Unit 4 Atomic Theory Structure of the Atom

Unit 4: Atomic Theory Structure of the Atom (& Radioactivity)

Early Atomic Theories

Early Atomic Theories p Atomists and Democritus n Greeks approx. 2, 500 years ago n Matter was made up of atoms “atomos” or “Indivisible” particles n Seashell experiment—broken into smaller & smaller pieces

Early Atomic Theories p John Dalton n 1766 -1844; returned to theory of atoms Atoms are like billiard balls (solid spheres) which can’t be broken down further 4 major postulates 1. 2. 3. 4. All elements are composed of atoms Atoms of the same element are identical Atoms can physically mix or chemically combine in simple whole number ratios Reactions occur when atoms separate, join, or rearrange

Dalton’s Model of the Atom No subatomic particles!

Early Atomic Theories p William Crookes n Developed Crookes tube (a. k. a cathode ray tube) in 1870’s n First evidence for existence of electrons because you could “see” electrons flow and confirm their existence. n Tube is precursor to today’s TV picture tubes

Building the Atom – The Electron p J. J. Thomson n Discovered electron in 1897 n Discovered positively charged particles surrounded by electrons n Found the ratio of the charge of an electron to its mass to be 1/1837

Thomson’s Cathode Ray Tube Experiment p J. J. Thomson Video: Cathode Ray Tube Demo

Building the Atom – The Electron p J. J. Thomson n n Cathode ray tube experiments – advancement of Crookes tube “plum-pudding model”

Thomson’s Model of the Atom (Plum Pudding Model)

Millikan’s Oil Drop Experiment p Robert Millikan n Oil drop experiment n Determined the charge and mass of an electron Video: Millikan's Oil Drop Experiment

Building the Atom – The Nucleus p Ernest Rutherford n Discovered nucleus (dense core of atom) in 1911 n Famous Gold foil experiment n Quote from E. R. ’s Lab Notebook p “It is about as incredible as if you had fired a 15 -inch shell at a piece of tissue paper and it came back and hit you. ” -ER

Rutherford’s Gold Foil Experiment Video Clip: Rutherford Gold Foil Experiment

Rutherford’s Model of the Atom

Building the Atom – The Neutron p James Chadwick n Discovered the neutron (no charge, but same mass as proton) n Neutrons help disperse the strong repulsion of positive charges § Nucleus diameter = 10 -5 nm § Atom diameter = 10 -1 nm § If Nucleus = basketball --> then Atom = 6 miles wide!

Building the Atom p Niels Bohr n Improved on Rutherford’s work n “Planetary model”- positive center is surrounded by electrons in defined orbits circling the center

Bohr Model of the Atom (Planetary Model)

Bohr Model of the Atom Vocab. p Energy level – the location where an electron is found at a set distance from the nucleus dependent on the amount of energy it has p Ground state – the typical energy level where an electron is found; lowest energy p Excited state – an energy level higher than the ground state for an electron; temporary condition

Schrödinger Model (Quantum Mechanical Model) p Quantum Mechanical Model n n n Erwin Schrödinger; Mathematical model Electron locations are based on probability Electrons are not particles, but waves! p Interactive Simulation: try it! Defined: n Orbital – region where an electron is likely to be found 90% of the time

Atom (Quantum Mechanical Model)

Atomic Theory p Atom – the smallest particle of matter that retains its properties. n Smallest individual unit of an element p One atom of hydrogen is different from one atom of carbon. p Subatomic particles – the component parts of an atom: proton, neutron, and electron

Atomic Theory p Ion - atom with the same number of protons but a different number of electrons i. e. an atom with a charge! n If the atom has a (+) charge it has more protons than electrons. n If the atom has a (-) charge it has more electrons than protons.

Subatomic Particles Subatomic Particle Mass and Abbreviation Proton p+ Mass =1 amu +1 Nucleus ---- Neutron n Mass =1 amu 0 Nucleus Chadwick in 1932 -1 Electron cloud (outside nucleus) Thomson in 1897 Electron e. Mass ≈ zero amu Charge Location Discoverer

Atomic Symbols Atomic symbol – the letter or letters that represent an element. 13 Al 26. 981 Atomic number Atom symbol Atomic mass or weight

Atomic Number Atomic number = the number of protons in the nucleus. (same for every atom of that element) 13 Al Atomic number Atom symbol 26. 981 Atomic mass or weight

Mass Number = # protons + # neutrons p. A Boron atom can have: 5 p + 5 n = 10 amu Mass number Atomic number Named as boron-10

Calculations w/ Subatomic Particles p Atomic number = # of protons p Mass number = # of protons + # of neutrons p (For a neutral atom): # of protons = # of electrons p (For a charged ion): Charge = #p+ - #e-

Isotope Notation p Isotope (Isotopic Notation) Mass # Atomic Symbol Example: Uranium-238 Z A X

Example p

Example p

Sample Problem Write the atomic symbols for the following: p The isotope of carbon with a mass of 13 p The nuclear symbol when A = 92 and the number of neutrons = 146. 31

Isotopes p Isotope – atoms of the same element with different numbers of neutrons (different mass numbers) n p Example: Carbon-12 Carbon-14 Atomic mass – weighted average of the masses of all the isotopes of an element

Atomic Mass The weighted average is the addition of the contributions from each isotope. Isotopic Abundance is the percent or fraction of each isotope found in nature. 33

Most Abundant Isotope Usually can round atomic mass on the periodic table to nearest whole number (but not always!!) 13 Al 26. 981 Atomic number Atom symbol Atomic mass or weight 34

Example: Determine the average atomic mass of magnesium which has three isotopes with the following masses: 23. 98 amu (78. 6%), 24. 98 amu (10. 1%), 25. 98 amu (11. 3%). 1) Multiply the mass number of the isotope by the decimal value of the percent for that isotope. 2) Add the relative masses of all of the isotopes to get the atomic mass of the element. 35

Example: Determine the average atomic mass of magnesium which has three isotopes with the following masses: 23. 98 amu (78. 6%), 24. 98 amu (10. 1%), 25. 98 amu (11. 3%). 36

Now You Try! If 90% of the beryllium in the world has a mass number of 9 and only 10% has a mass number of 10, what is the atomic mass of beryllium?

Radioactivity - Vocabulary p Radioactivity - the spontaneous emission of radiation from a substance p Radiation - rays and/or particles emitted from radioactive material p Nuclear reactions - reactions involving changes in an atom’s nucleus

Radioactivity p Radioactive n isotopes are unstable These isotopes decay over time by emitting particles and are transformed into other elements p Particles emitted: Alpha (α) particles: helium nuclei n Beta (β) particles: High speed electrons n Gamma (γ) rays: high energy light n 39

Types of Radiation – α particles p Alpha radiation - stream of high energy alpha particles n Alpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus. n Symbol: n Not much penetrating power, travel a few centimeters, stopped by paper, no health hazard 4 He 2+ 2

Types of Radiation – β particles p Beta radiation - High speed electrons To form beta radiation a neutron splits into a proton and an electron n The proton stays in nucleus and the electron propels out at high speed. n p Symbol: 0 e-1 0β -1 p 100 times more penetrating then alpha radiation, pass through clothing to damage skin

Types of Radiation – Radiation p Gamma radiation n n Similar to X rays Doesn’t consist of particles (instead, high energy light) p Symbol: 0 0 n Penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangerous

End of Unit 4!