Chapter 2 Matter is Made up of Atoms - My eCoach2章,物质是由原子-我开放

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-”
• They coined the word “atom” meaning cannot be broken.
“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
• 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
different from atoms of other elements.
Hypotheses, Theories, Laws (review)
• Hypothesis: testable prediction to explain an observation
• Theory: well tested explanation that explains many observations. May change over time
• THEN…JJ Thomson (1897) discovers the electron (e-)
JJ Thomson Cathode Ray Experiment
• Vacuum tube (no air inside) w/ electrode on each end, attached to a terminal.
– Compounds’ components are always in a specific proportion by mass
Dalton’s Atomic Theory, cont.
• John Dalton (1766-1844) 1. All matter is made of atoms 2. Atoms are indestructible and can’t be divided 3. All atoms of one element are exactly alike, but
• LET’S PRACTICE!
– Whiteboard – Marker – Paper towel
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.
Atomic Mass
• (definition) Weighted average of all the isotopes of an element. See p 68 of text. calculating atomic mass
• 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
had thought, but must contain charged particles.
Discovery of Protons & Neutrons
• Later scientists realized there was a second ray in the vacuum tube.
• In 1910 Thomson discovered that neon atoms have different masses.
• Conclusion: there must be another particle that has no charge, called a neutron.
Discovery of Nucleus
• Finding Atomic Mass on Periodic Table • Located below element symbol on periodic
table.
(Section 2):Electrons in Atoms
Energy of Electrons • Why electrons don’t crash into the nucleus:
– 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.
Discovery of Protons & Neutrons, cont.
(Kind of like planets in orbit around the sun.)
Energy of Electrons (cont.)
(Don’t write this!)
DISCUSS WITH YOUR NEIGHBOR: • You are an electron. If you have a lot of
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.”
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.
magnet. – “ “ bends away from a negative end of a magnet.
Cathode Ray Experiment
• Conclusion: 1. The cathode ray was actually ____ charged
particles. 2. The atom could not be ________ as scientists
Chapter 2: Mas
Early Ideas About Matter
• Greek philosophers (2500 y.a.) proposed matter was made of 4 elements: earth, air, fire, water.
– Some were deflected – Others bounced straight back!
The Nuclear Model of the Atom
Conclusion:
1. Atoms are nearly all empty space! 2. Atoms have small, densely packed central nucleus
atom is neutral in charge.)
– Ex: atomic number of carbon, C = 6 – Question: how many protons? How many
electrons? How many neutrons?
Slide 2.2
Isotopes
• The number of neutrons can vary from atom to atom in an element.
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.
• 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
• Law: fact of nature observed so often it is accepted as truth. Doesn’t change.
Discovery of Atomic Structure
• 1800s- scientists thought atom was a tiny solid ball.
atom.
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
burnt, eaten, etc. – Atoms are just rearranged into new matter.
• Proust (1799) discovered water was always 11% hydrogen, 89% oxygen – law of definite proportions
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 has. – Since p+ and n0 are the heavy parts, – mass # = # of p+’s + n0’s.
QUESTION: If the mass number of a carbon atom is 14,
• How many protons? • How many electrons? • How many neutrons?