The Atom: SECTION 4.2

Defining the Atom Elements are made of ATOMS Elements are made of ATOMS ATOM: The smallest particle of an element that retains the properties of the element ATOM: The smallest particle of an element that retains the properties of the element

BELIEVING IN THE ATOM For a long time, we had to INFER atoms existence through scientific experiments. For a long time, we had to INFER atoms existence through scientific experiments. Now, we have the technology to SEE atoms Now, we have the technology to SEE atoms SCANNING TUNNEL MICROSCOPE SCANNING TUNNEL MICROSCOPE

How small is an atom? Now I'd like you to picture yourself on the beach. Reach down and pick up a handful of sand. How many grains of sand do you see? Could you even begin to count the number of grains of sand you are holding in that one handful? Imagine picking out one single grain of sand. There are possibly more atoms within that single grain of sand than there are grains of sand on the entire beach. Now I'd like you to picture yourself on the beach. Reach down and pick up a handful of sand. How many grains of sand do you see? Could you even begin to count the number of grains of sand you are holding in that one handful? Imagine picking out one single grain of sand. There are possibly more atoms within that single grain of sand than there are grains of sand on the entire beach. About 5 million atoms could fit in the period at the end of a sentence. About 5 million atoms could fit in the period at the end of a sentence.

Dalton’s Flaws Two of his points were found to be FALSE Two of his points were found to be FALSE Atoms can be broken into subatomic particles Atoms can be broken into subatomic particles Some atoms of the same element can have different properties Some atoms of the same element can have different properties

Electrons Atoms are made up of smaller particles Electron: The negatively charged particles of an atom All atoms have e - (electrons)

Questions…

Cathode Ray Tube Scientists pass electricity through a glass tube with NO AIR inside (vacuum) Scientists pass electricity through a glass tube with NO AIR inside (vacuum) PURPOSE: to study the behavior of electricity without matter. PURPOSE: to study the behavior of electricity without matter. Helped DISCOVER THE ELECTRON Helped DISCOVER THE ELECTRON

Cathode Ray Tube The glass tube is hooked up to a battery. The glass tube is hooked up to a battery. Negative plate (cathode) hooks up to - part of battery Negative plate (cathode) hooks up to - part of battery Positive plate (anode) hooks to + side of battery Positive plate (anode) hooks to + side of battery

Cathode Ray Battery +- Vacuum tube Metal Disks

Sir William Crookes Saw a RAY going through tube Saw a RAY going through tube He observed that it originated from the cathode. He observed that it originated from the cathode. He called it the CATHODE RAY He called it the CATHODE RAY

Passing an electric current makes a beam move from the negative to the positive end Passing an electric current makes a beam move from the negative to the positive end BATTERY -+

Battery Adding charged plates outside the tube caused the beam to be deflected Adding charged plates outside the tube caused the beam to be deflected Why was the beam deflected towards the positive plate? Why was the beam deflected towards the positive plate? - +

WHAT IS THE BEAM?? The CATHODE RAY consists of CHARGED particles The CATHODE RAY consists of CHARGED particles Because they came FROM the negative end (cathode) TO the positive end (anode) MUST BE – Because they came FROM the negative end (cathode) TO the positive end (anode) MUST BE – Because they were deflected TOWARDS + plate, MUST BE – (opposites attract) Because they were deflected TOWARDS + plate, MUST BE – (opposites attract) Negatively charged particles = ELECTRONS ature=related

Oil-drop experiment - - Millikan - - Sprayed oil droplets that fell btw CHARGED plates (- plate is on the bottom) - - Different amounts of charge changed the droplet’s rate of fall - - More charge = falls more slowly (repelled by - plate)

Millikan’s Experiment Determined the charge of an Determined the charge of an electron is -1

Mass of an Electron He used the charge and the charge to mass ratio to mathematically figure out the mass of an electron. He used the charge and the charge to mass ratio to mathematically figure out the mass of an electron. Mass of electron = 9.1 x kg Mass of electron = 9.1 x kg 1840 times SMALLER than the smallest atom 1840 times SMALLER than the smallest atom

Structure of the Atom (a guess) Atoms are NEUTRAL so if they have electrons, they must also have + charge Atoms are NEUTRAL so if they have electrons, they must also have + charge How are electrons arranged in an atom?? How are electrons arranged in an atom??

