Reading valence on the periodic table is often relegated to soon forgotten introductory chemistry. However it is applicable to more complex concepts in pharmacology calculations like milliequivalent and osmolarity.
Unfortunately concepts are usually taught in silos. Being able to integrate concepts is necessary for advanced application rather than short term memorization. The goal is to create a solid foundation of valency that will be the basis for future topics.
Overview of the Periodic Table
The periodic table is organized into rows called “periods” (across) and columns called “groups” (down).
Within the periodic table there are several classes of elements that “behave” similarly. For the purpose of this unit, which is calculating valence, we will focus only on the sections of the periodic table that obeys the rules of valency.
Ptable provides an excellent, interactive periodic table.
For each element, the periodic table provides an atomic number and an atomic mass (mass number).
The atomic number is the number above the element. It increases in increments of 1 across the periodic table from left to right.
The atomic number tells us the total number of electrons in the atom. Carbon has an atomic number of 6, there are 6 electrons in a carbon atom.
The basic structure of the atom is a nucleus at the center surrounded by electrons. Electrons on each atom are arranged in several “shells” around the nucleus of the atom.
Structure of an Atom
The innermost shell on a stable can hold 2 electrons, the second shell can hold 8 electrons and the outermost shell can hold a maximum of 8 electrons. When the shells are occupied with the maximum number of electrons, this creates a stable atom. Stable atoms have low reactivity.
When the outermost shell is not maximized, those electrons are the most reactive. They are high energy electrons that undergo reactions in a attempt to achieve a stable atom or create a stable molecule. These high energy, outermost electrons are called the valence electrons.
Valence electrons react by either donating, accepting or sharing these outer shell electrons to create a stable atom or molecule. The number of electrons donated, accepted or shared (the valence) is the number of bonds that can be formed in a reaction.
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Valence Electrons and Groups
Reading valence on the periodic table centers around the organization of the groups (columns). The group numbers are listed above each column in the periodic table.
The elements in each group have similarities in the way they will react. This is because the elements in each group have the same number of energetic electrons in the outermost shell. They have the same number of valence electrons.
Groups 3-12 on the periodic table (the middle stripped square) has a more nuanced calculation of valence that is beyond the scope of the unit and my level of understanding honestly.
Valence Electrons and Valency
The number of valence electrons does NOT indicate the valency
This is a common misconception. Valence electrons refer to the number of electrons on the outermost shell.
Valency refers to the number of electrons the atom must gain, loss or share to form a stable atom.
Understanding this difference in terminology is key to making sense of valency calculations
How to Read Valency in 4 Steps
- Find the atomic number: total number of electrons in atom
- Assign the total number of electrons to each shell up to the maximum quantities of each: 2,8,8 (1st, 2nd, 3rd respectively)
- Determine the number of electrons in the outermost shell (valence electrons)
- Determine the number of electrons to gain, lose or share (the valency) to get an outer shell with the maximum number of electrons: 2,8,8 (1st, 2nd, 3rd respectively)
Less than 4 valence electrons: you will lose
Greater than 5 valence electrons: you will gain
4 electrons: you will share electrons
5 valence electrons: you can gain or lose
Exceptions
Hydrogen follows all the rules except gain/lose. Hydrogen has only 1 valence electron. While all other elements with less than 5 electrons will lose electrons to create a stable atom, hydrogen must gain to form a stable 1st outer shell of 2 electrons. It cannot lose the only electron it has!
Group 14 can neither gain nor lose electrons it will only share its 4 valence electrons to form a stable atom.
Group 15 is a point of transition in the periodic table in terms of valence. In group 15 the elements all have 5 valence electrons. They can gain OR lose their outermost electrons to achieve a stable outer shell of 8 electrons. They can gain 3 electrons OR lose 5 electrons to create a stable atom. Because of this the elements in group 15 can have a valence of 3 or 5.
Valency Calculations
(Swipe): The tables above provide a breakdown of the valency of elements in each group of the periodic table. This allows you to visualize the relationship between all the factors at play: atomic number, valence electrons and valency.
- Notice that valency correlates only to the number of electrons needed to create a stable atom.
- Notice that the valency correlates to the charge on the atom.
- Notice that the number of valence electrons does NOT equal valency.
All the values provided in these tables are derived using the 4 simple steps discussed above and summarized below:
The atomic number tells us the total number of electrons in the atom. Assign the electrons to their respective shells: 2,8,8 (1st, 2nd, 3rd respectively). Figure out the number of electrons on the outermost shell. This is your number of valence electrons. The valency is then simply the number of electrons it will take to maximize the electrons in the outer shell to form a stable atom.
I hope this unit will clear up the confusion about how we determine the valency. In future units you will see how a solid understanding of valency will allow you to track along with more complicated topics.
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