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Midterm open-ended exam

Study Guide

History of the Atom & Bonding

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Daltons Atomic Model Atom are solid and indivisible

Called the “Billiard Ball Model”

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Thomson’s Model He Found the negatively charged

electron existed in a positive framework.

Electron - Negatively charged particles of the atom. Mass = 9.1 x 10^-28 grams

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J. J. Thomson - English physicist. 1897

Made a piece of equipment called a cathode ray tube It is a vacuum tube - all the air has

been pumped out. A limited amount of other gases are

put in

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Thomson’s Experiment

Voltage source

+-

Metal Disks

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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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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

- By adding an electric field By adding an electric field

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Rutherford’s Atomic Model

Used Alpha particles - positively charged pieces given off by uranium.

Shot them at gold foil which can be made a few atoms thick.

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Rutherford’s model Discovered the nucleus small dense

and positive Electrons

moved around in Electron cloud

Rutherford’s Atomic Model (cont)

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Rutherford’s experiment When the alpha particles hit a

florescent screen, it glows.

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Lead block

Uranium

Gold Foil

Fluorescent Screen

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He Expected The alpha particles to pass through

without changing direction very much. Because…

The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles.

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What he expected

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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

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What he got

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The Rutherford Atomic Model

+

Atom is mostly empty. Small dense,

positive piece at center.

Alpha particles are deflected by

it if they get close enough.

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Rutherford’s experiment (cont.)

+

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Bohr’s Atomic Model Why don’t the electrons fall into the

nucleus? Electrons move like planets around

the sun. In circular orbits at different

levels. Energy separates one level from

another.

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The Bohr Ring Atomn = 3n = 4

n = 2n = 1

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Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

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Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

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Modern View

The atom is mostly empty space.

Two regions. Nucleus- protons

and neutrons. Electron cloud-

region where you might find an electron.

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The Quantum Mechanical Model Energy is quantized. It comes in chunks. Quanta - the amount of energy needed to move

from one energy level to another. Quantum is the leap in energy.

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Does have energy levels for electrons.

Orbits are not circular. It can only tell us the

probability of finding an electron a certain distance from the nucleus.

The Quantum Mechanical Model

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The electron is found inside a blurry “electron cloud”

An area where there is a chance of finding an electron.

Draw a line at 90 %

The Quantum Mechanical Model

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Chapter 8

Ionic Compounds

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8.1 Forming Chemical Bonds The force that holds two atoms together

is called a Chemical Bond. Chemical Bonds (ionic, covalent, metallic) : May form due to the attraction

between “+” charged nucleus and “–” charged electrons, or

May form due to the attraction between a Positive ion and a negative ion.

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Valence Electrons are responsible for the chemical properties of atoms. Located in the outer energy level Involved in the formation of

chemical bonds between two atoms.

Total number of valence electrons = group number. (Na has 1, (Group 1 element)

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Stable Electron Configurations All atoms react so as to achieve a

noble gas configuration of 8 valence electrons.

ls called the octet rule.

Ar

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8.2 IONIC BONDS Ionic Bond-The electrostatic force that holds

oppositely charged particles (anions and cations) together in an ionic compound.

Ionic compounds result when metals react with nonmetals. Ionic compounds are called salts.

Simplest ratio is called the formula unit (Ca3P2).

The ionic bond is formed through the transfer of electrons.

Electrons are transferred to achieve noble gas configuration (8).

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Ionic Bonds: One Big Greedy Thief Dog!

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Properties of Ionic Compounds Crystalline lattice structure.

A 3-dimensional geometric arrangement of particles. Regular repeating arrangement of ions in the solid.

Ions are strongly bonded. Structure is hard and rigid. High melting points and boiling points-

because of strong forces between ions.

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An ionic compound is an electrolyte if it conducts electricity when it is liquid or in aqueous solution.

Lattice energy- The strength of the force holding the ions together in an ionic compound. The more negative the number, the stronger the force.

Lattice energy must be exceeded in order to

break the ionic bond.

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Do they Conduct? Conducting electricity is allowing charges to

move. In a solid, the ions are locked in place. Ionic solids are insulators. When melted, the ions can move around. Melted ionic compounds conduct. First get them to 800ºC. Dissolved in water they conduct.

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Metallic Bonds How atoms are held together in the

solid. Metals hold onto their valence

electrons very weakly. Think of them as positive ions floating

in a sea of electrons.

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Sea of Electrons

+ + + ++ + + +

+ + + +

Electrons are free to move through the solid.

Metals conduct electricity.

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Metals are Malleable Hammered into shape (bend). Ductile - drawn into wires.

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Malleable

+ + + ++ + + +

+ + + +

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Malleable

+ + + +

+ + + ++ + + +

Electrons allow atoms to slide by.

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Ionic solids are brittle

+ - + -+- +-

+ - + -+- +-

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Ionic solids are brittle

+ - + -

+- +-+ - + -

+- +-

Strong Repulsion breaks crystal apart.

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Chapter 9

Covalent Bonding

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9.1 The Covalent Bond Covalent Bond – A chemical bond that

results from the sharing of valence electrons. Between nonmetallic elements of similar

electronegativity. A group of atoms united by a covalent bond

is called a molecule. A substance made up of molecules is called

a molecular substance. Molecules may have as few as two atoms.

(CO), (O2)

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Polar Covalent Bonds: Unevenly matched, but willing to share.

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The principle that describes covalent bonding is the Octet rule.

Atoms within a molecule will share electrons so as to acquire a full set (8) of valence electrons.

If each atom has 7 valence electrons, than they will share one so that they both will have 8. The sharing of that electron is a Single covalent bond.

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Single Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons Both end with full orbitals

F F

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Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons Both end with full orbitals

F F8 Valence electrons

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Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons Both end with full orbitals

F F8 Valence electrons

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If more than two atoms join in a covalent bond the atoms with the fewest valence electrons will pair up with the atom that has the most up to a total of 8.

If a pair of electrons is not shared they are called an unshared pair.

If two atoms share two pairs of electrons it is called a double covalent bond.

If two atoms share three pairs of electrons it is called a triple covalent bond.

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The Lewis structures that you have seen so far use a pair of dots to represent covalent bonds.

Another kind of Lewis structure uses dashes to represent covalent bonds.

A single dash represents a single covalent bond.

A double dash represents a double covalent bond

A triple dash represents a triple covalent bond.

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Properties of Covalent Compounds Made of nonmetals Poor or nonconducting as solid, liquid

or aqueous solution Low melting point