Thomson’s Model - Thought the atom was arranged like PLUM PUDDING - Electrons spread evenly throughout an atom’s positive interior

QUESTIONS… Draw the Plum Pudding model… Draw the Plum Pudding model… Why is this model of the atom called “plum pudding?” Why is this model of the atom called “plum pudding?”

Rutherford Ernest Rutherford studied alpha particles (+ charged) Ernest Rutherford studied alpha particles (+ charged) He shot a narrow beam of alpha particles at a thin sheet of gold foil He shot a narrow beam of alpha particles at a thin sheet of gold foil

Rutherford Because + and – charge were thought to be evenly distributed, + particles should go straight through foil (not attracted or repelled) Because + and – charge were thought to be evenly distributed, + particles should go straight through foil (not attracted or repelled)

Rutherford’s Experiment

Gold Foil Atoms  Alpha Particle Beam

WHAT HE SAW…Rutherford’s experiment - Most particles passed straight through the foil as if the foil was not there (as expected) - A very small fraction of alpha particles were deflected in ALL directions… some even completely reversed direction

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What this tells Rutherford Alpha (+) particles should not get deflected dramatically by even distribution of charge Alpha (+) particles should not get deflected dramatically by even distribution of charge There must be a dense, massive concentration of positive charge in the center There must be a dense, massive concentration of positive charge in the center

What was happening on an atomic level… +  Positive Center  Alpha beam deflected because positive charges repel

Rutherford’s Conclusion Most of atom is empty space Most of atom is empty space There is a concentrated positive center: NUCLEUS There is a concentrated positive center: NUCLEUS Nucleus is DENSE and has most of atom’s MASS (electrons are MUCH lighter than nucleus) Nucleus is DENSE and has most of atom’s MASS (electrons are MUCH lighter than nucleus)

Rutherford’s model Electrons stay with the atom because they are attracted to the positive center Electrons stay with the atom because they are attracted to the positive center - Electrons orbit the nucleus – similar to planets around the sun

Rutherford eventually concluded… - - Nucleus- - Contains POSIVITELY charged particles called PROTONS and NEUTRALLY charged particles called NEUTRONS

Neutrons Neutrons are the other particle in the nucleus Neutrons are the other particle in the nucleus They have no charge They have no charge Its mass is nearly equal to that of the proton Its mass is nearly equal to that of the proton

Homework… Pg 114 #7, 8, 9 and 10 Pg 114 #7, 8, 9 and 10

4.3 How Atoms Differ

Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the SAME number of protons) - The atomic number = the number of electrons in a neutral atom - Written just above the atomic symbol on the periodic table

Complete the Table: ElementAtomic ## Protons# Electrons 6 N Pb 3 19 Br

ElementAtomic ## Protons# Electrons C666 N777 Pb82 Li333 K19 Br35

Isotopes All atoms in an element have the same number of protons The number of neutrons vary Atoms of an element with different numbers of neutrons = ISOTOPES

Isotopes Elements in nature = a mix of isotopes The abundances of different isotopes are constant EX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons

Mass of atoms One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.atomic mass unit One amu is nearly, but not exactly, equal to one proton or one neutron.

Mass of subatomic particles

Mass Number # PROTONS + # NEUTRONS = MASS NUMBER EXAMPLE: What is the mass number for manganese if there are 30 neutrons in the nucleus?

Element Symbols Top is mass number Top is mass number Bottom is atomic number Bottom is atomic number Practice Practice

Symbols EXAMPLE 1: Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number F 19 9

Answer… protons neutrons electrons - - Atomic # = Mass # = What would the mass number be of a fluorine isotope with two additional neutrons?

