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What is Matter Made Up Of?
LESSON 1
Matter is Made Up of Atoms
 Democritus wondered how many times you could cut
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something in half and in half and in half until you
could not cut it any smaller
He called this the atom!
This was about 2400 years ago!
ATOM – Smallest unit of a pure substance that still
has the properties of the substance
It was not until the 1800’s when scientists were
finally able to use experiments, not just ideas, to
study these things
Atoms are TINY!
 How many atoms do you think is in one grain of
table salt?
 2,000,000,000,000,000,000!
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That’s 2 quintillion or 2 billion billion atoms!
 John Dalton used experiments to support the Atomic
Theory in the 1800’s, stating that matter is made up
of atoms
 He also believed that atoms were the smallest
particles of matter and cannot be divided
Atomic Structure
 J.J. Thomson showed that atoms are made of even
smaller particles called subatomic particles
 Plum Pudding Model
 Does anyone know an example of a subatomic
particle?
 The atoms is made up many things!:
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Nucleus
Proton
Neutron
Electron
The Nucleus
 The center of an atom which contains protons and
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neutrons
Ernest Rutherford’s famous experiment in 1911
showed that an atom had a nucleus
Gold Sheet Experiment:
https://www.youtube.com/watch?v=LMXNESnTc2c
The nucleus is tiny compared to the size of the atom
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It is only about 1/10,000 the diameter of the atom
So if the nucleus is a tennis ball, the atom would be the size of
a about size football fields!
Nucleus
 Despite it being a really small portion of the size, it
makes up almost the entire mass!
 The nucleus is about 99.9% of the mass of the atom
 This is because a proton or neutron has about 2000X
more mass than an electron!
 The number of protons contained in the nucleus
determines what kind of atom it is
Proton
 A proton is a subatomic particle that has a positive
charge
 Usually found in the nucleus
 Every atom’s nucleus contains at least one proton!
 The number of protons in an atoms nucleus
determines what type of atom it is
Neutron
 Subatomic particle that has the same mass as a
proton, but does not have electric charge
 Neutral charge
 Also found in the nucleus
Electrons
 Negatively charged subatomic particle
 Electrons are in constant motion around the nucleus
 Mass is MUCH smaller than proton or neutron
 First thought that they were in “orbit” around
nucleus
 Now we know they are in clouds around the nucleus
that define the size of the atom
Atomic Structure
Atomic Number
 The number of protons in an atom is called the
Atomic Number
 For example, the atomic number of Helium is 2, the
atomic number of Carbon is 6?
 Give me some of your own examples:
Isotopes
 All the atoms in an element have the same atomic
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number because they have the same number of
protons!
BUT… Atoms of the same substance may have different
masses.
Can anyone think of why this might be?
It is actually because they have a different amount of
neutrons!
Atoms that have the same number of protons, but a
different number of neutrons are called isotopes!
Atomic Mass Units
 Atoms are really really small right?!?
 How do you think we measure the mass of a single atom?
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Grams? Pounds? Other guesses?
We instead use atomic mass units or amu
Every proton has the same mass and it is defined as 1
amu
Neutrons have almost the same mass as a proton and so
it also has a mass of 1 amu
The atomic mass is the average of all the number of
protons and neutrons in all of the isotopes of a substance
AMU and Isotopes
 The atomic mass units of hydrogen isotopes are:
 1 amu
 2 amu
 3 amu
 An isotope’s names is usually followed by the
number of particles in the nucleus
 For example, a hydrogen with one proton and no
neutrons would be Hydrogen-1
Charges
 What is the charge on a proton?
 What is the charge on an electron?
 I thought opposite charges attract… so why do the
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electrons stay on the outside of the nucleus and the
protons are all next to each other?
The strong nuclear force or atomic force keeps the
nucleus together!
An ion is an atom that has an electric charge on it!
A neutral atom has no charge on it!
Let’s do some examples!
 http://ed.ted.com/lessons/just-how-small-is-an-
atom