds consist of a combination of cation(s) and an anion(s)• the formula is always the same as the empirical formula

• the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero

The ionic compound NaCl

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-

Examples of Older Names of Cationsformed from Transition Metals

(memorize these!!)From Zumdahl

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

• 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

• 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

HI hydrogen iodide

NF3 nitrogen trifluoride

SO2 sulfur dioxide

N2Cl4 dinitrogen tetrachloride

nitrogen dioxide TOXIC!NO2

N2O dinitrogen monoxide

Molecular Compounds

Laughing Gas

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

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

Mixed Practice1. Dinitrogen monoxide2. Potassium sulfide3. Copper (II) nitrate4. Dichlorine heptoxide5. Chromium (III) sulfate6. Ferric sulfite7. Calcium oxide8. Barium carbonate9. Iodine monochloride

1. N2O2. K2S3. Cu(NO3)2

4. Cl2O7

5. Cr2(SO4)3

6. Fe2(SO3)3

7. CaO8. BaCO3

9. ICl

Mixed Practice1.1. BaIBaI22

2.2. PP44SS33

3.3. Ca(OH)Ca(OH)22

4.4. FeCOFeCO33

5.5. NaNa22CrCr22OO77

6.6. II22OO55

7.7. Cu(ClOCu(ClO44))22

8.8. CSCS22

9.9. BB22ClCl44

1.1. Barium iodideBarium iodide2.2. Tetraphosphorus trisulfideTetraphosphorus trisulfide3.3. Calcium hydroxideCalcium hydroxide4.4. Iron (II) carbonateIron (II) carbonate5.5. Sodium dichromateSodium dichromate6.6. DiiodineDiiodine pentoxidepentoxide7.7. Cupric perchlorateCupric perchlorate8.8. Carbon disulfideCarbon disulfide9.9. DiboronDiboron tetrachloridetetrachloride