Chapter 02 - Atoms Molecules and Ions

8/13/2019 Chapter 02 - Atoms Molecules and Ions

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Chapter 2

Atoms, Molecules, and Ions

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Early Philosophy of Matter

Some early philosophers believed that matter consisted of

tiny, indivisibleparticlesatomos theoryLeucippusand Democritus

Other philosophers believed that matter was infinitelydivisiblePlato and Aristotle

Because there was no experimental way of proving whowas correct, the best debater was the person assumed

correct, i.e., Aristotle

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Scientific Revolution

For the next 150+ years, observations about naturewere made that could not easily be explained by the

infinitely divisible matter concept

In the late 17thcentury, the scientific approach tounderstanding nature became established

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Law of Conservation of Mass

Antoine Lavoisier

1743-1794

In a chemical reaction, matter is neither created nor

destroyed

Total mass of the material before and after a reaction must beequal: total mass of reactants = total mass of products

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In a reaction of sodium with chlorine to make sodium chloride

7.7 g Na + 11.9 g Cl2

19.6 g NaCl

only whole atoms combine and atoms are not changed ordestroyed in the process

the mass of sodium chloridemade must equal the total massof sodiumand chlorine atomsthat combine together

Law of Conservation of Mass

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Law of Definite Proportions

All samples of a given compound, regardless of

their source or how they were prepared, have thesame proportion of their constituent elements

Joseph Proust 1754-1826

decomposition of 18 gramsof water (H2O) always

gives 16 gramsof oxygen (O) and 2 gramsofhydrogen (H). An 8:1 ratio.

decomposition of 17 gof ammonia (NH3) always

gives 14 gof nitrogen (N) and 3 gof hydrogen. A4.7:1 ratio.

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Law of Multiple Proportions

When two elements (call them Aand B) form twodifferent compounds, the masses of Bthat combine

with 1 g of Acan be expressed as a ratio of small,

whole numbers

John Dalton 1766-1844

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Law of Multiple Proportions

Carbon combines with oxygen to form two different compounds,

carbon monoxide(CO) and carbon dioxide(CO2).

CO contains 1.33 gof oxygen for every 1.00 gof carbon .

CO2contains 2.67 gof O for every 1.00 gof C

oxygen mass ratio

= 2mass of O combining with 1 g of C in CO

mass of O combining with 1 g of C in CO2

2.67 g

1.33 g=

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Dalton proposed a theory of matter based on it havingultimate, indivisible particles to explain these laws

1. Each element is composed of tiny, indestructible particles

called atoms

2. All atoms of a given element have the same mass and other

properties that distinguish them from atoms of other

elements

3. Atoms combine in simple, whole-number ratios to form

molecules of compounds

4. In a chemical reaction, atoms of one element cannot change

into atoms of another elementthey simply rearrange the

way they are attached

Daltons Atomic Theory

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PracticeDecide if each statement is correct

according to Daltons model of the atom

Copper atoms can combine with zinc atoms to

make gold atoms

Water is composed of many identical molecules that

have one oxygen atom and two hydrogen atoms

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Cathode Ray Tube

Glass tube containing metal electrodes from which

almost all the air has been evacuated

When connected to a high voltage power supply, aglowing area is seen emanating from the cathode

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J.J. Thomson

Believed that the cathode ray was composed of tiny

particles with an electrical charge

He designed an experiment to demonstrate that the rays

were particles by measuring the amount of force ittakes to deflect their path a given amount

Similar to measuring the amount of force it takes tomake a turn

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Thomsons Experiment

+++++++++++

-------------

Power Supply- +

Cathode Anode

Investigating the effect of placing an electric field around tube

1. charged matter is attracted to an electric field2. lights path is not deflected by an electric field

()

electrically charged plates

(+)

(+)

()

filled with low pressure gas

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Fancy Cathode Ray Tube

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Thomsons Results

cathode rays are made of tiny particles

The particles have a negative charge because the beamalways deflected toward the + plate

The amount of deflection is related by the charge and the

mass of the particles

Every material tested contained these same particles

The charge:mass ratio of these particles was 1.76 x 108C/g

The charge:mass ratio of the hydrogen ion is +9.58 x 104C/g

the particle is almost 2000x smaller than a hydrogen atom! 15


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Millikans Oil Drop Experiment

ionizing radiation

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Millikans Conclusions

He combined his results with the ones from Thompsonsexperiments to determine the mass of an electron

Millikan determined that an electron (e

) has a charge of1.60 x 1019C

9.10 x 1028g=1.60 x 1019C

1.76 x 108C/g=e

mass = e

charge

charge to

mass ratio

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A New Theory of the Atom

Because the atom is no longer indivisible, Thomson

proposes a new model of the atom to replace the first

statement in Daltons Atomic Theory

rest of Daltons theory still valid at this point

Thomson proposes that instead of beinghard,marble-like unbreakable spheres, the way Dalton

described it, atoms actually had an inner structure

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Thomsons Plum Pudding Atom

atom structure contains many negatively charged electrons

electrons are held in the atom by their attraction to apositively charged electric field within the atom

atoms are neutral so there has to be some positive charge

Thomson assumed there were no positively charged piecesbecause none showed up in the cathode ray experiment

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Radioactivity

In the late 1800s, Henri Becquerel and Marie Curiediscovered that certain elements would constantly emitsmall, energetic particles and rays. [uranium, radium and

polonium]

Marie Curie

1867-1934

These energetic particles could penetrate matter

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Radioactivity

Ernest Rutherford discovered that radioactive elements emitthree different kinds of emissions

alpha, a, raysstream of particles with 4x the mass of H atom and a +

charge

beta, , raysmade of particles with a mass ~1/2000thH atom and

charge

gamma, , raysare energy rayshigh energy photons

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Rutherfords Gold Foil Experiment

Rutherford also wanted to know whether Thomsons idea

about the atom was correct. So he devised an experiment

in which he shot some of the newly discovered positively

charged alpha decay particles at a very thin piece of gold foil

most of the particles went through the foil, but some particleswhere scattered.

