History of the Atom

Chapter 3

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

Period

Group

Foundations of Atomic TheoryLaw of conservation of mass: Antoine Lavoisier

Mass is neither created nor destroyed.

HgO Hg + O433.2 g 401.2g + 32g

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

Ob-scertainer Solutions

#1 #2 #3 #4

#5 #6 #7 #8

#9 #10 #11 #12

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 Tube Experiment

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.

(Uranium compound)

Types of Radioactive Decay

Decay Type

Symbol

Charge

Reaction

Particle

Strength

Can be stopped

by…

Alpha

Beta

Gamma

Radioactive Decay

Ernest Rutherford’s Gold Foil Experiment

Set up Gold Foil with a detection sheet around it.

Set up radioactive source emitting alpha particles.

ALPHA PARTICLES shot at gold foil.

Rutherford’s Gold Foil Experiment

MOST particles went through the gold foil

SOME particles BOUNCED back

Rutherford: It’s like shooting a cannon at a piece of tissue paper and having it bounce back at you!

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

Other scientists later confirmed that the nucleus

was made up of protons and neutrons

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

SUBATOMIC PARTICLES

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Isotopes: atoms of the same element (X) with different numbers of neutrons in their nuclei

Isotopes

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?

Hyphen Notation

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

Element – Mass#Uranium – 235Calcium – 40

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

13 protons, 14 neutrons, 10 (13 – 3) electrons

34 protons, 44 neutrons, 36 (34 + 2) electrons

Do You Understand Ions?

How many protons and electrons are in Al2713 ?3+

How many protons and electrons are in Se7834

2- ?

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 or final grade calculation)

Relative Atomic Mass

Calculation AVERAGE Atomic Mass

Steps:1. Percent to a

decimal

2. Multiply by mass

3. ADD IT UP!

75% 133Cs20% 132Cs5 % 134Cs