CHEM1 Chapter 5 - Atom

Engr. Yvonne Ligaya F. Musico

Evolution of the Different Atomic TheoriesStructure of Atom and Properties of AtomThe Quantum NumbersThe Electron Configuration of Atoms

Evolution of Different Atomic Theories

The History of the Discovery of the Atom

The Greek Model of the AtomDalton’s Atomic TheoryThomson ModelRutherford ModelBohr ModelModern Atomic Theory

The Greek Model of the Atom

Beginning with Democritus, who lived during the late 5th and early 4th centuries BC,

Greek philosophers developed a theory of matter that was not based on experimental

evidence, but on their attempts to understand the universe in philosophical

terms.

According to this theory, all matter was composed of tiny, indivisible particles called atoms (from the Greek word atomos, meaning “indivisible”).

According to the ancient Greeks, atoms were all made of the same basic material,

but atoms of different elements had different sizes and shapes

The sizes, shapes, and arrangements of a material’s atoms determined the material’s

properties.

Other than the atoms, matter was empty space. Atoms and empty space were

believed to be the ultimate reality.

Dalton’s Atomic Theory

Late 1700’s - John Dalton-EnglandTeacher- summarized results of his

experiments and those of others.

Dalton’s Atomic TheoryCombined ideas of elements with that of

atoms.

All matter is made of tiny indivisible particles called atoms.

Atoms of the same element are identical, those of different atoms 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.

Just How Small an Atom

Think of cutting a piece of lead into smaller and smaller pieces

How far can it be cut?An atom is the smallest particle of an element

that retains the properties of that elementAtoms-very small

still observable with proper instruments

Thomson Model

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.

Thomson’s Experiment

Voltage source

Vacuum tube

Metal Disks

Voltage source

+-Voltage source

Voltage source

Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end

Voltage source

By adding an electric field

Voltage source

By adding an electric fieldBy adding an electric field

Voltage source

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

Other Particles

Proton - positively charged pieces 1840 times heavier than the electron – by E. Goldstein

Neutron - no charge but the same mass as a proton – by J. Chadwick

Rutherford Model

Ernest Rutherford -English physicist. (1910)

Believed in the plum pudding model of the atom

Wanted to see how big they are.

Rutherford’s Experiment

Used radioactivity.Alpha particles - positively charged

pieces- helium atoms minus electronsShot them at gold foil which can be

made a few atoms thick.When an alpha particle hits a

fluorescent screen, it glows.Here’s what it looked like

Lead block

Uranium

Gold Foil

Fluorescent Screen

What he expected??

The alpha particles to pass through without changing direction very much.

Because……

…the positive charges were thought to be spread out evenly. Alone they

were not enough to stop the alpha particles.

What he expected……..

Because…

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

How he explained it!

Atom is mostly empty.Small dense, positive piece

at center.Alpha particles

are deflected by

it if they get close enough.

Since most of the particles went through, it was mostly empty space.

Because the pieces turned so much, the positive pieces were heavy.

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

nucleus.

Bohr Model

Line-Emission Spectrum

ground state

excited state

ENERGY IN PHOTON OUT

Bohr Model

e- exist only in orbits with specific amounts of energy called energy levels

Therefore…

e- can only gain or lose certain amounts of energy

only certain photons are produced

Bohr’s Model

456Energy of photon

depends on the difference in energy levels

Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom

Other Elements

Each element has a unique bright-line emission spectrum.“Atomic Fingerprint”

HeliumBohr’s calculations only worked for

hydrogen!

Modern Atomic Theory

NUCLEUS ELECTRON

(-) CHARGEPROTONS NEUTRONS

(+) CHARGE NEUTRAL

QUARKSLEPTONS BOSONS

Most of the Atom’s MassAtomic number equals the number of….

Equal in a neutral atom

Subatomic Particles

Quarkscomponent of

protons & neutrons

6 types

3 quarks = 1 proton or 1 neutron

Summary of the Model of the Evolution of Atom

Structure and Properties of Atoms

Structure of Atoms

Electrons Negatively chargeMoving around the nucleus

Nucleus Protons – positively chargeNeutrons – neutral

Properties of Subatomic Particles

Name Symbol Charge Relative

Actual

mass (g)

Electron e- -1 1/1840 9.11 x 10-28

Proton p+ +1 1 1.67 x 10-24

Neutron n0 0 1 1.67 x 10-24

Counting the Pieces

Atomic Number number of protons in the nucleusnumber of protons determines kind of atom

(since all protons are alike!)the same as the number of electrons in the

neutral atom.Mass Number

the number of protons + neutrons.These account for most of mass

Number of Electrons

An atom is neutral The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons

Atomic Symbols

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

XNumberMass

NumberAtomic

Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

Atomic Number on the Periodic Table

Atomic Number

Symbol

All atoms of an element have the same number of protons

11 protons

Sodium

Subatomic Particles in Some Atoms

16 31 65 O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Learning Check

State the number of protons for atoms of each of the following:A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons

B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons

C. Barium1) 137 protons 2) 81 protons 3) 56 protons

Solution

State the number of protons for atoms of each of the following:

A. Nitrogen

2) 7 protons

B. Sulfur

2) 16 protons

C. Barium

3) 56 protons

Isotopes

Atoms with the same number of protons, but

different numbers of neutrons.

Atoms of the same element (same atomic number)

with different mass numbers

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37

Isotopes

Relative Atomic Mass

12C atom = 1.992 × 10-23 g

atomic mass unit (amu)

1 amu = 1/12 the mass of a 12C atom

1 p = 1.007276 amu1 n = 1.008665 amu1 e- = 0.0005486 amu

Average Atomic Mass

weighted average of all isotopeson the Periodic Tableround to 2 decimal places

(%)(mass(mass)(%) )

Avg.AtomicMass

Atomic Mass of Magnesium

Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

Learning Check

Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

Solution

Avg.AtomicMass

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Learning Check

Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.

Solution

Avg.AtomicMass

(37)(2)(35)(8)35.40 amu

Learning Check

Gallium is a metallic element found in

small lasers used in compact disc players.

In a sample of gallium, there is 60.2% of

gallium-69 (68.9 amu) atoms and 39.8% of

gallium-71 (70.9 amu) atoms. What is the

atomic mass of gallium?

Solution

Avg.AtomicMass

8)(70.9)(39.2)(68.9)(60.69.7 amu

Atomic Mass

Listed on the periodic table

Gives the mass of “average” atom of each element compared

to 12C

Average atom based on all the isotopes and their abundance %

Atomic mass is not a whole number

Atomic mass is the weighted average mass of all the atomic

masses of the isotopes of that atom.

Na22.99

Atomic Mass on Periodic Table

Atomic Number

Symbol

Atomic Mass

Learning Check

Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Solution

Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium _40.1 amu _

B. aluminum _27.0 amu _

C. lead _207.2 amu_

D. barium _137.3 amu_

E. iron _55.8 amu__

Finding an Isotopic Mass

A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?

Assign X and Y values:X = % 10B Y = % 11B

Determine Y in terms of XX + Y = 100Y = 100 - X

Solve for X:X (10.0) + (100 - X )(11.0) = 10.8

100 100

Multiply through by 10010.0 X + 1100 - 11.0X = 1080

Collect X terms

10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B

Y = 100 - X

% 11B = 100 - 20% = 80% 11B

Learning Check

Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check periodic table for atomic mass)

1) 30% 2) 70% 3) 100%

Solution

2) 70%

Solution

62.9X + 6490 = 64.9X = 6350-2.0 X = -140

X = 70%

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