History of the Atomic Model Chapter 4

Sir William Crookes (1879) Invented the cathode ray tube and investigated electrical charges in gases. Postulated a negatively charged particle that was eventually named the electron.

Schematic of a Cathode Ray Tube

Henri Becquerel (1896) Noticed that certain substances emitted radiation without any external energy source. Two types of radiation called alpha (  ) and beta (  ) due to their penetrating power. Third was later added on – gamma (  )

John Joseph (JJ) Thomson (1903) Using the cathode ray tube invented by Crookes, discovered the electron, e. Also calculated the charge to mass ratio of an electron. Q e /m e = x10 11 C/kg Proposed the “plum pudding” model. Postulated positive particles. 0

9.11x kg Robert Millikan (1909) Working with Harvey Fletcher, Determined the charge on a single electron, Q = x C Since Thomson found Q/m, they were able to calculate an electron’s mass Which is 1,836 times lighter than a hydrogen atom. What is an Electron’s Mass?

Millikan’s Oil Drop Experiment

Ernest Rutherford (1909) Discovered the positively charged dense central portion of the atom using his “gold foil experiment”  Nucleus Atoms are mostly empty space! Discovered the proton ( p) in

Rutherford’s experiment

Results of foil experiment if Plum Pudding model had been correct Failure!

Actual Results

An atom viewed in cross section The nucleus is 100 thousand times smaller than the atom! If the nucleus were the size of a marble, the atom would be as big as a football stadium!

Timeline So Far 1803 – Dalton’s Atomic Theory 1879 – Crookes postulates a negatively charged particle  electron – Bequerel discovers radioactive materials – Thompson defines the charge to mass ratio of the electron (thereby “discovering” it) – Millikan discovers the charge (and therefore mass) of the electron 1909 & 1918 – Rutherford discovers that the atom is mostly empty space and eventually the proton.

Problem to Solve If electrons are negatively charged and protons are positively charged, what keeps them separated? Why don’t they simply smash together like magnets? Hang on to your hats, it’s about to get weird!

Updated Atomic Model (post 1909) Discarded Plum Pudding Electrons must orbit around central “nucleus”  Planetary Model Nearly all of the mass is located in the dense, central, positively charged nucleus. Still, why don’t the electrons fall into the nucleus like opposite magnetic poles?

Enter the Dane Niels Bohr (1885 – 1962) Danish physicist Danish physicist Went to Cambridge with the complete works of Dickens Went to Cambridge with the complete works of Dickens (to learn the language, duh!). Worked on the electron/nucleus problem Worked on the electron/nucleus problem Often thought of as the Father of Quantum Mechanics Often thought of as the Father of Quantum Mechanics

Bohr’s Solution (1913) Electrons are located in specific energy levels Electrons move in a definite orbit around the nucleus Areas between orbits are not allowed!

More Problems to Solve Why are electrons limited to specific orbits? Quantum mechanics has all the answers… uh…sort of…maybe…probably…kind of… Quantum mechanics has all the answers… uh…sort of…maybe…probably…kind of… With exception of hydrogen, atoms weigh more than sum of protons and electrons – there must be another particle …

Walther Bothe & Herbert Becker (1928) Aimed alpha radiation at light elements like boron. Found it gave off an extremely penetrating radiation. Thought it produced high energy gamma rays. Bothe

Radiation Comparison Alpha Radiation – positively charged, easily stopped (paper, skin, etc) Beta Radiation – negatively charged, stopped by a sheet of aluminum Gamma radiation – high energy light (no charge), penetrates a lot of material New Radiation - ??? – penetrates even more

Irene Joliot-Curie ( ) Daughter of Marie & Pierre Curie Continued work of Bothe and Becker (1932) Aimed Bothe’s new beam at paraffin Aimed Bothe’s new beam at paraffin Ejected high energy protons Ejected high energy protons Misinterpreted results – cost her a Nobel Prize Misinterpreted results – cost her a Nobel Prize Also found a way to transmute elements! (1934) Inexpensive way to create radioisotopes for medicine Inexpensive way to create radioisotopes for medicine Just like her mother, died from radiation exposure

Did not believe the work of Joliot-Curie She said beam was light waves She said beam was light waves He thought not enough mass He thought not enough mass Discovered neutron in No charge. Mass slightly more than a proton. Jimmy Neutron James Chadwick ( )

Subatomic Particles NameAbbrevLocation Mass (kg) Charge (C) Effective Charge Electron e - or e Orbit 9.019x x Proton p + or p Nucleus 1.673x x Neutron n or n Nucleus 1.675x

Comparison (cont’d) Protons: Heavy, positive charge  repelled by the nucleus. Electrons: Nearly massless, negative charge  repelled by other electrons surrounding an atom. Neutrons: Heavy, no charge  no interactions with nucleus or electrons. Can pass through a lot of material. Can pass through a lot of material. If fast enough, can break nucleus apart! If fast enough, can break nucleus apart!