Polonium-214

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Rutherfords Results

nucleusincoming

a-particles

Interpretation

Plum Pudding Atom Nuclear Atom

nucleus

containing

protons and

neutrons

volume

occupied byelectrons

The structure of an atom

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Discovery of the Neutron

James Chadwick did essentially the same experiment as

Rutherford using Beryllium foil and discovered the neutron.

Atoms are composed of positive protons and neutral neutrons

in the center (nucleus) held together by nuclear forces

surrounded by a fuzzy cloud of electrons

1 amu = 1.66054 x 1024g

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El l S b l A i N b d M N b


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The number of protons defines an elementand is called

the atomic number(Z)

Elemental Symbols: Atomic Numbers and Mass Numbers

the number of protons + neutrons in the nucleus is called

the mass number (A)

Each element is identified by a unique chemical symbol, a

one or two letter abbreviation of its name with the first letter

capitalized and when needed the second lower case.

Zelement symbolsX

Amass number

atomic number27

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Isotopes

Isotopes have different numbers of neutrons.

Isotopes are atoms of the same element with the same

atomic number (Z), but different atomic masses (A).

ZX

A

6C

13

6C

14

6C

12

6C

11

All carbon atoms have 6 protons.

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Atomic Mass

A mass spectrometer can measure atomic and molecular

masses with great accuracy.

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Average Atomic Mass

The atomic weightis the average atomic mass of all of the

isotopes of a particular element.

S[(isotope mass) x (fractional isotope abundance)]

over all the isotopes

atomic

weight

=

naturally occurring carbon is composed of 98.93% carbon-12

and 1.07% carbon-13with masses of 12.0 amu (exactly)and

13.00335 amu, respectively.

(12.0) x (0.9893 amu)+ (0.0107) (13.00335 amu)= 12.01 amu

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Periodic Table

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Periodic Table

Transition Metals

Lanthanides

Actinides

alkali metals

alkaline earth metals

halogens

chalcogens

noble gases(inert gases)

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M l l d Ch i l F l


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Molecules and Chemical Formulas

chemical formulatells what type and how many atoms

are in a molecule

moleculeis two or more atoms bonded together

a diatomic moleculehas only two atoms

compoundcomposed of atoms of two or more differentelements.

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M l l d Ch i l F l


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Molecules and Chemical Formulas

The subscript to the right of an elements symbol tells the

number of that elements atoms in one molecule of the compound.

Molecular compoundsare composed of molecules and

almost always contain only nonmetals.

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Di i M l l


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Diatomic Molecules

These seven elements occur naturally as diatomic

molecules containing two atoms:Hydrogen

Nitrogen

Oxygen

Fluorine

Chlorine

Bromine

Iodine

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T f F l


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Types of Formulas

Empirical formulasgive the lowest whole-number ratio

of atoms of each element in a compound.

Molecular formulasgive the exact number of atoms of

each element in a compound.

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T f F l


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Types of Formulas

Structural formulasshow the order in which atoms are

bonded.

Perspective drawingsalso show the three-dimensional array

of atoms in a compound.

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I L i G i i El t


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Ions: Losing or Gaining Electrons

Ions are atoms or molecules which have gained () or lost

(+) electrons

(+) cations and () anions

sodium Na Na+ + 1e sodium ion

chlorine Cl + 1 eCl chlorine ion

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Ions

When atoms lose or gain electrons, they become ions.

Cationsare positive and are formed by elements on the leftside of the periodic chart.

Anions are negative and are formed by elements on the right

side of the periodic chart. 39

I i B d


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Ionic Bonds

Ionic compounds(such as NaCl) are generally formed

between metals and nonmetals.

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Ch i l P k M l


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1 atom of C-12 weighs exactly 12 atomic mass units (amu)

1 Mole= the number of atoms in exactly 12 g of C-12

1 mole of C-12 weighs exactly 12 g

1 mole of C atoms weighs 12.01 g and has

6.022 x 1023atoms

the average atomic massof a C atom is 12.01 amu

Chemical Packages - Moles

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The number of particles in 1 mole is called

Avogadros Number (NA) = 6.0221421 x 1023

anything

mol

U it F th P i di T bl


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Units From the Periodic Table

The periodic table gives the average atomic mass of carbon

as 12.01 amu

1 mole = 6.022 x 1023carbon atoms

1 amu = 1.6654 x 1024g

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12.01 amu

C atom

6.022 x 1023 C atoms

mol

1.6654 x 1024 g

amu=

12.01 g

mol

this means the periodic table gives units of g/mol or amu

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The mass of one mole of atoms is called the molar mass

The molar mass of an element, in grams, is numerically

equal to the elements atomic mass, in amu

The lighter the atom, the less a mole weighs

The lighter the atom, the more atoms there are in 1 g

Relationship Between Moles and Mass

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M l d M R l ti hi


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Mole and Mass Relationships

1 mole

sulfur

32.06 g

1 mole

carbon44

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Chapter 02 - Atoms Molecules and Ions