Symbols EXAMPLE: Find the –number –number of protons of neutrons of electrons –Atomic –Atomic number –Mass –Mass Number Br 80 35

Answer –Number of protons: 35 –Number of neutrons: 45 –Number of electrons: 35 –Atomic number: 35 –Mass Number: 80

Symbols If an element has an atomic number of 34, a mass number of 78, find… –number of protons –number of neutrons –number of electrons And WRITE the Complete symbol

Answer… protons electrons neutrons - - Symbol: Se 78 34

Symbols n if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol

Pa

Symbols n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol

Pt

Naming Isotopes Put the mass number after the name of the element Put the mass number after the name of the element carbon- 12 carbon- 12 carbon -14 carbon -14 uranium-235 uranium-235

Mass of the atom Atomic mass on the periodic table is NOT an even # The atomic mass of an element is the weighted average mass of the isotopes of that elementatomic mass Depends on abundance of each isotope

Atomic Mass - How heavy is an atom of oxygen? - The MASS NUMBERS we’ve been using are measured in amu - 1 amu (atomic mass units) is ABOUT the mass of 1 proton or 1 neutron (or 1/12 of a carbon -12 atom) - On the periodic table, the MASS of each element is NOT reported as a whole number… - We can’t have HALF a proton or HALF a neutron in an atom… so why aren’t these masses whole numbers??

Atomic Mass - - Chlorine-35 is MORE common in nature than chlorine-37…so the mass of 1000 Cl atoms will be CLOSER to amu - - To take the average of the two isotopes, we need to use the WEIGHTED AVERAGE: - - In nature, 75.53% of Cl atoms are chlorine-35, and % are chlorine- 37… - - (% is called % abundance)

Calculating the WEIGHTED AVERAGE - - Write each percent as a decimal - - Multiply that decimal by the isotope’s mass - - Find the average by adding together Average = % as decimal x mass + % as decimal x mass - Average Mass of a Chlorine atom: (0.7553)(35) + (0.2447)(37) = amu (0.7553)(35) + (0.2447)(37) = amu % abundance isotope 1 Mass isotope 1 % abundance isotope 2 Mass isotope 2

Atomic Mass: EXAMPLE - Calculate the atomic mass of copper if copper has two isotopes. 69.1% have a mass of amu and the rest have a mass of amu…

ANSWER… Mass of Cu = (.691)(62.93) + (.309)(64.93) = amu

Atomic Mass: EXAMPLE 2 - Magnesium has three isotopes % magnesium-24, 10.00% magnesium-25, and the rest is magnesium-26 amu. What is the atomic mass of magnesium?

Answer… Mass of Magnesium = 7899 x 24) + (.1000 x 25) + (.1101 x 26) = ( x 24) + (.1000 x 25) + (.1101 x 26) = amu

4.4 Unstable Nuclei and Radioactive Decay

Radioactive decay In the late 1890s, scientists noticed some substances spontaneously emitted radiation, a process they called radioactivity.radioactivity

Radioactive Decay Nuclear reactions can change one element into another element.

Radioactive decay radiation is rays and particles given off. Reactions in a nucleus is nuclear reactions.

Unstable nuclei Elements that give off radiation are trying to make themselves more stable. They need a form where it does not take effort to exist

Alpha Particles Alpha particles contain 2 p + and 2 n. Alpha radiation is alpha particles being given off.

Alpha radiation The atomic number decreases by 2, and the mass number decreases by 4.

Alpha Equation

Beta Radiation Each beta particle is an electron with a 1– charge.beta particle

Beta Radiation Beta emission converts a neutron to a proton Beta emission converts a neutron to a proton The mass number remains the same, but the atomic number increases by one. The mass number remains the same, but the atomic number increases by one.

Beta Equation

Electric Field Deflection

Gamma Rays Gamma rays are high- energy radiation with no mass and are neutral.Gamma rays Gamma rays account for most of the energy lost during radioactive decay.

Characteristics of Radiation

Unstable atoms Atoms that contain too many or too few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus. Few exist in nature—most have already decayed to stable forms.