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The Atomic
Nature of Matter
An overview of the inner-cosmos
Physics 1100, Fall 2011
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
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
Physics 1100, Fall 2011
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)
Physics 1100, Fall 2011
Chemical elements are defined by the number
of protons in the atom’s nucleus
• Hydrogen: 1 proton & 1 electron
Cloud of
“electron probability”
10-10 m
Physics 1100, Fall 2011
Carbon has 6 protons
Cloud of 6 electrons
6 protons
6 or 7 neutrons
Physics 1100, Fall 2011
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
The Periodic Table
Elements in columns (groups) have similar
outer-electron configurations,
and so tend to behave similarly.
transition metals
Physics 1100, Fall 2011
noble gases
The Periodic Table: the Bulk Earth
A small number of elements make up >99% of the solid Earth.
O = oxygen
Na = sodium
Mg = magnesium
Al = aluminum
Si = silicon
S = sulfur
Ca = calcium
Fe = iron
Ni = nickel
Physics 1100, Fall 2011
Physics 1100, Fall 2011
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.
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.
Physics 1100, Fall 2011
But that’s not all!
– 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
The Standard Model of Particle Physics
Fundamental forces
are mediated by
photon, gluons, W’s
and Z’s (bosons)
Basic Ingredients are
quarks and the electronlike objects (leptons)
Physics 1100, Fall 2011
Structure of the Atom
Physics 1100, Fall 2011
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)
Physics 1100, Fall 2011
The Essence of Matter
• 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
Physics 1100, Fall 2011
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?
Physics 1100, Fall 2011
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?
Physics 1100, Fall 2011
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.
Physics 1100, Fall 2011
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
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
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
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
D) the planet would annihilate.
Physics 1100, Fall 2011