Unit II : Atoms, Molecules, Ions and Nuclear Chemistry

Early History of ChemistryBefore 16th Century Alchemy: Attempts (scientific or

otherwise) to change cheap metals into gold

17th Century Robert Boyle: First “chemist” to

perform quantitative experiments

18th Century George Stahl: Phlogiston flows out of a

burning material. Joseph Priestley: Discovers oxygen

gas, “dephlogisticated air.”

Law of Conservation of Mass

-Discovered by Antoine Lavoisier

-Mass is neither created nor destroyed

-Combustion involves oxygen, not phlogiston

Law of Definite Proportions- Sometimes called the law of constant

composition- John Proust (1799)- A given compound always contains exactly the

same proportion of elements by mass.

- Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.

Law 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 in H2O and H2O2 will be a small whole number (“2”).

Dalton’s Atomic Theory

1 Each element is made up of tiny particles called atoms.

2 The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

3 Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.

4 Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Avagadro’s Hypothesis (1811)At the same temperature and

pressure, equal volumes of different gases contain the same number of particles.

5 liters of oxygen5 liters of nitrogen

SAME NUMBER OF PARTICLES

Chemical SymbolsThe chemical symbols used today were

developed by Jons Jakob Berzelius.

They consist of one or two letters. The first letter is always capitalized. The second, if there is one, is never

capitalized. The second letter is often a letter prominent in the pronunciation.

Co Cu He H

Early Experiments to Characterize the Atom

DemocritisDaltonThomsonRutherfordBohrSchrodinge

r

Cathode Ray

• Streams of negatively charged particles were found to emanate from cathode tubes.• J. J. Thompson is credited with their discovery (1897).

Deflection of Cathode Ray by an Applied Electric Field

02_21

Metalelectrode

Metalelectrode

(-)

(+)

(+)

(-)

Appliedelectric field

Rutherford Gold Foil Experiment

Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Rutherford Gold Foil Experiment

02_25

Electrons scatteredthroughout Diffuse

positivecharge

(a) (b)

-

--

-

-

-

--

-

-

-

--

-

-

-

--

--

n+

(a)Expected results if Thomson’s Model was correct

(b)Actual results

Summary of AtomDemocritis - first idea of the atomDalton - ~1807 Atomic Theory (atom = ball)Thomson - Experiment with Cathode Ray

Discovered the electron“Plum pudding model”

Rutherford - Gold Foil ExperimentAtom is mostly empty space with a dense,

positively charged nucleusBohr - “Solar System” Schrodinger - Quantum Mechanical Model

Modern View of Atomic StructureThe atom contains 3 types of subatomic

particles:Electrons

Negatively charged, found outside the nucleus… very small mass… would take ~2000 electrons to equal the mass of 1 of the other subatomic particles

Protons Positively charged, found in the nucleus

Neutrons Found in the nucleus… like a proton but with no

charge

Subatomic Particle Summary

Mass (g)

Charge Atomic Mass Scale

Relative Mass

Relative Charge

Proton (p+) 1.6726x1

0-24

+1.602x10-19 c

1.0073 amu

1 +1

Neutron (n0)

1.6726x10-24

0 1.0087 amu

1 0

Electron (e-) 9.109 x

10-27

-

1.602x10-19 c

5.486 x 10-4 amu

1/1836

-1

Useful UnitsAtomic Diameter:10-10m (1 Å) = 100 pm = 1x10-8cmNuclear Diameter: 10-13cm1 amu (atomic mass unit) = 1.66054 x10-24 kg

Based off of C-12… relative mass of O w/ 8 protons and 8 neutrons is 15.9949

1 picometer (pm) = 1 x 10-12 m

NOTE:Heaviest atom has a mass of only 4.8x10-22g and a diameter of

only 5x10-10mBiggest atom is 240 amu and is 50 Å across.Typical C-C bond length 154 pm (1.54 Å)Molecular models are 1 Å /inch or about 0.4 Å /cm

Atomic Number

K39

19

Element Symbol

Z = atomic number = number of protons

A = mass number = number of protons + number of neutrons

A = Z + NMass number = # p+ + # n0

IsotopesIsotopes are atoms of the same element

with different masses.Isotopes have different numbers of

neutrons.

