The Atom
Chapter 4 Sect 1
V. Parts of the Atom:
A. Nucleus – the solid dense core of the atom that
contains the protons and neutrons
1. Proton- what can you find out about them?
A. positively charged particle
B. mass = 1 amu (atomic mass unit)
C. The number of protons identifies the
element
D. Number of protons = the atomic
number
E. Quark – 3 small particles that make up
a proton
Parts of the Atom:
2. Neutron – what can you find out about them?
A. particles with no charge
B. mass = 1 amu (same as proton)
C. # of protons + # of neutrons = atomic
mass
D. Adding or taking away neutrons DOES
NOT change the atom, it makes different
isotopes
E. Quark – 3 quarks make up a neutron
Parts of the Atom:
B. Electron cloud – surrounds the nucleus and
contains electrons
1. Electrons – what can you find out about them?
A. Negatively charged particle
B. Mass very small amu = 0
C. It takes 1800 electrons to equal the
mass of 1 proton
D. # of electrons = # of protons in a
neutral atom
VI. Atomic Number – what is it and why is it
important?
The atomic number is the number of
protons in the nucleus of the atom.
It is important because the atomic number
identifies the element.
Table #1
Proton number Atomic number Element Symbol
8 8 Oxygen O
1 1 Hydrogen H
6 6 Carbon C
7 7 Nitrogen N
10 10 Neon Ne
13 13 Aluminum Al
VII. Sub atomic Particles
Subatomic Atomic mass in charge location
particle atomic mass units
Proton 1 amu + nucleus
positive
Neutron 1 amu 0 nucleus
neutral
Electron 0 amu _ Orbits around
nucleus in
negative electron
cloud
VIII. What is mass number and why is it
important?
The mass number is the number of
protons plus the number of neutrons in the
nucleus.
The mass number is important because
you can calculate the number of neutrons
to identify isotopes
Mass number – Atomic Number = Neutrons
Recall
**The only thing you need to know to identify an element is the
number of protons.
**Each protons weighs 1 amu and each neutron weighs 1 amu.
**When you add up the number of protons and the number of
neutrons you get the mass number.
Why don’t you add in the number of electrons?
Electrons are too small to calculate into the equation
Remember! Electrons are 1/1800 of a proton or neutron
So, if your atomic number is 11 – what element are you?
Atomic Number 11 = Sodium (Na)
X. Isotopes
Most elements have naturally occurring
isotopes. What is an Isotope?
Atoms with the same number of protons but
different numbers of neutrons are isotopes.
If you gain a neutron, you will be heavier (more
massive)
Why? You are adding a neutron which has a mass
of 1 amu
If you lose a neutron, you are lighter (less
massive)
Why? You are taking away 1 amu
Isotopes or different elements?
D and F are different elements
different # of protons
J and L are isotopes
same # of protons, different # of neutrons
X and Y are different elements
different # of protons
Q and R are isotopes
same # of protons, different # of neutrons
Isotopes or different elements?
T has 20 protons and 20 neutrons
Z has 20 protons and 21 neutrons
T and Z are isotopes
same # of protons, different # of neutrons
A has 31 protons and 39 neutrons
E has 32 protons and 38 neutrons
A and E are different elements
different # of protons
The atomic mass that you see on the
Periodic Table is the average of all the
isotopes of that element. How does this
explain why there are no whole atomic
mass numbers on the Periodic Table?
There are no whole numbers on the
Periodic Table because averages usually
don’t equal a whole number.
How to read a Periodic Table
Atomic Number
6
Atomic
C Atomic
Symbol
12.011
Mass
How to read a Periodic Table
Atomic Number
3
Atomic
Li Atomic
Symbol
6.941
Mass
How to read a Periodic Table
Atomic Number
8
Atomic
O Atomic
Symbol
15.999
Mass
Middle of page 5
Before we move on, let’s review protons
and neutrons. Where are they located? In
the nucleus. They each have a mass of 1
amu. An electron is much smaller than a
proton or a neutron and has a mass of 0
amu. Electrons are located in the
electron cloud of the atom. Protons have
what charge? Positive (+) Electrons
have what charge? Negative (-)
Table 3 (Use inside back cover)
Element Atomic Proton Atomic Neutrons Electrons Charge of
number number mass atom
(rounded)
C 0
Carbon
6 6 12 6 6 Neutral
Na 0
Sodium
11 11 23 12 11 neutral
Si 0
Silicon
14 14 28 14 14 neutral
O 0
Oxygen
8 8 16 8 8 neutral
3 Main Parts
Protons
with 1 atomic mass unit or amu and a + charge
Neutrons
with 1 atomic mass unit or amu and a 0 charge
Electrons
with 0 atomic mass unit or amu and a - charge
Unlike protons and neutrons in an atom, the
electrons are arranged in a particular order. The
electrons fill the energy shells closest to the
nucleus first and then fill outward:
The first energy shell can hold up to 2 electrons
The second energy shell can hold up to 8
electrons
The third energy shell can hold up to 18
electrons
The fourth energy shell can hold up to 32
electrons
Electron Shell Diagram –
Phosphorous
First Energy Level
Second Energy
Level
Third Energy
Level
Electron Shell Diagram –Phosphorous
First Energy Level
Second Energy
Level
Third Energy
Level
XI. Forces that hold the atom together
1. Gravity - even in an atom… depends on:
A. How big (massive) the objects are
B. how far apart they are
2. Electromagnetic forces…like charges do
what? Unlike charges do what?
A. Like charges repel
B. Unlike (opposite) charges attract
C. Electrons repel electrons, but attract protons
D. Protons repel protons, but attract electrons
3. Strong force (nuclear force) – holds protons together in the
nucleus
4. Weak force – plays a role in radioactive (unstable) atoms when a
neutron changes into a proton and an electron
IV. History of the Atom
A. Democritus (400 BC)
1. Said elements are invisible particles called
atoms
2. The atoms were “indivisible” or “uncutable”
B. Aristotle (384 – 322 BC)
1. Disagreed with Democritus
2. All matter was made up of the 4 elements:
Air, Earth, Water, and Fire
History of the Atom (cont.)
C. John Dalton (late 1700’s)
1. Atoms cannot be created, divided or
destroyed.
2. Atoms of the same element are alike.
3. Atoms join with other atoms to make new
substances
D. J.J. Thompson (1897)
1. Found that atoms are made of smaller parts.
2. Discovered a negative charge – later called
the electron.
History of the atom (cont.)
E. Ernest Rutherford (1909)
1. Proved atoms are not solid
2. They are mostly empty space, but with a
solid nucleus
F. Neils Bohr (1913)
1. Suggested that electrons traveled around the
nucleus in definite paths (Sun and planets
model for atoms)
2. Electron can jump between levels.