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Chapter 2: Atoms, Moleculesand Ions

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Web Resources (Atomic Structure)

• Chem Team: Atomic Structure• http://dbhs.wvusd.k12.ca.us/webdocs/

AtomicStructure/AtomicStructure.html• http://www.visionlearning.com/library/

module_viewer.php?mid=50

• Discovery of the electron• http://www.3rd1000.com/history/

electrons.htm

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I. History of the Atomic TheoryA. DemocritusB. AristotleC. LavoisierD. ProustE. Dalton

F. Modern Atomic TheoryII. History of Atomic Structure

A. ThomsonB. MilikanC. RutherfordD. BohrE. ChadwickF. Quantum Atom

III. Subatomic ParticlesA. Atomic NumberB. Mass Number and IsotopesC. Electrons and IonsD. Nuclear and Hyphenation NotationE. Average Atomic Mass

IV. The Periodic TableV. Naming and Formula Writing

A. Formula Units B. MoleculesC. Oxidation Number

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History of the Atomic Theory: Greeks

• Original idea Ancient Greece (400 B.C..)

• Democritus and Leucippus Greek philosophers

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Democritus (400 BC)– Matter is made up of “atoms”– Atoms are always moving– Atoms join together– He was right– Problem: no evidence and

not as popular– Coined the word: Atomos

History of the Atomic Theory: Greeks

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Aristotle (350 BC)– All matter is continuous– Air Water Fire Earth– He was very well known in his day– But he was completely wrong

History of the Atomic Theory: Greeks

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History of Atomic Theory: XVII-XVIII Centuries

Boyle (1627-1691): defined the concept of an element. Did tremendous work with gases

Newton(1642-1727) advocated the corpuscular( corpuscle – particle) theory of matter.

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History of Atomic Theory: Late XVIII Century

• Combination Laws are formulated

– Law of Conservation of Mass

– Law of Definite Proportions

– Law of Multiple proportions

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Antoine Lavoisier (1770s)– Experiment:

tin + oxygen tin oxide

mass before rxn = mass after rxn

– Develops Law of Conservation of Mass

• Matter cannot be created or destroyed (in a chemical or physical change)

Law of Conservation of Mass

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Joseph Proust (1779)– In each compound elements combine in

definite proportions to each other by mass

– Example: carbon dioxide contains 27.3% carbon and 72.7% oxygen

– Example: water, 11.2 % hydrogen and the rest oxygen

Law of Definite Composition

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Law of Multiple Proportions (Dalton)

• If two elements form more that one compound, the ratio of one element that combines with 1 gram of the first element in each of the compounds is a small whole number.

• Examples: 1.00 g of NO will combine with 1.14 g of O to form nitrogen monoxide, and

1.00 g of N will combine with 2.28 g of O to form nitrogen dioxide. The ratio of masses of oxygen in both compounds is

2.28: 1.14 = 2:1

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• Dalton, unlike Democritus, collects evidence and develops:

• Dalton’s Atomic Theory

History of the Atomic Theory: Dalton

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Dalton’s Atomic Theory: Postulates

All matter is made of tiny indivisible particles called atoms.

Atoms of the same element are identical in their properties, those of different elements are different.

Atoms of different elements combine in whole number ratios to form compounds

Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

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Explain the Combination Laws using Dalton’s Atomic Theory

• Explain the combination laws using Dalton's Theory : check textbook.

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History of the Development of Modern Atomic Structure

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Discovery of Radioactivity

– Becquerel: 1896. Discovered radioactivity

– Maria Curie Sklodowska: discovered radium and polonium (1898)

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The Discovery of Electrons

• J. J. Thomson(1856-1940) - English physicist. – Used cathode ray tube.– It is a vacuum tube - all the air has been

pumped out.– Discovered the electron (1897) and the proton

(1923)

– Calculated the ratio between the charge of the

electron and its mass: e/m

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Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

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Thomson’s Experiment

Voltage source

+-

Vacuum Tube before the experiment: the tube is evacuated and power is about to be applied.

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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 endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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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 in straight linepositive end in straight line

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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Voltage source

Thomson’s Experiment

• When external electric field (outside the tube) is added:

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Voltage source

Thomson’s ExperimentThomson’s Experiment

External electric fieldExternal electric field

+

-

External electrodes

anode

cathode

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an external electric fieldBy adding an external electric field

+

-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an external electric fieldBy adding an external electric field he found that the he found that the moving pieces were moving pieces were negativenegative: they bend towards the : they bend towards the positive anode.positive anode.

