Atoms - Loudoun County Public Schools

ATOMS Unit 2: History and Structure ATOMIC STRUCTURE SUBATOMIC PARTICLES Particle Charge Electron Negative Proton Neutron

Positive Neutral Location Mass (amu) Outside the 1/1840 nucleus/ele atomic ctron cloud mass unit (negligible) Nucleus 1 amu Nucleus 1 amu

ATOMIC NUMBER the number of protons in an atom This is unique to each element Found at the top of each element cell 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) 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 AVERAGE ATOMIC MASS The mass of an atom considers the abundance of the all naturally occurring isotopes Weighted average How to solve 1. Make a table of values that include A. Isotope B. Mass C. Percent 2. Covert the percent to decimals (divide by 100) 3. Multiple by the mass of the isotope

4. Add the answers together SAMPLE PROBLEM 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 (0.692) (62.93)amu = 43.55

(0.308) (64.93)amu = 19.998 63.55 amu 30.8 % 0.308 64.93 amu 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

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 PARTICLES

TRANSMUTATIONS 23993Np 14) 15) Pu + 239 94 0

1 _________ 238 92 U He + _________ 4 2 235 92

Pu + 240 94 U n + __________ 1 0 236

U 92 1 n + 0 1 0 n 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 HALF LIFE EXAMPLE A rock that originally had a mass of 1.00 gram of uranium-238 now has only 0.50 grams. How old is the rock if the half-life of uranium-238 is 4.5 billions of years. Half life

Time Mass (g) 0 1.00 grams 4.5 billion years 0.50 grams 2 THE ANCIENTS Democritus

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 DALTON

Had 5 principles to his Atomic theory 1. All matter is composed of particles called atoms 2. Atoms of a given elements have the same properties, atoms of different elements have different properties 3. Atom cannot be subdivided, created, or destroyed 4. Atoms of different elements combined in small whole numbers 5. Chemical reactions are when atoms are combined, separated, and rearranged. DISPROVING DALTON Not all of Daltons principles were

correct Atoms can be subdivide in to subatomic particles Neutron Proton Electron Elements can combine with themselves to form diatomic molecules J.J. THOMPSON The discovery of the first subatomic

particle took place in the late 1800s. 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. Electric Current Electric Current cathode ray cathode Cathode

R anode ay OBSERVATIONS AND CONCLUSIONS 1. Cathode rays were deflected by a magnetic field 2. The rays were deflected away from a negatively charged object

Scientists determined the ray was negatively charged. Electron negatively charged Must be a positive particle Mg

+ - + - + + - + + + + + + + + -

Negative particles embedded in a sphere of positive plasma-like matter. THINK Chocolate Chip Cookie 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 OBSERVATION Alpha particles 1. Went straight through 2. Deflected significantly at an angle 3. Bounced back 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 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 BOHR Why dont 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

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 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 10-12 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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