Chapter 2Chapter 2

Atoms, Molecules, and Ions Atoms, Molecules, and Ions

LawsLaws Conservation of MassConservation of Mass Law of Definite Proportion-Law of Definite Proportion- compounds have a constant compounds have a constant

composition.composition.

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

They react in specific ratios by mass.They react in specific ratios by mass. Multiple Proportions-Multiple Proportions- When two elements form more than When two elements form more than

one compound, the ratios of the masses of the second one compound, the ratios of the masses of the second element that combine with one gram of the first can be element that combine with one gram of the first can be reduced to small whole numbers.reduced to small whole numbers. The ratio of the masses of oxygen in HThe ratio of the masses of oxygen in H22O and HO and H22OO22 will will

be a small whole number (“2”).be a small whole number (“2”).

Dalton’s Atomic TheoryDalton’s Atomic Theory1)1) Elements are made up of atomsElements are made up of atoms

2)2) Atoms of each element are identical. Atoms of each element are identical. Atoms of different elements are different.Atoms of different elements are different.

3)3) Compounds are formed when atoms Compounds are formed when atoms combine. Each compound has a specific combine. Each compound has a specific number and kinds of atom.number and kinds of atom.

4)4) Chemical reactions are rearrangement of Chemical reactions are rearrangement of atoms. Atoms are not created or atoms. Atoms are not created or destroyed.destroyed.

Gay-Lussac- under the same conditions of Gay-Lussac- under the same conditions of temperature and pressure, compounds temperature and pressure, compounds always react in whole number ratios by always react in whole number ratios by volume.volume.

Avagadro- interpreted that to mean at the Avagadro- interpreted that to mean at the same temperature and pressure, equal same temperature and pressure, equal volumes of gas contain the same number of volumes of gas contain the same number of particles.particles.

(called (called Avagadro’s HypothesisAvagadro’s Hypothesis))

A Helpful ObservationA Helpful Observation

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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.positive end.

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

Voltage source

Thomson’s ExperimentThomson’s Experiment

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

+

-

Thomson’s ModelThomson’s Model Found the electron.Found the electron. Couldn’t find positive Couldn’t find positive

(for a while). (for a while). Said the atom was like Said the atom was like

plum pudding.plum pudding. A bunch of positive A bunch of positive

stuff, with the stuff, with the electrons able to be electrons able to be removed. removed.

Millikan’s ExperimentMillikan’s Experiment

Oil

Atomizer

Oil droplets

Telescope

-

+

Millikan’s ExperimentMillikan’s Experiment

X-rays

X-rays give some electrons a charge.

Millikan’s ExperimentMillikan’s Experiment

Some drops would hover

From the mass of the drop and the charge on the plates, he calculated the mass of an electron

RadioactivityRadioactivity Discovered by accidentDiscovered by accident BequerelBequerel Three types Three types

– alpha- helium nucleus (+2 charge, large alpha- helium nucleus (+2 charge, large mass)mass)

– beta- high speed electronbeta- high speed electron

– gamma- high energy lightgamma- high energy light

Rutherford’s ExperimentRutherford’s Experiment Used uranium to produce alpha particles.Used uranium to produce alpha particles. Aimed alpha particles at gold foil by Aimed alpha particles at gold foil by

drilling hole in lead block.drilling hole in lead block. Since the mass is evenly distributed in Since the mass is evenly distributed in

gold atoms alpha particles should go gold atoms alpha particles should go straight through.straight through.

Used gold foil because it could be made Used gold foil because it could be made atoms thin.atoms thin.

Lead block

Uranium

Gold Foil

Florescent Screen

What he expected

Because

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

What he got

How he explained it

+

Atom is mostly emptyAtom is mostly empty Small dense,Small dense,

positive piecepositive pieceat center.at center.

Alpha particlesAlpha particlesare deflected byare deflected by it if it if they get closethey get close enough. enough.

+

Modern ViewModern View The atom is mostly empty space.The atom is mostly empty space. Two regionsTwo regions Nucleus- protons and neutrons.Nucleus- protons and neutrons. Electron cloud- region where you might Electron cloud- region where you might

find an electron.find an electron.

The Mass and Change of the The Mass and Change of the Electron, Proton, and NeutronElectron, Proton, and Neutron

Particle Mass (kg) Charge

Electron 9.11 10 31 1

Proton 1.67 10 27 1+

Neutron 1.67 10 27 0

Sub-atomic ParticlesSub-atomic Particles Z - atomic number = number of protons Z - atomic number = number of protons

determines type of atom.determines type of atom. A - mass number = number of protons + A - mass number = number of protons +

neutrons.neutrons. Number of protons = number of electrons if Number of protons = number of electrons if

neutral.neutral.

SymbolsSymbols

XA

Z

Na23

11

Covalent Chemical BondsCovalent Chemical Bonds The forces that hold atoms together are The forces that hold atoms together are

called chemical bonds.called chemical bonds. Covalent bonding - sharing electrons.Covalent bonding - sharing electrons. Makes molecules.Makes molecules. Chemical formula- the number and type of Chemical formula- the number and type of

atoms in a molecule. atoms in a molecule. CC22HH66 - 2 carbon atoms, 6 hydrogen atoms, - 2 carbon atoms, 6 hydrogen atoms, Structural formula shows the connections, Structural formula shows the connections,

but not necessarily the shape.but not necessarily the shape.

Ionic BondingIonic BondingCationCation: A positive ion: A positive ion

MgMg2+2+, NH, NH44++

AnionAnion: A negative ion: A negative ion

ClCl, SO, SO4422

Ionic BondingIonic Bonding: Force of attraction between : Force of attraction between oppositely charged ions.oppositely charged ions.

HydratesHydrates Some salts trap water crystals when they Some salts trap water crystals when they

form crystals.form crystals. These are hydrates.These are hydrates. Both the name and the formula needs to Both the name and the formula needs to

indicate how many water molecules are indicate how many water molecules are trapped.trapped.

In the name we add the word hydrate with a In the name we add the word hydrate with a prefix that tells us how many water prefix that tells us how many water molecules. molecules.

HydratesHydrates In the formula you put a dot and then write In the formula you put a dot and then write

the number of molecules.the number of molecules. Calcium chloride dihydrate = CaClCalcium chloride dihydrate = CaCl2222 Chromium (III) nitrate hexahydrate = Chromium (III) nitrate hexahydrate =

Cr(NOCr(NO33))33 6H 6H22O O