In chemistry, valence electrons are often the focus of study because they play a vital role in the bonding behavior of an atom. Dr. Nivaldo Tro defines valence electrons as those that exist in the outermost energy shell of an atom. Quickly identifying the number of valence electrons is important in mastering chemistry. According to the University of Colorado at Boulder, valences are used to determine an element's place in the standard periodic table of elements. Clackamas Community College points out that major elements commonly used in chemical reactions, such as carbon and oxygen, exhibit unique behaviors due to the number of valence electrons they possess. The easiest method of determining valences is to just look at the periodic table.
Place a copy of the periodic table elements on a surface where all of the atoms represented on the table are easily reached. It can often help to physically point at each row as you examine it in order to develop an instinctive feel for the arrangement of the chart.
Look at the leftmost column on the periodic table. It is the start of the table and the top element is hydrogen -- the building block of matter as we understand it. This column is called "Group 1" and using a pen or pencil you should label a notecard with the heading "Column 1." The periodic table is arranged according to valences, and the first column of elements contains all the elements with one valence shell electron. Add this fact to your notecard and include any other devices to help you remember it, as well as the names of the seven elements listed under Column 1.
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Examine the column immediately to the right of Column 1. Label a notecard "Column 2" and make a note that this column contains the Group 2 elements -- the family of elements with two valence electrons. The first two columns are known as the metals, and are commonly found in salts and ionic compounds. There are six elements in this group and their names should be on your notecard.
Label a notecard with the heading of "transition metals" and set it aside. The next 10 columns to the right of group 2 do have valence electrons but the rules that apply to them do not exactly follow the pattern of those elements in the remainder of the periodic table. It is easier to group them together as the transition metals and ignore their group numbers.
Find neon (Ne) on the far right hand side of the periodic table and count left six spots from there. You will arrive at boron (B) and the elements under it on the periodic table fall under the category of Group 3 elements. From here the columns are numbered sequentially according to the same pattern as Columns 1 and 2 and so those elements falling under carbon (C) are group 4 elements with 4 valence electrons, those under nitrogen (N) are group 5 elements with 5 valence electrons and so on. Complete your notecards accordingly, noting the elements that fall under each column.
Make a note on the final card that the elements in column 8 are unique. These elements, the group 8 elements, are also known as the Noble gases. They all have full valence electron shells and so are not as important as groups 1 to 7. Keep your notecards in numeric order and refer to them often -- the number of valence electrons for each element on a given card are equal to the group number also on that card.
TL;DR (Too Long; Didn't Read)
The periodic table in its current form was constructed by Russian scientist Dmitri Mendeleev in order to take advantage of the periodicity of valences among the elements. Atomic elements in the same group behave in a similar manner to others of the same group and the number of valence electrons determines how reactive an element is, what other elements it will bond with and even how acidic it tends to be when in a compound.
Although many periodic tables do include group numbers for each column of the transition metals, they must not be viewed in the same way as the rest of the elements, which are known as the Main Group Elements. Although they generally do follow the same behavior as the group to which they are assigned, their outermost electron shells often exhibit aberrant behavior, and are the focus of study in a completely separate, specialized part of chemistry.