ORGANIZATION OF THE PERIODIC TABLE
Period Table Overview
The periodic table organizes the different types of atoms, or chemical elements. The elements
are arranged by atomic number in rows and columns, from left to right and top to bottom.
Vertical columns are called groups or families of elements while periods of elements consist of
horizontal rows.
Periodic table: Groups similar elements together; represents elements by their symbols; orders
elements based on increasing number of protons.
Periodic Law – Properties of elements tend to change in a regular pattern when elements are
arranged in order of increasing atomic number, or number of protons in their atoms.
Using the periodic table to determine electronic arrangement
Period – a horizontal row of elements in the periodic table. The period number gives you the
highest energy level that contains electrons.
Just as the number of protons an atom has increases by one as you move from left to right across
a period, so does its number of electrons.
Group (family) – a vertical column of elements in the periodic table
Atoms of elements in the same group have the same number of valence electrons, so these
elements have similar, but not exactly alike, properties. For groups 1 and 2, the number of the
group is the number of valence electrons. For groups 13 through 18, subtract 10 to get number
of valence electrons.
Elements in the same group have similar properties
Group 1
16
17
18
Lithium
Oxygen
Fluorine
Neon
Sodium
Electron Configurations for Selected Elements
SOME ATOMS FORM IONS
All atoms will be stable with a full outermost energy level; if atoms do not have a full outermost
energy level, they will react with other atoms to achieve a full outermost energy level. All
energy levels are considered full with eight electrons except the first level, which is full with two
electrons.
Atoms of Group 1 elements have one valence electron, so they are reactive. Atoms without an
octet may undergo ionization – the process of adding electrons to or removing electrons from an
atom or group of atoms. Atoms can either gain electrons to achieve an octet or lose electrons to
reveal the full outermost energy level.
Ionization
Atoms are electrically neutral; the number of negatively charged electrons is equal to the number
of positively charged protons.
An ion is an atom that has an electrical charge, either positive or negative. Ionization is the
process whereby gaseous atoms become electrically charged.
If an atom undergoes ionization, it no longer has the same number of electrons as it does protons.
Thus, the charges don’t cancel. The result is an ion – an atom or group of atoms that has lost or
gained one or more electrons and therefore has a net electric charge.
Ion
An ion is an atom, radical, or molecule that has gained or lost one or more electrons and has a
negative or positive charge.
Cations are ions that have a positive charge.
Anions are ions that have a negative charge.
Example: The element sodium has one electron in its third and outermost energy level. The
third energy level can hold up to eight electrons. Since energy is required to gain or lose
electrons, it requires less energy for the sodium atom to remove one electron from the outer
energy level than to add seven electrons. If sodium loses its one outer electron, it would then
drop from three to only having two energy levels, and its outermost energy
level would be filled. The removal of one electron forms a positively charged sodium ion, Na+.
A lithium atom loses one electron to form a 1+ charged ion
Example: Lithium is in Group 1, so it has one valence electron. It is in the second period, so it
has two levels with electrons. Two fill the first level, and one is in the outer level, which takes
eight to be full. What will take less energy to make Lithium stable: gain seven to complete an
octet in the second level, or lose one electron to reveal the full first level? Removing this
electron is easier, and it forms a cation – an ion with a positive charge. The lithium cation is
written Li+. It’s very stable.
Hydrogen Ion Simulation
A fluoride atom gains one electron
Example: Fluorine is in Group 17, so it has seven electrons. It is in the second period, so it has
two levels with electrons. Two fill the first level, and seven are in the outer level, which takes
eight to be full. What will take less energy to make Fluorine stable: gain one to complete an
octet in the second level, or lose seven electrons to reveal the full first level? Adding this
electron is easier, and it forms an anion – an ion with a negative charge. The fluorine anion is
written F-. It’s very stable.
HOW DO THE STRUCTURES OF ATOMS DIFFER?
Atomic structure animation table
How Do the Structures of Atoms Differ?
Atomic number (Z) equals the number of protons. Atoms are always neutral because they have
an equal number of protons and electrons. The atomic number also equals the number of
electrons. Each element has a different atomic number, which never changes.
Mass number (A) equals the number of protons plus the number of neutrons.
Isotopes of an element have different numbers of neutrons
Isotopes and Nuclides
Isotopes are atoms of the same element that have different masses.
Nuclide is the general term for any isotope of any element, based on the number of protons and
neutrons in its nucleus.
Many elements have only one stable form, while other elements have different “versions” of
their atoms. These isotopes, or “versions” have the same number of protons and electrons as all
other isotopes but have a different number of neutrons.
Isotope – any atoms having the same number of protons but different numbers of neutrons.
What will vary in isotopes of an element, A or Z?
Simulated hydrogen and helium atoms
Hydrogen Isotope Simulation
Some isotopes are more common than others
Calculating the number of neutrons in an atom.
The mass of an atom
Average Atomic Mass
The average atomic mass is the weighted average of the atomic masses of the naturally
occurring isotopes of an
element.