Download Notes # ____ - The Periodic Table

Notes # ____ - The Periodic Table
Periodic Trends
The elements on the periodic table are grouped based on their physical and chemical
properties. Elements are arranged in order of their atomic number, which is equal to the
number of protons in the nucleus of the atom. The term periodic refers to the fact that the
characteristics repeat in a regular pattern. The table is read in rows, called “periods” and
columns, called “groups” or “families”.
Periods are read from left to right and the atomic number increases by one for each
element. One way to remember this is that a “period” is often found at the end of a
sentence, and sentences are read left  right. The period number refers to the number of
electron shells the atom has. For example, in period 1, the elements have only 1 shell. In
period 2 the elements have 2 shells, period 3 has 3 shells, and so on. The outer shell fills
up as you move from left  right because each time a proton is added (atomic number
increases by 1) an electron is added as well. By the time you get to the far right, the outer
shell of the elements is completely full.
The number of electrons in the outer shell has a lot to do with how reactive elements are.
Elements that have one electron in the outer shell are found in group 1, elements with 2
electrons in the outer shell are in group 2. Elements in the same group share physical and
chemical properties because they have the same number of electrons in their outer most
shell. As you move down a column the atoms get larger and more reactive (with the
exception of group 18).
Atomic # = # of protons
Atomic mass = # protons + # of neutrons
# of protons = # of electrons
# of neutrons = atomic mass – atomic #