Atoms, Molecules and Ions Chapter 2

Foundations of Atomic Theory Law of conservation of mass: Antoine Lavoisier Mass is neither created nor destroyed. The total mass of a compound must be the same as the total mass of individual elements. HgO  Hg + O 433.2 g 401.2g + 32g

Dalton’s Atomic Theory (1808) Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

JJ Thomson’s Cathode Ray Tube Negatively Charged Electrode Positively Charged Electrode

Cathode Ray Tube Scientists studied the flow of electric current in a glass vacuum tube with electrodes at each end. The electrode is named by what type of particle it attracts Cathode: Negative (-) Anode: Positive (+) When connected to electric current the remaining gas glowed forming a BEAM OF LIGHT. The beam always originated at the NEGATIVE electrode and toward the POSITIVE electrode.

JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED THE ELECTRON! 1897 JJ Thomson used magnets to deflect the beam proving that particles had a NEGATIVE CHARGE. JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED THE ELECTRON!

Now with the knowledge of electrons, and knowing the atom is neutral, there must be a particle that is positive to balance the negative charge. A small paddle wheel was placed inside and it rolled toward the anode, providing evidence that some PARTICLE MUST BE STRIKING THE WHEEL to make it move.

CRT Video

Plum Pudding Model

Plum Pudding OR Chocolate Chip Cookie

Robert A. Millikan Performed the Oil Drop Experiment Determined the exact charge of an electron

Thomson’s charge/mass of e- = -1.76 x 108 C/g Measured Charge of e- (1923 Nobel Prize in Physics) e- charge = -1.60 x 10-19 C Thomson’s charge/mass of e- = -1.76 x 108 C/g e- mass = 9.10 x 10-28 g

Oil Drop Experiment Video

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.

(Uranium compound)

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 Rutherford: It’s like shooting a cannon at a piece of tissue paper and having it bounce back at you! MOST particles went through the gold foil SOME particles BOUNCED back

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

Gold Foil Experiment Video

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

**DISCOVERED THE PROTON!! Goldstein and Wien - 1886 Cathode Ray Tube with perforated cathode 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

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 X A Z Mass Number Element Symbol Atomic Number Isotopes: atoms of the same element (X) with different numbers of neutrons in their nuclei H 1 H (D) 2 H (T) 3 U 235 92 238

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

Period Group

diatomic molecule: contains only two atoms Molecule: an aggregate of two or more atoms in a definite arrangement held together by chemical bonds H2 H2O NH3 CH4 diatomic molecule: contains only two atoms H2, N2, O2, Br2, HCl, CO polyatomic molecule: contains more than two atoms O3, H2O, NH3, CH4

cation – ion with a positive charge An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl- 17 protons 18 electrons Cl 17 protons 17 electrons

monatomic ion: contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- polyatomic ion: contains more than one atom OH-, CN-, NH4+, NO3-

Do You Understand Ions? Al How many protons and electrons are in ? 27 13 ? 3+ 13 protons, 14 neutrons, 10 (13 – 3) electrons How many protons and electrons are in Se 78 34 2- ? 34 protons, 44 neutrons, 36 (34 + 2) electrons

Relative Atomic Mass Atomic Mass Unit One atom is the standard – Carbon Mass of other elements are based off of the standard Carbon: 6 p and 6 n = 12 amu Atomic Mass Unit 1/12 mass of Carbon atom Periodic table lists weighted average atomic masses of elements (like a GPA calculation)

Calculation AVERAGE Atomic Mass 75% 133Cs 20% 132Cs ??% 134Cs Steps: 1. Percent to decimal 2. Multiply by mass 3. Add it up!