+

-

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Thomson’s Model

• Thomson proposed the following:– the atom was like

plum pudding– Ball of positively

charged matter– electrons scattered

throughout and can be easily removed

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Millikan’s Oil Drop Experiments

Robert Milikan (1909)– Oil Drop Experiment – Measured the electrical charge on the

electron– Mass can be calculated (Thomson

determined the e/m ratio)– Mass is 1/1840 the mass of a hydrogen

atom– electron has a mass of 9.11 x 10-28 g

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Oil-Drop Experiment: Animation

• http://wps.prenhall.com/wps/media/objects/602/616761/MillikanOilDropExperiment.html

• http://www.britannica.com/nobel/cap/omillik001a4.html

• http://www.daedalon.com/oildrop.html

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• So, at this point we know:– Dalton’s Atomic Theory– Electrons are negatively charged (Thomson)– Atoms are neutral– The mass of an electron is very small

(Millikan)– Where is the mass of the atom?

Discovery of the Nucleus

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Ernest Rutherford (1909): Gold Foil Experiment– Alpha particles (+2 charge) beamed at thin gold foil– Progress followed on a scintillating screen– When alpha particles (+2) hit screen, the screen lights

up– Most particles went through– Some particles bounced to the sides and at 180º– In process: discovered the proton– Proton is positively charged

Discovery of the Nucleus: Rutherford

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Lead block

Uranium

Gold Foil

Florescent Screen

THE SET-UP

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Rutherford Expected

• The alpha particles to pass through without changing direction

Because• The positive charges were spread out

evenly. Alone they were not enough to stop the alpha particles

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Because

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

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

Lead block

Uranium

Gold Foil

Fluorescent Screen

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Rutherford’s results

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Rutherford’s Gold Foil Experiment

Here’s what it looked like (pg 72)

• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

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Discovery of the Nucleus: Rutherford

• Discovered the:– Proton: positively charged piece 1840 times

heavier than the electron

• Predicted the existence of – Neutron: no charge but the same mass as a

proton.

• Where are the pieces?

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Discovery of the Nucleus: Density of the Atom

• Since most of the particles went through, the atom is mostly empty.

• Since some of the alpha paricles were strongly deflected, the positive pieces were heavy.

• Small volume, big mass, big density• This small dense positive area is the

nucleus

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Rutherford: Explanation

Atom is:• mostly empty• small dense,

positive piece at center: nucleus

• Alpha particles are deflected by it if they get close

enough

+

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Rutherford: Explanation

+

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Rutherford’s Contributions

• Discovered the nucleus.• Did not know where to place the electrons• Proposed the NUCLEAR MODEL of the atom also

called the PLANETARY ATOM• Proposed the existence of the neutron• Determined the size of the nucleus (1x10-15 m) and

the atom (1 x 10-11 m for hydrogen atom, larger for other atoms)

• Elucidated the structure of the alpha-particles• Performed the first nuclear reaction

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• At this point in 1909, we know:p+ = 1.67 x 10-24 ge- = 9.11 x 10-28 g The charges are balance!Size of atom: 1x 10-11m (for hydrogen atom, the

smallest in the periodic table)Size of nucleus: 1 x 10-15 m

• But,How are the electrons arranged?There is still mass that is unaccounted for

Discovery of the Nucleus

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Niels Bohr (1913)– Electrons orbit nucleus in predictable paths– Much more on him later

Structure of the Atoms: Niels Bohr

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Chadwick (1935)– Discovers neutron in nucleus– Neutron is neutral– n0

– Mass is 1.67 x 10-24 g

Discovery of the Neutron: Chadwick

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• Charges balanced (protons in nucleus + electrons)

• Mass accounted for (protons + neutrons)

• Placement of electrons: uncertain

• Today, we subscribe to the Quantum Atom Theory to describe the atomic structure

Structure of the Atom: 1913

Quantum Atom Theory

• The atom is mostly empty space

• Two regions:► Nucleus- protons and neutrons► Electron cloud- region where you have a 90%

chance of finding an electron

Quantum Atom

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History of the Atomic Theory: Summary

1803 1897 1909 1913 1935 Today

solid

Particle

electron Proton, nucleus

Random placed

electrons

e- orbit nucleus: Planetary

model, quanta

neutron Quantum Atom theory

Dalton Thomson Rutherford Bohr Chadwick Schrodinger and others

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1. All matter is made up of small particles called atoms.

2. Atoms of the same element have the same chemical properties while elements of different elements have different properties (isotopes)

3. Not all atoms of an element have the same mass, but they all have a definite average mass which is characteristic.(isotopes)

Atomic Theory: Modern View

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4. Atoms of different elements combine to form compounds and each element in the compound loses its characteristic properties.

