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Unit 2: History and Structure

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1 Unit 2: History and Structure
Atoms Unit 2: History and Structure

2 Atomic Structure

3 Subatomic particles Particle Charge Location Mass (amu) Electron
Negative Outside the nucleus/electron cloud 1/1840 atomic mass unit (negligible) Proton Positive Nucleus 1 amu Neutron Neutral

4 Atomic Number the number of protons in an atom
This is unique to each element Found at the top of each element cell

5 Mass Number proton + neutrons This is NEVER a rounded number
Cannot split neutrons or protons Is not on the period table In isotope notation it is the top number (A)

6 Isotope Same element with DIFFERENT numbers of neutrons
Neutrons = mass number – atomic number N= 1-1 N=0 N= 2 -1 N=1 N= 3 -1 N=2

7 Average atomic Mass The mass of an atom considers the abundance of the all naturally occurring isotopes Weighted average How to solve Make a table of values that include Isotope Mass Percent Covert the percent to decimals (divide by 100) Multiple by the mass of the isotope Add the answers together

8 Sample Problem (0.692) (62.93)amu = 43.55 (0.308) (64.93)amu = 19.998
The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for a mass of 62.93amu and 30.8% for a mass of 64.93amu. Calculate the average atomic mass of copper. Isotope Percent Mass Cu - 63 69.2 % 0.692 62.93 amu Cu – 65 30.8 % 0.308 64.93 amu (0.692) (62.93)amu = 43.55 (0.308) (64.93)amu = 63.55 amu

9 ions Same elements with different number of electrons
Cation: positive ion lower number of electrons than protons Anion: negative ion higher number of electrons than protons

10 Nuclear Decay Isotopes can have an unstable nucleus
Stable nuclei have a 1:1 ratio of protons and neutrons Nucleus will emit different particles until it becomes stable stability can take many transmutations Nuclear reactions During nuclear decay, the identity of the element can change

11 Particles

12 transmutations −1 0 β 92 238 U 92 235 U 23993Np  23994Pu + _________
14) 42He + _________  Pu n n 15) 10n __________  U U U

13 Half Life The amount of time it takes for half the nuclear material within a substance to transmute/ decay Calculated using a time and mass chart Always start with time zero

14 Half Life example A rock that originally had a mass of 1.00 gram of uranium-238 now has only grams. How old is the rock if the half-life of uranium-238 is 4.5 billions of years. Time Mass (g) 1.00 grams 4.5 billion years 0.50 grams Half life 2

15 The Ancients Aristotle Democritus Matter is continuous
All matter was composed of tiny particles Atomos “particles “ were thought to be indivisible Aristotle Matter is continuous Everything in the universe is indefinitely indivisible

16 Dalton Had 5 principles to his Atomic theory
All matter is composed of particles called atoms Atoms of a given elements have the same properties, atoms of different elements have different properties Atom cannot be subdivided, created, or destroyed Atoms of different elements combined in small whole numbers Chemical reactions are when atoms are combined, separated, and rearranged.

17 Disproving Dalton Not all of Dalton’s principles were correct
Atoms can be subdivide in to subatomic particles Neutron Proton Electron Elements can combine with themselves to form diatomic molecules

18 J.J. Thompson The discovery of the first subatomic particle took place in the late 1800’s. A power source was attached to two metal ends of an evacuated glass tube, called a cathode ray tube. A beam of “light” appears between the two electrodes called a cathode ray.

19 Electric Current cathode ray cathode anode Cathode Ray

20 Observations and Conclusions
Scientists determined the ray was negatively charged. Electron negatively charged Must be a positive particle Cathode rays were deflected by a magnetic field The rays were deflected away from a negatively charged object

21 Negative particles embedded in a sphere of positive plasma-like matter.
THINK… Chocolate Chip Cookie Mg + -

22 Ernest Rutherford Gold Foil Experiment
directed a narrow beam of alpha particles at a very thin sheet of gold foil. Alpha particles (a) are He atoms that have been stripped of their electrons

23

24 Observation Alpha particles Went straight through
Deflected significantly at an angle Bounced back

25 Conclusions Scientists determined
Atom is made up of mostly empty space There is a positive particle (like repels like) Proton positively charged There is a dense mass in the center of an atom Nucleus Must be another particle to account for the mass

26 Chadwick In 1932, the English physicist James Chadwick discovered yet another subatomic particle. the neutron is electrically neutral Its mass is nearly equal to the proton Therefore the subatomic particles are the electron, proton, and neutron

27 Bohr Why don’t the electrons crash into the nucleus?
Energy levels –Planetary model Electrons orbit on specific energy levels Electrons can absorb/release a specific amount of energy, quanta, to move up or down an energy level Produces a emission spectra

28 Emission Spectra An electron absorbs a photon of energy and jumps to a higher energy level. Becomes excited moves to an excited state An electron releases a photon in the form of wavelength De-excites back down to it normal energy state Ground state The wavelength is seen as color

29 Forces Strong: works over small distances, so can bring about interactions between particles colliding at high kinetic energy Weak: involved in certain decays and interactions, and its involvement is signaled by interactions over seconds or longer Electromagnetic: acts between all charged particles and is signaled by photon emissions or absorption. It may be involved in the internal re-arrangement of electron within an atom.


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