4.15. Atomic Number
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Printed: October 8, 2014
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. 4.15. Atomic Number
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4.15. Atomic Number
• Define the atomic number.
• Relate the number of electrons in an element to the atomic number for that element.
What is unique about each one of us?
For the vast majority of people, it is not their name, because it is quite possible for others in the world to have the
same name (check it out by doing an internet search for your name and see how many other of “you” there are). It
is not your physical description. Eye-witnesses to crime scenes often pick the wrong person when trying to identify
the criminal.
There may be some unique identifiers for us. If you have a cell phone in your name, nobody else in the world has
that number. Email addresses are different for each of us, which is a good thing since we can email almost anywhere
in the world. Our DNA is unique, but getting a DNA analysis is expensive and time-consuming, so we really don’t
want to have to explore that.
Organizing the Elements
One of the goals of science is to discover the order in the universe and to organize information that reflects that
order. As information about the different elements was made known, efforts were made to see if there were patterns
in all of the data. An early attempt to organize data was made by Mendeleev, who developed the first periodic
table. His data set was based on atomic weights and was instrumental in providing clues as to the possible identity
of new elements. Once we learned the details of the atomic nucleus, the table was based on the number of protons
in the nucleus, called the atomic number of the element.
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Atomic Number
FIGURE 1.1
How can you determine the atomic number of an element?
The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. This
means that the number of protons is the characteristic which makes each element unique compared to all other
elements. Elements are different because of their atomic number. The periodic table displays all of the known
elements and is arranged in order of increasing atomic number. In this table, an element’s atomic number is indicated
above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen
atom has one proton in its nucleus. Following on the table is helium, whose atoms have two protons in the nucleus.
Lithium atoms have three protons, and so forth.
Since atoms are neutral, the number of electrons is equal to the number of protons. Hydrogen atoms all have one
electron occupying the space outside of the nucleus. Manganese (atomic number 25) would have twenty-five protons
and twenty-five electrons.
The classification of elements by atomic number allows us to understand many properties of the atom and makes it
possible to predict behaviors instead of just having to memorize everything.
Summary
• The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element
• The number of electrons is equal to the number of protons in an element.
Practice
Questions
Use the link below to answer the following questions:
http://en.citizendium.org/wiki/atomic_number
1.
2.
3.
4.
2
What letter is used by convention to designate the atomic number?
What determines the chemical properties of an element?
What are the atomic numbers of the elements that appear in nature?
How many elements were known in John Dalton’s day?
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Chapter 1. 4.15. Atomic Number
FIGURE 1.2
The periodic table classifies elements by atomic number.
Review
Questions
1.
2.
3.
4.
5.
6.
Name two unique identifiers of people.
Who developed the first periodic table?
What was this table based upon?
What is the current periodic table based upon?
What does the atomic number represent?
How many protons are in the following elements?
(a) Ne
(b) Ca
(c) Pt
7. Write the symbol for the element with the following atomic number:
(a)
(b)
(c)
(d)
18
41
82
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• periodic table: This table displays all of the known elements and is arranged in order of increasing atomic
number.
• atomic weight: Each chemical element has an atom with a given mass.
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• atomic number : The number of protons in the nucleus of each atom of that element.
References
1. Image copyright CLIPAREA l Custom media, 2014. http://www.shutterstock.com .
2. Laura Guerin, based on image by User:Materialscientist/Wikimedia Commons. CK-12 Foundation (original i
mage: http://commons.wikimedia.org/wiki/File:Atomic_number_depiction.jpg) .
3. User:Cepheus/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Periodic_table.svg .
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