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Atoms and the Periodic Table
The Atom
All matter in the universe is made of atoms. Atoms are the
smallest component of an element that still have all the
properties of that element. Atoms are so small that they are
assigned their own unit of mass, called the Atomic Mass
Unit, or AMU. One AMU is equal to
0.000000000000000000000001.7 grams!
Atoms are mostly empty space, and if one was drawn to
scale, it would look something like the picture to the right.
The center cluster of the atom is called the nucleus. Two
particles reside within the nucleus:
- Protons, which have a positive charge and a mass
of 1 AMU.
-
Neutrons, which have no charge and a mass of 1
AMU.
Orbiting around the nucleus in the electron cloud (or orbitals) at extremely high speeds are negativelycharged particles called electrons. Electrons are much lighter than protons and neutrons, with a mass of
1/1847AMU. Electrons are so light that when we calculate the mass of an atom, we ignore them (Mass of
electrons = 0 AMU).
How many electrons, protons, and neutrons are present in this atom of carbon?
Protons: ___________
Neutrons: ____________
Electrons: _____________
Normally, atoms have the same number of protons and electrons. Since
they have no net charge, they are sometimes referred to as neutral
atoms.
Fill out the following chart:
Charge
Protons
Neutrons
Electrons
Mass (in AMU)
Location
Periodic Table
Each particle of the atom determines something different about its characteristics.
-
Protons determine the identity of the element.
Neutrons determine the atom’s mass and stability.
Electrons determine how reactive the atom is and how it bonds.
The Periodic Table is a chart that lists each of the known elements in the universe, starting with hydrogen,
which only has one proton, and ending with Ununoctium, which has 118 protons in its nucleus.
Each element has its own box in the periodic table,
containing four pieces of information. Label each of
these on the box for helium on the right.

The element name.

The element’s chemical symbol.

The atomic number (abbreviation = Z).

The mass number (abbreviation = A)
The atomic number gives the number of protons. In a neutral atom, there will also be an equal number of
electrons.
Helium has an atomic number of 2. Therefore, it has ______ protons and __________ electrons.
Helium has a rounded mass number of 4. Therefore, it has ___________________ neutrons. You can
only have whole numbers of neutrons. Fractions of neutrons do not exist, so round the element’s mass
before you calculate the number of neutrons.
Practice
Identify the number of protons, neutrons, and electrons in a neutral atom of each of these elements.
Isotopes
Isotopes and Atomic Mass
All atoms of the same element will always have the same number of protons. Protons determine the
identity of the element. Different atoms of an element may have different numbers of electrons; this
forms ions. Atoms may also differ in their number of neutrons, creating isotopes.
Isotopes of the same element only differ from each other by their atomic mass and the number of
neutrons. Atoms with more neutrons will be heavier, while atoms with less neutrons will be lighter.
Look at the three isotopes of hydrogen:
How many protons are present in each isotope?
How many electrons are present in each isotope?
How many neutrons are present in each isotope?
Isotope Notation
When writing the chemical symbol or name of an element, it is important to indicate if
it is not the most common isotope. The most common isotope is the one that you get
by rounding the atomic mass shown by the periodic table.
Which isotope of hydrogen is the most common? (Circle below)
Hydrogen-1
Hydrogen-2
When writing the chemical symbol for an unusual isotope, it is important to indicate the atomic mass.
This will tell the reader which isotope you are referring to, and they will be able to calculate the number
of neutrons in that isotope.
How many neutrons are in 14C?
How many neutrons are in 41K?
There are two ways to write out isotopes:
Element Name – mass number
𝑀𝑎𝑠𝑠 𝑛𝑢𝑚𝑏𝑒𝑟
Ex.) Carbon – 12
Ex.) 12𝐶
𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑆𝑦𝑚𝑏𝑜𝑙
Practice
Circle the most common isotope for each of the elements below. You can find this by comparing the
atomic masses given to the rounded mass number from the periodic table of each element.
1. Helium-3
Helium-4
2. Barium-135
Barium-136
Barium-137
3. Argon-36
Argon-38
Argon-40
4. Chlorine-35
Chlorine-37
5. Nitrogen-14
Nitrogen-15
6. Silicon-28
Silicon-29
Silicon-30
7. Uranium-234
Uranium-235
Uraniuim-238
Barium-138
For each of the isotopes given below, add the correct symbol (using isotope notation), the name (showing
atomic mass), and the number of protons, neutrons, and electrons.
Symbol
Name
# of Protons
11B
Boron-11
5
10B
Boron-10
# of Neutrons
# of Electrons
5
18O
131Xe
Xenon-131
Sulfur-35
Titanium-48
60Co
51
72
43
56
42
HINT




Protons = atomic number
Neutrons = Mass number – atomic number
Electrons = atomic number (only when it is a neutral atom)
Mass number = protons + neutrons
33
Isotope and Ion Notation
Extra Practice
Background
Isotopes are atoms of the same element with different numbers of neutrons. Ions are atoms of the same
element with a charge due to the gain or loss of electrons. Ions are isotopes are both expressed with
specific forms of notation.
Ions will have a + or – sign next to the chemical symbol of the element.
Magnesium has a 2+ charge, indicating that it has lost two electrons. Bromine has
a – charge, indicating that it has lost one electron.
Isotopes must be accompanied by the mass number, since the number of neutrons
has changed. The atomic number may also be included. Each of these isotopes
of carbon has 6 protons. Carbon-12 has 6 neutrons, while carbon-14 has 8
neutrons.
Practice
For each of the element symbols below, indicate whether it represents an ion, isotope, or neutral atom.
Use a periodic table to identify element name and the number of protons, neutrons, and electrons present.
Symbol
Name
Classification
# of Protons
# of Neutrons
# of Electrons
Al3+
Aluminum
Cation
13
14
10
Hydrogen-3
Neutral Atom
Chlorine
18
Ag+
18
O
Neutral Atom
Scandium
21
34
19
46
S-2
41
K
19
Gold-198
P-3
15
Modified from James Dauray