Carbon has two natural isotopes: 12C and 13C

How does this change things?

IsotopesOn the periodic table the given atomic

mass is the AVERAGE massWe use percentage to help with this math.For Carbon, there is 98.89% 12C and 1.11%

of 13CNow what?

12 (mass of 12C) x 0.9889 = 11.866813 (mass of 13C) x 0.011 = 0.144311.8668 + 0.1443 = 12.0111

IsotopesOn the periodic table the given atomic

mass is the AVERAGE mass

We use percentage to help with this math.

For Carbon, there is 98.89% 12C and 1.11% of 13C

Mass Spectrometer

Used to experimentally determine mass and percent abundances of isotopes

Prentice-Hall © 2002General Chemistry: Chapter 2Slide 23 of 25

The Periodic TableAlkali Metals

Alkaline Earths

Transition Metals

Halogens

Noble Gases

Lanthanides and Actinides

Main Group

Main Group

Periodicity

When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.

The Periodic Table1869- Dmiti Mendeleev organized 1st

periodic table by lining elements up in horizontal rows in order of increasing atomic weightLeft spaces when an element was not

known but should exist and have properties similar to the element about it in his table.Periodicity – periodic repetition of the

properties of elements

1913- H.G.J. Moseley organized elements according to increasing atomic numberLaw of chemical periodicity

The properties of the elements are periodic functions of atomic numbers.

Features of the Periodic TableVertical Columns: Groups or FamiliesA and B groups. “A” main group, “B”

transition elementsHorizontal Rows: called periodsPeriodic Law: when the elements are arranged

in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties

Metals~80% of all elements are

metalsHigh electrical

conductivity and have a high luster when cleaned

Ductile (can be made into wires) and Malleable

Can form alloysAre solid at room

temperature except for one… which one is it?

NonmetalsGenerally

nonlustrous… which means not shiny

Generally poor conductors of electricityWith the

exceptions of carbon (graphite), none conduct electricity

•Allotropes•A particular element can often exist in several different forms

•Carbon as graphite or diamond… oxygen as O2 or O3 (ozone)

MetalloidsSemi-metals

Overview of GroupsGroup 1A

Alkali Metals Metals, solid at room temp, and VERY REACTIVE

Only found in nature combined in compounds

Group 2AAlkaline Earth Metals

Metals, solid at room temp, and also only found in nature in compounds

Except for Be, all react with water to produce alkaline solutions

Overview of GroupsGroup 7A

HalogensAll exist in diatomic moleculesCombine violently with alkali metals to form salts

Group 8ANoble Gases (Inert Gases)

Least reactive elementsAll are gases

Overview of GroupsGroup B Metals (Groups 3-12):

Transition Metals & Inner Transition MetalsLanthanides- shiny metals similar in reactivity to

alkaline earth metalsActinides- unique in nuclear structures – nuclei are

unstable and therefore radioactiveInner Transitions Metals – rare-earth metals

Prentice-Hall © 2002General Chemistry: Chapter 2Slide 33 of 25

The Periodic TableAlkali Metals

Alkaline Earths

Transition Metals

Halogens

Noble Gases

Lanthanides and Actinides

Main Group

Main Group

Chemical FormulasThe subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.

Molecular compounds are composed of molecules and almost always contain only nonmetals.

Chemical Formulas: Diatomic Molecules

These 7 elements occur naturally as diatomic molecules

Types of Formulas

Molecular Formula – CH4

Structural Formula

Ball and Stick Model

Space-Filling Model

Types of FormulasMolecular Formula

Exact number of each type of atomC6H12O6

Empirical FormulaLowest whole number ratio of each type of

atomCH2O