History of the Atom

Chapter 3

History of Atom Part 1

Dalton’s Atomic Theory (1808)

1. All matter is made of indivisible and indestructible atoms.

2. All atoms of the same element are identical in their physical and chemical properties.

3. Atoms of different elements have different properties.

4. Atoms of different elements combine in simple whole-number ratios to form compounds.

5. Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions.

Changes to Dalton’s Theory

Are atoms actually indivisible?

Are all atoms identical?

The Atom

the smallest unit of an element that retains that elements properties

Made of protons, neutrons, and electrons Can you see inside an atom?

How did they know it contains protons, neutrons and electrons?

Obscertainer Activity

History of Atom Part 2

JJ Thomson’s Cathode Ray Tube

Cathode:Negatively Charged

Anode:Positively Charged

Cathode Ray Tube

When connected to electric current the remaining the gas forms a BEAM OF LIGHT.

The beam always started at the NEGATIVE electrode and flowed to the POSITIVE electrode.

The electrode is named by what type of particle it attracts Cathode: Negative (-) Anode: Positive (+)

Cathode Ray Conclusions

1. The beam was attracted to a POSITIVE magnet.

2. The beam must be made of NEGATIVE particles.

JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED

THE:

ELECTRON!

Plum Pudding Model

Plum Pudding OR Chocolate Chip Cookie

Radioactivity

Becquerel discovered RADIATION

uranium would expose photographic plates in the dark

The properties of an element changed as it gave off radiation

Curie Discovered radium and polonium

The radioactive emissions of alpha, beta and gamma rays were identified.

Types of Radiation

Radiation Type

Greek Symbol Charge

Alpha

Beta

Gamma

Ernest Rutherford’s Gold Foil ExperimentSet up Gold Foil with a detection sheet around it.

Set up radioactive source emitting alpha particles.

ALPHA PARTICLES shot at gold foil.

MOST particles went through the gold foil

SOME particles BOUNCED back

Rutherford’s Gold Foil Experiment

alpha particles shot through the gold foilbouncing back – like hitting the target

going through – missing the target

Gold Foil Conclusions

1. The atom is made up of mostly EMPTY SPACE

2. The center of the atom contains a POSITIVE CHARGE

3. Rutherford called this positive bundle of matter the NUCLEUS

Rutherford’s Model of the Atom

–

+ NUCLEUS

EMPTY SPACE

History of Atom Part 3

Goldstein and Wien - 1886

Used a cathode ray tube Discovered collection of positively

charged particles

**DISCOVERED THE PROTON!!

Chadwick’s Experiment - 1932

Found that alpha particles shot at beryllium made a beam form

The beam had the same mass of a proton but was electrically neutral

**DISCOVERED THE NEUTRON!!

Niels Bohr – 1913

Developed a new diagram of the atom Electrons can only be at certain energies Electrons must gain a specific amount of

energy to move to a higher level, called a quantum

**DISCOVERED ENERGY LEVELS!!

Bohr’s Model of the Atom

+ NUCLEUS

ENERGYLEVELS

–

Bohr Models

Energy Level(n)

# of Electrons

1

2

3

P+

no

n=3n=2n=1

Nucleus = correct number of protons & neutrons

How to Fill in Bohr Models

Element Symbols

Element Symbols

Rule 1 First letter is capitalizedH Hydrogen C Carbon O Oxygen

Rule 2 Second letter is lower caseCl Chlorine He Helium Ne Neon

Rule 3 Some are Latin namesNa Sodium (natrium) Fe Iron (ferrum)

Rule 4 Some elements are named for places, scientists,

greek gods…Eu Europium Am Americium

Hyphen Notation

The mass number is written with a hyphen after the name of the element.

Element – Mass#Uranium – 235Calcium – 40

Nuclear Symbol

The superscript indicates the mass number and the subscript indicates the atomic number.

C12

6

XAZ

Mass Number

Atomic NumberElement Symbol

Subatomic Particles

Subatomic Particles

PARTICLE SYMBOL CHARGE LOCATION RELATIVE MASS

PROTON

NEUTRON

ELECTRON

ALL ATOMS ARE ELECTRICALLY NEUTRAL

Must have same number of protons and

electrons

Atomic Number: number of protons

NUMBER OF PROTONS NEVER CHANGES!

Every atom of an element has the same

number of protons which makes the element

unique

Examples

Lithium: atomic number of 3How many protons?

Hydrogen: atomic number of 1How many protons?If you add another proton, does it become positive?

6.94

3

Atomic Number

Li

Mass Number

Protons + NeutronsThis is not the actual mass of the atomMasses on the periodic table are NOT

whole numbers

Examples: Silicon 14 protons 14 neutrons

Mass Number = ______

Fluorine 9 protons ____ neutronsMass number = 19

ELEMENT SYMBOL ATOMIC NUMBER PROTONS NEUTRONS ELECTRONS MASS

NUMBER

Nitrogen 15

Cu 35

38 88

12 12

30 26

Isotopes and Ions

Isotopes

Atoms with the same number of protons and different number of neutrons

Hydrogen – 1 Hydrogen

Hydrogen – 2 Deuterium

Hydrogen – 3 Tritium

Hydrogen has 3 isotopes:

1 p 0 n 1 p 1 n 1 p 2 n

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

ION : an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargewhen an atom loses an electron

anion – ion with a negative chargewhen an atom gains an electron

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

How many protons and electrons are in Al2713 ?3+

How many protons and electrons are in Se7834

2- ?

Do You Understand Ions?

Summary

Particle ProtonsNeutron

sElectro

ns

Atom Same Same Same

Isotope SameDIFFERE

NTSame

Ion Same SameDIFFERE

NT

NEVER CHANGES!!

!

Average Atomic Mass

Atomic Mass Unit

Carbon is the standard for all masses on the periodic table.

Carbon: 6 p and 6 n = 12 amu

Periodic table lists weighted average atomic masses of elements (like a GPA calculation)

Relative Atomic Mass

Final Grade Calculation

Calculation AVERAGE Atomic Mass

Steps:1. Percent to a

decimal

2. Multiply by mass

3. ADD IT UP!

75% 133Cs20% 132Cs5 % 134Cs