chapter 2: matter is made up of atoms. early ideas about matter greek philosophers (2500 y.a.)...
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Chapter 2: Matter is Made up of Atoms
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Early Ideas About Matter
• Greek philosophers (2500 y.a.) proposed matter was made of 4 elements: earth, air, fire, water.
• They coined the word “atom” meaning cannot be broken.
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“Modern” Atomic Theory• Lavoisier (1743-94) law of conservation of matter
– Matter is neither created nor destroyed– Matter never “goes away,” even when an object is
burnt, eaten, etc.– Atoms are just rearranged into new matter.
• Proust (1799) discovered water was always 11% hydrogen, 89% oxygen – law of definite proportions– Compounds’ components are always in a specific
proportion by mass
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Dalton’s Atomic Theory, cont.
• John Dalton (1766-1844)1. All matter is made of atoms2. Atoms are indestructible and can’t be divided3. All atoms of one element are exactly alike,
but different from atoms of other elements.
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Hypotheses, Theories, Laws (review)
• Hypothesis: testable prediction to explain an observation
• Theory: well tested explanation that explains many observations. May change over time
• Law: fact of nature observed so often it is accepted as truth. Doesn’t change.
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Discovery of Atomic Structure
• 1800s- scientists thought atom was a tiny solid ball.
• THEN…JJ Thomson (1897) discovers the electron (e-)
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JJ Thomson Cathode Ray Experiment
• Vacuum tube (no air inside) w/ electrode on each end, attached to a terminal.
• He send electricity through the tube and saw– A bright ray travelling from the negative end (cathode)
to the positive end (anode). “cathode rays”– picture of cathode ray tube– Cathode ray bends toward a positive end of a magnet.– “ “ bends away from a negative end of a magnet.
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Cathode Ray Experiment
• Conclusion: 1. The cathode ray was actually ____ charged
particles.2. The atom could not be ________ as scientists
had thought, but must contain charged particles.
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Discovery of Protons & Neutrons
• Later scientists realized there was a second ray in the vacuum tube.– It bent toward the – end of a magnet.– It bent away from the + end of a magnet.– Therefore, this ray was made of _____ charged
particles.
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Discovery of Protons & Neutrons, cont.
• In 1910 Thomson discovered that neon atoms have different masses.
• Conclusion: there must be another particle that has no charge, called a neutron.
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Discovery of Nucleus
• 1909-scientists now believe the atom is like chocolate chip cookie dough (see Fig 2.8 p 63 of text)
• Rutherford’s Gold Foil Experiment• Shot “alpha Particles” (helium nuclei) at gold foil.• Hypothesis: they would pass through unaffected.• Data: most did pass through
– Some were deflected– Others bounced straight back!
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The Nuclear Model of the Atom
Conclusion:1. Atoms are nearly all empty space!2. Atoms have small, densely packed central nucleus
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Composition of Atoms
• Protons: positive charge, in nucleus, are heavy, “p+”
• Neutrons: no charge, in nucleus, are heavy, “n0”
• Electrons: negative charge, outside nucleus “electron cloud”, very light (1/1840 of a proton or neutron), “e-”
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Charges in an Atom
• The + charge on a proton is equal to the - charge on an electron.
• Atoms are neutral (have no overall charge)• Therefore, the # of protons = # electrons in an
atom.
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Slide 2.2
Atomic number– determines the identity of the atom. – It tells us the number of protons in the atom. – It also tells us the number of electrons (b/c an atom is neutral in charge.)
– Ex: atomic number of carbon, C = 6– Question: how many protons? How many electrons? How many neutrons?
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Isotopes• The number of neutrons can vary from atom to
atom in an element.• Atoms of the same element w/different #s of
neutrons are called ISOTOPES.• In order to know how many neutrons in an atom
you must be told.• The mass number tells you how much mass the atom has.
– Since p+ and n0 are the heavy parts, – mass # = # of p+’s + n0’s.
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QUESTION: If the mass number of a carbon atom is 14,
• How many protons?• How many electrons?• How many neutrons?
• LET’S PRACTICE!– Whiteboard– Marker– Paper towel
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Atomic Mass Units
• Atoms are weighed in a.m.u.• 1 a.m.u. is based on the mass of a Carbon-12
atom. – it has 6 p+ and 6 n0, – 1 a.m.u = 1/12 the mass of a carbon-12 atom.
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Atomic Mass
• (definition) Weighted average of all the isotopes of an element. See p 68 of text.calculating atomic mass
• Finding Atomic Mass on Periodic Table• Located below element symbol on periodic
table.
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(Section 2):Electrons in Atoms
Energy of Electrons• Why electrons don’t crash into the nucleus:
they have enough energy to keep them away.• Why e-s (usually) don’t fly off of atoms: they
have enough attraction to the nucleus to keep them in “orbit.”
(Kind of like planets in orbit around the sun.)
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Energy of Electrons (cont.)
(Don’t write this!)
DISCUSS WITH YOUR NEIGHBOR:• You are an electron. If you have a lot of
energy, will you stay close to the nucleus or will you move further from it?
Answer: you may still stay in “orbit” but you will be able to move further away from the nucleus.
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Bohr’s Model of Atom
• Neils Bohr studied w/Rutherford
• His model is also called the planetary model
• He discovered that e-s could only exist at certain distances from the nucleus.
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Bohr’s Model of Atom
• See p 75 of text: electron energy levels are like rungs of a ladder.
• Ladder– To climb to a higher level, you can’t put your foot
at any level, – you must place it on a rung
• Electron energy levels– e-s must move to higher or lower e.l.’s in specific
intervals
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Electron Cloud Model of Atom
• Electrons aren’t in perfect orbits.
• Energy levels are regions of space in which an e- is likely to be found most of the time.
• The area in which they move is like a cloud, an area of space surrounding the nucleus.
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Electrons in Energy Levels
• Atoms are arranged in energy levels (e.l.’s), at different distances from nucleus
• Close to nucleus = low energy• Far from nucleus = high energy• e-s in highest occupied level are “valence e-s”• Only so many e-’s can fit in energy levels• e-s fill lower e.l.’s before being located in higher
e.l.’s*(* There are exceptions we will learn later!)
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Electrons in Energy Levels
• Only so many e-’s can fit in energy levels
Energy Level # of electrons1st 22nd 8
3rd 18* 4th 32*
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KEY CONCEPT!!!
•VALENCE ELECTRONS DETERMINE HOW
ELEMENTS BEHAVE!!!
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Drawing Bohr Models
Let’s practice drawing some atoms/ions
In your teams, pick up enough of the following for your team:
1 white board per person 1 marker per person1 paper towel per team (Please save a tree & share!)
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Drawing Bohr Models
• Show # of protons and neutrons in the nucleus
• Draw e.l.’s and show each electron in the proper e.l.
• Ex: Bohr Model of BORON-11
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Practice
• Hydrogen-2 (Practice together)• Helium-4• Lithium-6• Beryllium-8• Carbon-12• Magnesium-24
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Lewis-Dot DiagramsHave 2 parts1.Chemical symbol of element2.Valence e-s, represented by dots
– Are placed in one of four locations• Above• Below• Right• left
– Are not paired unless there is 1 e- in each location.– Ex: Oxygen
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Practice Lewis Dot DiagramsTEACHER DEMONSTRATION• Hydrogen• Helium• Lithium
STUDENT PRACTICE• Beryllium• Boron• Carbon
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PRACTICE WORKSHEET
• Bohr Models• Lewis dot diagrams