Physics 1100, Fall 2011 1

The AtomicNature of Matter

An overview of the inner-cosmos

Physics 1100, Fall 2011 2

The Search for Basic Constituents Traces Back to Greek Times

• Atomic Hypothesis from Democritus

• How many “basic” constituents should we expect?– Air – Fire– Earth– Water

Say the Greeks

Physics 1100, Fall 2011 3

Atoms make up the everyday material world

• Everything is made of atoms!• Crystalline arrays – solids• Loose atoms rattling around – gases• Sloppy arrangements of atoms – liquids

Physics 1100, Fall 2011 4

Physics 1100, Fall 2011 5

Atoms Are Composite Objects

• Protons (+ electric charge), p• Electrons (– electric charge), e• Neutrons (no charge), n• Proton and Neutron have about the same mass• Electron is about 2000 times less massive than proton• Electrical Forces produce attraction between electrons and the

protons in the nucleus (they are oppositely charged)

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Cloud of “electron probability”

Chemical elements are defined by the number of protons in the atom’s nucleus

• Hydrogen: 1 proton & 1 electron

proton

10-10 m

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Carbon has 6 protons

Nucleus:6 protons

6 or 7 neutrons

Cloud of 6 electrons

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Atomic Number

• We distinguish one element from another on the basis of the

atomic number, which is the number of protons.

• So, an atom of any element can have a variable number of

electrons and neutrons, but given the number of protons, the

fundamental properties of the element are unchanged.

This is the basis for Dmitri Mendeleev’s

organization of the Periodic Table of the Elements.

The table is a way of organizing elements on

physical grounds, but also serves to group

elements with consistent chemical properties.

Physics 1100, Fall 2011 9

Elements in columns (groups) have similar outer-electron configurations,

and so tend to behave similarly.

The Periodic Tablea

lkal

is

alkali earths

rare earths

halogens

noble gases

transition metals

actinides

Physics 1100, Fall 2011 10

The Periodic Table: the Bulk Earth

A small number of elements make up >99% of the solid Earth.

O = oxygenNa = sodiumMg = magnesiumAl = aluminumSi = silicon

S = sulfurCa = calciumFe = ironNi = nickel

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Physics 1100, Fall 2011 12

Atomic Weight: It’s all in the Nucleus

• Since electrons weigh virtually nothing, the mass of an atom is

concentrated in its nucleus. • Each atom can be described by its atomic weight (or mass), which is

the sum of the protons and neutrons.

lithium: atomic number = 3 3 protons 4 neutrons atomic weight = 3 + 4 = 7

BUT... although each element has a defined number

of protons, the number of neutrons is not fixed.

Atoms with the same atomic number but variable

numbers of neutrons are called isotopes.

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But that’s not all!• Antimatter

– Each elementary particle has an “antimatter” counterpart

Electron – Positron

Proton – Antiproton

Neutron – Antineutron

etc. anti-etc. – E=mc2 says matter and energy are interchangeable– If they find one another – major fireworks!

• Dark Matter– Invisible, but must be there for gravitational attraction

• Dark Energy– Invisible, but must be there for repulsion (universe expanding at an

increasing rate!

Physics 1100, Fall 2011 14

The Standard Model of Particle Physics

Basic Ingredients are quarks and the electron-like objects (leptons)

(Fermilab)

Fundamental forces are mediated by photon, gluons, W’s and Z’s (bosons)

Physics 1100, Fall 2011 15

Structure of the Atom

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What holds the nucleus together?

• Electrical charges interact, and like charges repel

– Opposites attract, of course

• The closer they get, the more protons in the nucleus should be repelled from each other!

• Something must serve as the glue to hold the nucleus together

– The “strong” nuclear force: overcomes the electrical “Coulomb” force at short distances

– Felt by protons and neutrons, not by electrons or their cousins (collectively called leptons)

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• Classification of matter?– • Elements,

– • compound,

– • mixture, ...

• Three (actually 4) states of matter.– • Solid-fix shape, fix volume

– • Liquid- shape container, fix volume

– • Gas- shape container , volume container

The Essence of Matter

Physics 1100, Fall 2011 18

Class Problem

• Oops! Those "harmless" germanium capsules you just swallowed may have an extra proton in each nucleus.

Is this good news or bad news? Why?

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Class Problem

• Suppose you could add or subtract protons from oxygen nuclei. To turn oxygen into gas that would glow red when an electric current flows through it, would you add or subtract protons? How many?

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Class Problem

• Suppose you could add or subtract protons from oxygen nuclei. To turn oxygen into gas that would glow red when an electric current flows through it, would you add or subtract protons? How many?

• Add two protons to each nucleus of oxygen and you increase the atomic number from 8 to 10. You then have neon, which will glow a very nice red when a current flows through it.

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Class Problems

1. Which of the following statements is true?

A) There are thousands of different kinds of atoms that account for a wide variety of substances.

B) A large atom can be photographed with the aid of an ordinary microscope.

C) An atom is the smallest particle known to exist.

D) There are only about 100 different kinds of atoms that combine to form all substances.

E) None of these statements are true.

2. What makes an element distinct?

A) the number of electrons

B) the number of protons

C) the number of neutrons

D) the total mass of all the particles

E) none of these

Physics 1100, Fall 2011 22

Class Problems

3. Which of the following is not a compound?

A) air

B) salt

C) ammonia

D) water

E) All are compounds.

4. Solid matter is mostly empty space. The reason solids don't fall through one another is because

A) of electrical forces.

B) of nuclear forces.

C) atoms are constantly vibrating, even at absolute zero.

D) of gravitational forces.

E) none of these

Physics 1100, Fall 2011 23

Class Problems

5. If an astronaut landed on a planet made of antimatter, there would be an explosion and

A) the astronaut would annihilate.

B) an amount of planet matter equal to that of the astronaut would annihilate.

C) the astronaut and an equal amount of the planet would both annihilate.

D) the planet would annihilate.