Atomic Requirements All atoms of a given element must have the same number of protons! 1 Proton ~1 Dalton (1 Da = x kg) 1 Proton ~1 Dalton (1 Da = x kg) In order to be neutral, atoms must have same # of electrons as protons (charges must cancel out to zero) Electrons do not add weight (compared to p + & n) Electrons do not add weight (compared to p + & n) Total weight comes from additional neutrons. 1 Neutron ~ 1 Da 1 Neutron ~ 1 Da

Solving the Mass Problem Total mass is sum of protons, neutrons, and electrons. 1 Atom of Helium has 2 protons, 2 electrons, and 2 neutrons = 4 Da But, it turns out that some atoms of elements weigh more (or less) than the others…

Elements and Neutrons Atoms can have more (or less) neutrons Isotopes are atoms with the same # of protons, but different # of neutrons Since protons determine chemical identity, neutrons just add mass. Isotopes occur in different ratios in nature.

Two isotopes of Sodium.

Specifying Isotopes Two ways to write an isotope: C or simply C C or simply C Carbon-12 Carbon-12 These isotopic symbols tell you how many of each particle is in the isotope. The Mass number (A) is the sum of p + & n The Z number is the number of p + The Charge, Q, is # p + - # e

Uranium-235 Uranium has 92 protons Needs 92 electrons to cancel them out Its mass number is 235 (92 p n) U = 0, so no charge!

Nitrogen-14 ion N Mass Number (A) # p + + # n Z Number # p + (nuclear charge) Charge # p + - # e - Atomic Symbol Nitrogen has 7 protons and 7 neutrons. It tends to gain 3 extra electrons when it forms an ion. Let’s Practice!

Mass of Atoms Mass of an atom is sum of its p +, n, & e -. Subatomic particles are really (really) light. Use the Dalton (Da) to measure the mass – also called the unified mass unit (u) or atomic mass unit (amu) 1 Da = 1/12 th mass of a Carbon-12 atom 1 Da = x kg

Particle Weights Electrons – lightest of subatomic particles Da (usually ignored) Da (usually ignored)Protons Da Da Neutrons – heaviest of subatomic particles Da Da p + & n are usually rounded to 1 Da, while e - are assumed to be 0 Da

Atomic Mass Mass of a specific isotope. Carbon-12 = Da Carbon-12 = Da Carbon-13 = Da Carbon-13 = Da Oxygen-16 = Da Oxygen-16 = Da Must be measured experimentally due to Mass Deficit 6 p n 6 p n 8 p n???

Mass Deficit When subatomic particles combine, their masses change (don’t ask why…) Sometimes, the combination weighs less, other times, it weighs more (depends on numbers of each combining). Gain/lose mass according to Einstein’s famous equation: E = mc 2 Atomic & Hydrogen (Nuclear) bombs release this energy!

Carbon-12 Was chosen as the “standard” atom. All atomic masses are based on this specific isotope. Mass of C-12 defined as exactly Da. Masses of other atoms are relative to C-12.

Atomic Masses of Isotopes Uranium Isotopes Oxygen Isotopes Uranium Isotopes Oxygen Isotopes Isotope Mass (Da) Isotope

Abundance of Isotopes Isotopes naturally occur in different proportions. Oxygen isotopes: Isotope Mass (Da) Abundance O % O % O %

Atomic Weight Different from Atomic Mass Also called “Relative Atomic Mass” Average weight of all naturally occurring isotopes. Provides more accurate weight of a typical sample A weighted average

Calculating Atomic Weight Atomic Mass * Abundance = mass fraction Carbon has 2 stable isotopes: Carbon-12 = 12.00Da – is 98.93% Carbon-12 = 12.00Da – is 98.93% Carbon-13 = 13.00Da – is 1.07% Carbon-13 = 13.00Da – is 1.07% 98.93/100*12.00 Da = Da 1.07/100*13.00 Da = Da 1.07/100*13.00 Da = Da Da Da

Oxygen’s Atomic Wt Da * /100 = Da Da * / 100 = Da Da * / 100 = Da Total = Da That’s the # on the Periodic Table! Isotope Mass (Da) Abundance % % %

Summary 3 subatomic particles electron (-1, nearly massless) electron (-1, nearly massless) proton (+1, ~1 Da) proton (+1, ~1 Da) neutron (0, ~1 Da) neutron (0, ~1 Da) Isotopes – same protons, different # neutrons Written two ways Written two ways Atomic Mass – mass of 1 atom Atomic Weight – average weight of all isotopes