Chapter 2Chapter 2

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

Early Atomic Theory HistoryEarly Atomic Theory History Democritus – “Atomos” (has idea of atoms)Democritus – “Atomos” (has idea of atoms) John Dalton & His LawsJohn Dalton & His Laws J.J. Thompson – Discovered electronJ.J. Thompson – Discovered electron Rutherford – Discovered Nucleus in atomRutherford – Discovered Nucleus in atom

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.

Dalton’s LawsDalton’s Laws Conservation of MassConservation of Mass – Matter cannot be – Matter cannot be

created or destroyedcreated or destroyed Law of Definite Proportion-Law of Definite Proportion- compounds compounds

have a constant composition.have a constant composition. The react in specific ratios by mass.The react in specific ratios by mass. Water always has 2 hydrogen atoms and 1 Water always has 2 hydrogen atoms and 1

oxygen atom.oxygen atom.

Law of Multiple Proportions-Law of Multiple Proportions- When two elements form more than one When two elements form more than one

compound, compound, the ratios of the masses of the the ratios of the masses of the second elementsecond element that combine with one that combine with one gram of the first can be reduced to small gram of the first can be reduced to small whole numbers.whole numbers.

What?!What?! Water (HWater (H22O)has 8 g of oxygen per 1.0 g of O)has 8 g of oxygen per 1.0 g of

hydrogen.hydrogen. Hydrogen peroxide (HHydrogen peroxide (H22OO22) has 16 g of ) has 16 g of

oxygen per 1.0 g of hydrogen.oxygen per 1.0 g of hydrogen. 16/8 = 2/116/8 = 2/1 The ratios of the masses of oxygen is small The ratios of the masses of oxygen is small

whole number.whole number.

Example 2:Example 2: Sulfur and oxygen react to form both SOSulfur and oxygen react to form both SO22

and SOand SO33. In SO. In SO22 there are 32.02g of sulfur there are 32.02g of sulfur and 32.00g of oxygen. In SOand 32.00g of oxygen. In SO33, 32.06g of , 32.06g of sulfur are combined with 48.00grams of sulfur are combined with 48.00grams of oxygen.oxygen.

What is the ratio of the weights of oxygen What is the ratio of the weights of oxygen that combine with 32.06g of sulfur?that combine with 32.06g of sulfur?

b) How does this illustrate the law of b) How does this illustrate the law of multiple proportions?multiple proportions?

SOSO22 32.00 g Oxygen = 0.999g O32.00 g Oxygen = 0.999g O32.02 g Sulfur32.02 g Sulfur

SOSO33 48.00 g Oxygen = 1.497 g48.00 g Oxygen = 1.497 g

32.06 g Sulfur32.06 g Sulfur

Shows law of multiple proportions because Shows law of multiple proportions because ratio of 1.497g/0.999g = 1.5 ratioratio of 1.497g/0.999g = 1.5 ratio

Which is 3/2 ratio!Which is 3/2 ratio!

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 +-

J. J. Thomson- used Cathode ray tubesJ. J. Thomson- used Cathode ray tubes

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

+

-

Thomsom’s ModelThomsom’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 ExperimentSome drops would hover

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

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 The atom is mostly

empty space.empty space. Two regionsTwo regions Nucleus- protons Nucleus- protons

and neutrons.and neutrons. Electron cloud- Electron cloud-

region where you region where you might find an might find an electron.electron.

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

Using Atomic Number and Mass Using Atomic Number and Mass Numbers to determine the Numbers to determine the

number of protons, neutrons number of protons, neutrons and electronsand electrons

19 K39Mass #

Atomic #

Element Symbol

Number of Protons = Atomic number = 19 protons

Number of Neutrons = Mass # - Atomic #

= 39 – 19 = 20 neutrons

Number of electrons = # protons = 19 electrons

(In a neutral atom!)

How Can Atoms of the Same How Can Atoms of the Same Element be Different?Element be Different?

Isotopes — Atoms of the same element that have different number of neutrons

Since neutrons do not have a charge, adding more does not affect the atom.

This only adds to the mass of the atom (increases mass number).

Each particle in the nucleus has a mass of one. That means . . . The number of particles in the nucleus = Mass Number

The number of electrons will still be the same as the protons.

IonsIons — Atoms of the same element that have — Atoms of the same element that have a different number of electrons than protonsa different number of electrons than protons ..

•Having more or less electrons creates an imbalance in the charge of the atom.

•The number in the charge is how many more or less electrons there are.

•If there are more protons than electrons, then the charge is positive. (Loss of electrons)•If there are more electrons than protons, the charge is negative. (Gain of electrons)

IonsIons

8 O16 -2

Charge

Protons

Neutrons

Electrons

8

810

(-2 )charge means that there was a gain of 2 electrons, so you add 2 to the number of protons.

Practice – Finding the number of pPractice – Finding the number of p++, , nnoo and e and e--

1.)

56 Ba142

2.)

13 Al27

+3

3.)

17 Cl

38 -1

4.)

27 Co

60

p+ ___ no___ e-___

p+ ___ no___ e-___

p+ ___ no_____ e-___

p+ ___ no___ e-___

56

86 56

17 21 18

13

14 1027 33 27

* (-) charge means add to p+

* (+) charge means subtract from p+