Properties of Atoms
and the Periodic Table
Chapter 19
Sections 1 and 2
Section 1
STRUCTURE OF THE ATOM
Structure of the Atom
•
•
•
An element of matter is
made up of one type of
atom
Atoms are composed of
particles called protons,
neutrons, and electrons
•
Protons-positive charge
•
Electrons-negative charge
•
Neutrons-no charge
Protons and neutrons are in
centrally-located nucleus
surrounded by a cloud
containing electrons
Quarks
•
•
•
Scientists hypothesize that electrons are not
composed of smaller particles
Protons and neutrons are made up of smaller
particles called quarks
The search for the exact composition of protons and
neutrons is ongoing
Atomic Models
•
Many different models of the
atom were proposed
throughout the years
•
•
Aristotle argued that atoms
didn’t even exist
John Dalton’s provided
evidence that atoms existed
•
•
•
•
Dalton’s Model
Thomson Model
His model was a simple sphere
Joseph Thomson proposed the
“plum-pudding” model
Rutherford Model
Ernest Rutherford’s model
included a nucleus
Niels Bohr suggested that
electrons travel in fixed orbits
around the nucleus
Bohr Model
The Electron Cloud Model
•
•
•
•
By 1926, scientists
developed the electron
cloud model of an atom
This is the currently
accepted model
The electron cloud is the
area around the nucleus
where its electrons are most
likely to be found
The cloud is 100,000 times
larger than the diameter of
the nucleus
Section 2
MASSES OF ATOMS
Atomic Symbols
•
•
•
All elements are represented by a chemical symbol
The symbols consist of one capital letter or a capital
letter and one or two lowercase letters
Some symbols are the first letter of the elements
name
•
•
•
•
O
Some are derived from Latin
•
•
Oxygen
Silver
Argentum
Ag
Some are name in honor of scientists
Some are named for where they were discovered
The symbols are recognized worldwide
Atomic Number
•
•
The number of protons in an atoms is known as the
atomic number
The number of protons in an atom is what gives an
atom its identity
•
Carbon will always have 6 protons
•
•
Neon will always have 10 protons
•
•
Atomic number = 6
Atomic number = 10
The atomic number is on the periodic table and is
used to identify each element
Atomic Mass
•
•
The nucleus contains most of the mass of an atom
The mass of a proton and a neutron are very similar
•
•
•
•
1.67 x 10-24 g
The mass of a proton (or a neutron) is 1,836 times
greater than that of an electron
Because the mass of proton or neutron is awkward to
use, a standard measurement is used.
Each proton or neutron has a mass of approximately
1 atomic mass unit (1 amu)
•
Carbon has 6 protons and 6 neutrons
•
6(1) + 6(1)= 12 amu (mass of a carbon atom)
•
Why didn’t we use the mass of an electrons?
Mass Number
•
The mass number of an atom is the sum of the
number of protons and neutrons
•
•
Remember: This is because each proton or neutron has a
mass of 1 amu
If you know the mass number and the atomic number
of an atom, you can determine the number of
neutrons
•
•
# of Neutrons = Mass Number – Atomic number
Example: How many neutrons are in an Carbon atom that
has a mass number of 14?
•
What is the atomic number of Carbon?
•
•
6
14 – 6 = 8
There are 8 neutrons in the atom.
Isotopes
•
•
•
•
•
•
•
•
•
All atoms of an element will have the same number
of protons
But not all the atoms will always have the same
number of neutrons
Atoms of the same element that have different
numbers of neutrons are called isotopes
Isotopes are usually identified by their mass number
Carbon-12 (6 protons, 6 neutrons)
Carbon-14 (6 protons, 8 neutrons)
Boron-10 (5 protons, 5 neutrons)
Boron-11 (5 protons, 7 neutrons)
How many neutrons does Oxygen-18 have?
Average Atomic Mass
•
•
•
•
Because not all the atoms of an element have the
same mass, an average mass is used
To figure out the average mass, we need to know
how many of each isotope is present in a sample
Scientists do this by using weighted averages
For example:
•
80% of all Boron atoms are Boron-11
•
20% of all Boron atoms are Boron-10
•
.80(11) + .20(10) = 10.8 amu