Chapter 2: Matter is Made up of Atoms

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.

“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

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.

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.

Discovery of Atomic Structure

• 1800s- scientists thought atom was a tiny solid ball.

• 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.

• 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.

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.

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.

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.

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!

The Nuclear Model of the Atom

Conclusion:1. Atoms are nearly all empty space!2. Atoms have small, densely packed central nucleus

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-”

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.

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?

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.

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

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

• 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:

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.)

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.

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.

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

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.

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!)

Electrons in Energy Levels

• Only so many e-’s can fit in energy levels

Energy Level # of electrons1st 22nd 8

3rd 18* 4th 32*

KEY CONCEPT!!!

•VALENCE ELECTRONS DETERMINE HOW

ELEMENTS BEHAVE!!!

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!)

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

Practice

• Hydrogen-2 (Practice together)• Helium-4• Lithium-6• Beryllium-8• Carbon-12• Magnesium-24

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

Practice Lewis Dot DiagramsTEACHER DEMONSTRATION• Hydrogen• Helium• Lithium

STUDENT PRACTICE• Beryllium• Boron• Carbon

PRACTICE WORKSHEET

• Bohr Models• Lewis dot diagrams