Chapter 2 Atoms Molecules Ions

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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 tomos, which means uncuttable, or indivisible, or something that cannot be divided further.

The AtomEarliest Theory attributed to Democritus (460 370 B.C.)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. 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. Aristotles idea was: matter is continuous Therefore, 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.


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 todays Chemistry. Its 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 Lavoisiers 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 some enemies 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.

FIRST ATOMIC THEORY 1808(Well, one of the first anyway.)John Dalton (1766 1844) Dalton was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research 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.(Well talk about Daltons Law of Partial Pressures later in this course)

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

Above: Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).



16 X



8 X2Y


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 electrons mass as 9.109382 x 10-28 grams.

(Uranium compound)

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

(1908 Nobel Prize in Chemistry)

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

1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

Rutherfords Model of the Atom

atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Chadwicks Experiment (1932)H atoms - 1 p; He atoms - 2 p mass He/mass H should = 2 measured mass He/mass H = 4

+ 9Be


+ 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 nucleiMass Number Atomic Number

A ZX 2 1H

Element Symbol

1 1H 235 92

(D)238 92

3 1H




Do You Understand Isotopes?14 in 6 C ?

How many protons, neutrons, and electrons are

6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are11 in 6 C ?

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

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





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 MOLECULESRemember: 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 charge If a neutral atom loses one or more electrons it becomes a cation. Na11 protons 11 electrons


11 protons 10 electrons

anion ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl17 protons 17 electrons


17 protons 18 electrons

Forming Cations & AnionsA CATION forms when an atom loses one or more electrons. An ANION forms when an atom gains one or more electrons

Mg --> Mg2+ + 2 e-

F + e- --> F-

A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3-

A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3-

Do You Understand Ions?27 3+ 13 Al

How many protons and electrons are in


13 protons, 10 (13 3) electrons How many protons and electrons are in78 234 Al


34 protons, 36 (34 + 2) electrons


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 molecular H2O C6H12O6 O3 N2H4 empirical H2O CH2O O 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 Compounds2 x +3 = +6 3 x -2 = -6

Al3+1 x +2 = +2


O22 x -1 = -2

Ca2+1 x +2 = +2


Br1 x -2 = -2




Examples of Older Names of Cations formed from Transition Metals (memorize these!!)From Zumdahl

Chemical Nomenclature Ionic Compounds often a metal + nonmetal anion (nonmetal), add ide to element name

BaCl2 K2O Mg(OH)2 KNO3

barium chloride potassium oxide magnesium hydroxide potassium nitrate

Transition metal ionic compounds indicate charge on metal with Roman numerals

FeCl2 FeCl3 Cr2S3

2 Cl- -2 so Fe is +2 3 Cl- -3 so Fe is +3

iron(II) chloride iron(III) chloride

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

Molecular CompoundsHI NF3 SO2 N2Cl4 NO2 N2O hydrogen iodide nitrogen trifluoride sulfur dioxide dinitrogen tetrachloride nitrogen dioxide TOXIC! Laughing Gas

dinitrogen monoxide

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 H2CO3 H2SO4 nitric acid carbonic acid sulfuric acidHNO3

A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. NaOH KOH Ba(OH)2 sodium hydroxide potassium hydroxide barium hydroxide

Mixed Practice1. 2. 3. 4. 5. 6. 7. 8. 9. Dinitrogen monoxide Potassium sulfide Copper (II) nitrate Dichlorine heptoxide Chromium (III) sulfate Ferric sulfite Calcium oxide Barium carbonate Iodine monochloride 1. 2. 3. 4. 5. 6. 7. 8. 9. N2O K2S Cu(NO3)2 Cl2O7 Cr2(SO4)3 Fe2(SO3)3 CaO BaCO3 ICl

Mixed Practice1. 2. 3. 4. 5. 6. 7. 8. 9. BaI2 P4 S3 Ca(OH)2 FeCO3 Na2Cr2O7 I2O5 Cu(ClO4)2 CS2 B2Cl4 1. 2. 3. 4. 5. 6. 7. 8. 9. Barium iodide Tetraphosphorus trisulfide Calcium hydroxide Iron (II) carbonate Sodium dichromate Diiodine pentoxide Cupric perchlorate Carbon disulfide Diboron tetrachloride