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Getting to Know: Periodic Table
Scientists often use a table to record the data they collect during
an investigation. A data table allows scientists to collect data in an
organized way. It makes it easier to identify patterns and trends and
to compare the results of multiple trials, for example.
A table uses a logical system to organize information. Back in the 1800s,
scientists were researching the elements. Only about 60 elements had
been discovered by this time. Dmitri Mendeleev, a Russian chemist,
developed a system to organize all the known elements. To do this, he
had to decide what features of the elements he would use to organize
them. Should he classify the elements by their state at a certain
temperature? Should he group them by color? Which system would
be most useful? Mendeleev decided to group the elements by specific
chemical properties. After much research, he used the properties and
atomic masses of the elements to arrange them in a table.
Recording data in
tables helps organize a
scientist’s observations.
Like a data table used for an experiment, this table had rows and
columns. Not every part of the table was filled in. There were holes.
Mendeleev predicted that there would be additional elements to
“fill” those holes. He was right. Many more elements have been
discovered.
The modern periodic table is based on Mendeleev’s periodic table. In the modern periodic table, the elements are arranged in order of increasing atomic number. An element’s atomic number is the number of protons in the nucleus of an atom of that element. Why is the periodic table called a “table”?
The periodic table is arranged like a table with each element in a separate box. The boxes form horizontal rows and vertical columns. The rows in the periodic table are called periods. The period an element is in tells you something about how its atom’s electrons are arranged.
The columns of the periodic table are called groups. The group an element is in tells you something about the element’s properties. Elements in the same group have similar properties. For example, the elements in Group 2 are all metals. Group 2 metals all react strongly with water and air. What other kinds of information
can I find on the periodic table?
Aluminum, used to make this soft
drink can, was on e of the elements
Mendeleev included in his table of
the elements.
Each box in the table contains the chemical symbol for an element. Above the symbol is the
atomic number for that element. Below the symbol is the element’s atomic mass.
Concept: Periodic Table
Getting to Know
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The modern periodic table groups elements by their atomic number.
What’s the difference between an element’s atomic number and its
atomic mass?
Remember that the atomic number tells you the number of protons in the nucleus of an atom. An element’s atomic mass is the average mass of atoms of that element. Misconception: Why is the atomic mass defined as the “average” mass of atoms of
an element? Aren’t all atoms of the same element identical?
All atoms of the same element have the same number of protons. All atoms of the same
element do not have the same number of neutrons. For example, most atoms of the
element carbon, C, have 6 neutrons in their nucleus. There are some atoms of carbon
that have 7 or even 8 neutrons in their nucleus. Atoms of the same element that have a
different number of neutrons are called isotopes.
Remember that the atomic number tells you the number of protons in the nucleus of an
atom. Atomic mass of an isotope is the combined mass of all of the protons and neutrons of
an atom. Because each proton and neutron has a mass of approximately one unit (called an
atomic mass unit), the atomic mass of an isotope is approximately equal to the number of
protons and neutrons. It is possible for atoms of the same element to have a different number
of neutrons in their nucleus. These different possible versions of each element are isotopes.
Because isotopes exist, the atoms of the same element can have different numbers
of neutrons. The atomic mass that is written on the Periodic Table of the Elements is
actually the average atomic mass of all of the isotopes for that element. The average
atomic mass can be calculated by multiplying the atomic mass of an isotope by its
natural abundance, the percentage it is found a nature, and then adding the atomic
mass for each isotope together.
Concept: Periodic Table
Getting to Know
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.