Chapter 3 History of the Atom

History of Atom Part 1

Dalton’s Atomic Theory (1808) All matter is made of indivisible and indestructible atoms. All atoms of the same element are identical in their physical and chemical properties. Atoms of different elements have different properties. Atoms of different elements combine in simple whole-number ratios to form compounds. 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 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 Curie 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 Experiment Set 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 foil bouncing back – like hitting the target going through – missing the target

Gold Foil Conclusions The atom is made up of mostly EMPTY SPACE The center of the atom contains a POSITIVE CHARGE Rutherford called this positive bundle of matter the NUCLEUS

Rutherford’s Model of the Atom – + NUCLEUS EMPTY SPACE If the atom is the size of Giants Stadium Then the nucleus is a marble on the 50 yard line

History of Atom Part 3

**DISCOVERED THE PROTON!! 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!!

**DISCOVERED ENERGY LEVELS!! 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 – ENERGY LEVELS + NUCLEUS

Bohr Models P+ no n=3 n=2 n=1 Energy Level (n) # of Electrons Nucleus = correct number of protons & neutrons

How to Fill in Bohr Models

Element Symbols

Element Symbols Rule 1 First letter is capitalized H Hydrogen C Carbon O Oxygen Rule 2 Second letter is lower case Cl Chlorine He Helium Ne Neon Rule 3 Some are Latin names Na 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 – 235 Calcium – 40

Nuclear Symbol The superscript indicates the mass number and the subscript indicates the atomic number. X A Z Mass Number Element Symbol Atomic Number C 12 6

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

Atomic Number Examples 3 Li 6.94 Lithium: atomic number of 3 How many protons? Hydrogen: atomic number of 1 If you add another proton, does it become positive?

Mass Number Protons + Neutrons This is not the actual mass of the atom Masses on the periodic table are NOT whole numbers Examples: Silicon 14 protons 14 neutrons Mass Number = ______   Fluorine 9 protons ____ neutrons Mass number = 19

ELEMENT SYMBOL ATOMIC NUMBER PROTONS NEUTRONS ELECTRONS MASS NUMBER Nitrogen 15 Cu 35 38 88 12 30 26

Isotopes and Ions

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

Hydrogen has 3 isotopes: 1 p 0 n 1 p 1 n 1 p 2 n Hydrogen – 1 Hydrogen Hydrogen – 2 Deuterium Hydrogen – 3 Tritium

Do You Understand Isotopes? How many protons, neutrons, and electrons are in C 14 6 ? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are in C 11 6 ? 6 protons, 5 (11 - 6) neutrons, 6 electrons

cation – ion with a positive charge when an atom loses an electron ION : an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge when an atom loses an electron Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge when an atom gains an electron Cl- 17 protons 18 electrons Cl 17 protons 17 electrons

How many protons and electrons are in Do You Understand Ions? How many protons and electrons are in Al 27 13 ? 3+ 13 protons, 10 (13 – 3) electrons How many protons and electrons are in Se 78 34 2- ? 34 protons, 36 (34 + 2) electrons

Summary Particle Protons Neutrons Electrons Atom Same Isotope NEVER CHANGES!!! Summary Particle Protons Neutrons Electrons Atom Same Isotope DIFFERENT Ion

Average Atomic Mass

Relative Atomic Mass Atomic Mass Unit Carbon is the standard for all masses on the periodic table. Carbon: 6 p and 6 n = 12 amu Atomic Mass Unit Periodic table lists weighted average atomic masses of elements (like a GPA calculation)

Final Grade Calculation

Calculation AVERAGE Atomic Mass Steps: 1. Percent to a decimal 2. Multiply by mass 3. ADD IT UP! 75% 133Cs 20% 132Cs 5 % 134Cs