Chapter 4 Atomic Theory

Matter Matter All matter is made of atoms o Alone as elements Au, Na, O, He o In combination of elements as compounds H 2 O, NaCl, LiO 2

Democritus ( B.C.) proposed & believed that o Matter was not infinitely divisible o Made up of tiny particles called atomos or “indivisible” o Atoms could not be created, destroyed, or further divided o He did not do any experiments that backed up his theory. Early Theories of Matter

John Dalton, 19 th Century School teacher Dalton revised Democritus's ideas based upon the results of scientific research he conducted Dalton’s atomic theory o Not totally correct

Dalton’s First Atomic Theory: o Elements are made up of small indivisible particles called atoms o Atoms of the same element are identical, different elements are different - (Same size, mass and chemical properties) o Atoms are not created or destroyed in a chemical reaction o A compound always has the same relative numbers and kinds of atoms

What is an atom? An atom is the smallest particle of an element that retains the properties of the element

Atomic Models Thomson: Plum Pudding Model Based off experiments using cathode rays

Atomic Models Based off of his Gold foil experiments Rutherford: Electron Cloud Model

Rutherford: The Nuclear Atom His model consisted of the following ideas: o an atom consists mostly of empty space through which electrons move o electrons are held within the atom by their attraction to the positively charged nucleus o small, dense, positive charged nucleus

Bohr Model Based on Rutherford’s model “Planetary” model Adds idea of “quantized” energy levels

Quantum Model Schrodinger (1920’s) came up with the modern atomic model: proposing that electrons travelled in wave-like patterns

Inside an atom: subatomic particles These particles have mass and charge Nucleus holds protons & neutrons o Protons: + charge o Neutrons: Ø charge Outside of nucleus is the electron cloud o Electron: - charge

Parts of the Atom NameSymbolChargeMassLocation protonp+1 1 amu nucleus neutronn0 nucleus electrone- ~ 0 amu outside nucleus

Atomic Mass Unit (amu) Small mass #’s are not easy to work with, so the atomic mass unit (amu) was developed The mass of 1 amu is nearly equal to the mass of one proton or neutron

Reading the Periodic Table

Atomic Number The number of protons determine the element (ALWAYS!!!) Number of protons are unique to each element Examples: o Carbon (C) has 6 protons Atomic number is 6 o Copper has 29 protons Atomic number is 29

Atomic Number In uncharged atoms, atomic number is also the number of electrons o Why? o If an atom is charged, then it is an ion Uncharged atom: Atomic number = # of protons = # of electrons Charge = # protons - # electrons

Mass Number To find the Mass number # protons + # neutrons = mass number To find # neutrons mass number – proton (or atomic number) = neutrons Mass numbers are always WHOLE #’s!!

Symbols for Atoms X= symbol of element A= mass number Z= number of protons X or X -Can also be written “element - A” -i.e. carbon - 12 AZAZ A

Mg-25 Zn atomic # proton neutron electron mass #

Turn to your partner and summarize… How does the number of protons affect the identity of the atom? How do the number of protons and electrons relate to each other in a neutral atom? What is an ion?

Isotopes and Mass Number C C carbon-12 carbon-13 Isotopes are atoms with the same number of protons but different number of neutrons

Isotopes and Mass Number Example: 3 types of Potassium All 3 types contain 19 protons and __ electrons # of Protons # of NeutronsMass Number

What’s the difference between mass number and average atomic mass(weight)? C carbon-12 But if you look on the periodic table, the number states 12.01… Atomic Mass (atomic weight) – a weighted average of the masses of all of the isotopes of that element. It is not the same as the mass number. 12 6

What’s the difference between mass number and average atomic mass? Mass number- specifically about one isotope; the number of protons + neutrons Average atomic mass- includes the masses of all the different isotopes for that element

Mass of Individual Atoms Average atomic mass: o The weighted average mass of the isotopes of an element o Example: Chlorine Mixture of 75% chlorine-35 and 25% chlorine-37 Atomic mass = (0.75)* (0.25)*37.0 = 35.5 amu

Try this one 3 isotopes of neon: o Ne-20 (90.92%) o Ne-21 (0.25%) o Ne-22 (8.83%) What is the average atomic mass of Ne? (20)*(0.9092) + (21)*(0.0025) + (22)*(0.0883) = amu

c f wl

Energy of light is: - directly related to the frequency of the light - inversely related to the wavelength. - As frequency, energy - As wavelength, energy Energy has units of Joules (J) E = hf h = Planck’s constant = x J. s h E f

What is the energy and wavelength of radiation whose frequency is x s -1 ? 3.00 x10 8 m/s = 4.44 x m x10 18 s -1 E = hf = (6.626 x Js) (6.775 x10 18 s -1 ) = x J

1.What is the wavelength of light that has a frequency of 2.27 x10 17 s -1 ? 2.What is the frequency of light that has a wavelength of 9.13 x m? 3. What is the wavelength of light with a frequency of 4.27 x10 18 s -1 ? 4.Radiation has an energy of 1.23 x J. What is the frequency and wavelength of the radiation?

1. wavelength = 1.32 x10 -9 m 2. frequency = 3.29 x10 17 s wavelength = 7.03 x m 4. frequency = 1.86 x10 17 s -1 wavelength = 1.62 x10 -9 m

Radioactivity – when the nucleus of an atom is unstable causing it to decompose into another nucleus There are three types of radioactive decay: 1.Alpha Decay 2.Beta Decay 3.Gamma Decay

Shielding: Alphas, Betas, Gammas and Neutrons

Alpha Decay An alpha particle (α ) is produced An alpha particle is just a helium nucleus, He

Beta Decay An e- is kicked out of the nucleus (a neutron breaks up into a proton and e - ), e 0

Gamma Decay Only ENERGY is released from the nucleus the nucleus itself does not change, but almost always accompanies alpha and beta decay

Alpha Decay Th  4 2 He Ra Ra  Beta Decay Th  0 -1 e Pa I  Gamma Decay U  4 2 He Th + energy 4 2 He Rn 0 -1 e Xe

What are the products of Po-218 after it undergoes alpha decay followed by a beta decay, followed by beta decay followed by alpha decay? Po  4 2 He Pb Pb  0 -1 e Bi Bi  0 -1 e Po Po  4 2 He Pb

Where Does the Radiation Come From? The radiation you receive can be either Natural or Man- made

Turn to your partner and summarize… List the 3 types of radioactive decay and the particle that is released in each Which type of radioactive decay is most penetrating?

Half-Life (t 1/2 ) - time required for one half of the original sample of nuclei to decay. The half-life of Ra-223 is 12 days. If you start with grams of Ra-223, how much will be left after 36 days? g  g  g  g The half life of Ra-225 is 15 minutes. If you have 10.0 grams now, how much did you start with 60 minutes ago? 10.0 g  20.0 g  40.0 g  80.0 g  160.g

Fission – splitting a nucleus into two or more smaller nuclei 1 n + U  Kr + Ba + 3 n + energy This is what takes place in a nuclear reactor or an atomic bomb

F ission chain reaction - self-propagating reaction

Fusion – combining of two nuclei to form one nucleus of larger mass H + H  He + 1n + energy This is how all of the elements were created in nature and occurs naturally in stars