5. Atoms cannot be subdivided by chemical or physical changes.

(nuclear reactions)

Atomic Theory: Modern View

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Structure of the Atom: Modern View

• There are two regions

• The nucleus– With protons and neutrons

– Positive charge

– Almost all the mass

• Electron cloud- Most of the volume of an atom

– The region where the electron can be found

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Size of an Atom

• Atoms are small.

• Measured in picometers, 10-12 meters• Hydrogen atom, 32 pm radius• Nucleus tiny compared to atom• IF the atom was the size of a stadium, the

nucleus would be the size of a marble.

• Radius of the nucleus near 10-15m.

• Density near 1014 g/cm

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Subatomic Particles: Found in the Nucleus

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

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Structure of the Atom: Basic Definitions

►ATOMIC NUMBER : ►number of protons►# of protons determines kind of atom►the same as the number of electrons in the neutral

atom

►MASS NUMBER : ►the number of protons + neutrons

►NUCLEONS: fundamental particles found in the atom (electrons, neutrons, protons)

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Structure of the Atom: Isotopes

• Dalton was wrong.

• ISOTOPES: Atoms of the same element can have different numbers of neutrons, BUT THE SAME NUMBER OF PROTONS.

• different mass numbers• Same chemical properties, different

physical properties

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Contain the symbol of the element, the mass number and the atomic number

Notation

Nuclear Notation contains the symbol of the element, the mass

number, and the atomic number

Symbols of the Nuclei

XMass

number

Atomic numbe

r

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NotationNuclear Notation in

How many protons? How many neutrons? How many electrons?

Symbols of the Nuclei

Na23

11

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Symbols of the Nuclei

• Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass Number

F19 9

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Symbols of the Nuclei

Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

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• blackboard

Symbols of the Nuclei

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• blackboard

Symbols of the Nuclei: Hyphenation Notation

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Hyphenation Notation– Symbol or name of element – mass number

Fluorine-19Protons? Neutrons? Electrons?

C-12 Protons? Neutrons? Electrons?

Symbols of the Nuclei

Measuring Atomic Mass

• Unit is the Atomic Mass Unit (amu)

• One twelfth the mass of a carbon-12 atom.

• Each isotope has its own atomic mass we need the average from percent abundance.

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Average Atomic Mass

– Weighted average mass of all known isotopes

– Weighted means that the frequency of an isotope is considered

Average Atomic Mass

Average Atomic Mass

• Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.

Average Atomic Mass

• Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium?

• If not told otherwise, the mass of the isotope is the mass number in amu

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Introduction to Periodic Table

Periodic Table:

• an arrangement of elements, in order of increasing atomic number, in horizontal rows of such lengths that elements with similar chemical properties fall directly beneath one another in vertical groups.

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Period Table - Structure

• Structure:

– Group (family) vertical columns. Numbered 1-18

– Period: horizontal rows; numbered from 1 to 7

• Lanthanides

• Actinides

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The Periodic Table of Elements

• 18 columns (group or family) containing elements with similar properties

• 7 rows (periods) containing elements with same number of electron shells

• I abbreviate it…PTOE

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Elements in the Periodic Table

• Metals (about 80 elements): gray, silvery (exception: copper and gold); conduct electricity and heat, malleable, ductile, all solid at room temperature (exception: mercury)

• Nonmetals (about 20 elements): poor conductors of electricity and heat; have different colors; can be gases, liquids, or solids at room temperature; some found pure in nature (sulfur)

• Metalloids: have properties of both metals and nonmetals

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PTOE: Metals

Types– Alkali earth metals (group 1)– Alkaline earth metals (group 2)– Transition metals (group 3-12)– Lanthanides (bottom period 6)– Actinides (bottom period 7)– Others (some in groups 13-16)– You don’t need to memorize these group names

right now

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Properties– Luster– Conductors of heat and electricity– Solids (except Hg)– Malleable– Ductile– Tensile strength

PTOE: Metals

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• Properties (clip)– Vary according to metalloid– All are semi-conductors– B, Si, Ge, As, Sb, Te, Po

PTOE: Metalloids

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• Properties (clip)– Poor conductors– Brittle– Solids: C, P, S, Se, I, At– Liquid: Br– Gases: N, O, F, Cl

PTOE: Non Metals

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Br

I

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• Properties– All are gases– Stable, do not react naturally– Radon, heaviest

PTOE: Noble gases