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Variable Valency & Covalency in Chemistry

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we first review the meaning of valence and covalence for the elements. We then describe how some elements can have multiple different valences and covalences. In other words, some elements do not always form the same number of bonds or have the same charge (if they form ions). We will explain the differences for several elements and some of the compounds they form, and how this relates to atomic structure.

Introduction

Imagine that you---yes, you---are elected to public office, maybe even as president! You may spend much of your time at the office doing political things as part of your job, but at the end of the day you probably come home to friends and family and are expected to act differently in that setting. The chemical elements are similar: depending on 'who' they are around, they take on different roles. Specifically, we think about some elements having different valences. A valence is the number of electrons an atom of an element either donates or accepts from another element in order to achieve an octet, 8 valence electrons, or 2 valence electrons if the element is H, He, Li, or Be. In the discussion below, we will ignore the transition metals in groups 3-12 and only talk about the main group elements, that is, groups 1-2 and 13-18.

Valence: What Does it Mean?

Let's first discuss elements that almost always have the same valences. These are the elements in the first two periods of the Periodic Table, shown below:


Periodic Table


We could say that these elements spend all of their time at the office (as it were) and have no life outside of work. Whenever we see Na in compound, we know that its valence will be a +1, meaning that Na will always give one electron when forming a chemical bond. For example, in the compound Na2 CO3, Na will definitely give one electron, so we assign it a +1 valence. Remember: the reason Na is willing to do this is to form a full octet, thereby attaining the electron configuration of its nearest noble gas, Ne. Note that H, an element in group 1 of the periodic table, is not an alkali metal; it is a non-metal. Because of this fact, H usually has a valence of +1, but occasionally it has a valence of -1, particularly when it combines with metals. This is because metals are better at giving electrons--that is, having a positive valence. The group 2 elements have a valence of +2. The group 13 elements have a valence of +3.

The trend starts to reverse, in a way, when we reach group 14. For group 14, which includes C, note that the elements in this group can either give or take electrons in order to reach an octet. In our Na2 CO3 example, we hold off on assigning the valence to C just yet. Instead, we note that O is in group 16. Since it's easier to just gain two electrons to fill its octed, rather than lose 6, O has a valence of -2. O easily gains these electrons because it is a fairly electronegative element. Similarly, elements in group 15, 17, and 18 usually have valences of -3, -1, and 0. The noble gases usually prefer not to react at all! C has a choice to either give or take electrons, but considering that we have 3 O atoms, 2 Na atoms, and 1 C atom in Na2 CO3, we determine that C must have a valence of +4 in this compound such that the total valence equals the total charge on the (neutral) compound.

Covalence and Chemical Bonds

In the previous example, we assumed that atoms either give or take electrons to form chemical bonds. In reality, many compounds exist where electrons are not completely transferred from one element to another. We call this type of chemical bonding covalent bonding, in which electrons are shared between atoms of various elements. We still assign valence to elements in the normal way. When talking about atoms in a compound, we must understand that assigning valences is basically a 'bookkeeping' device for keeping track of where the electrons spend most of their time. An atom's covalency tells us how many electrons the atom can donate in order to form covalent bonds. When drawing Lewis structures, each atom participating in a covalent chemical bond donates one electron to that bond. Therefore, the most covalent bonds we might expect for an element is 8. In reality, however, we remember the trend in valence as we move from group 1 to group 4 of the Periodic Table: a maximum of four electrons can be given or taken. Whether as an individual atom or as a participant in a chemical bond, an element 'wants' to do the least amount of electron giving/taking as possible to achieve its octet. Thus, elements in groups 1-4 will usually form 1, 2, 3, or 4 chemical bonds in the molecules they form; they have covalences of 1, 2, 3, or 4. Group 15-17 elements typically form 3, 2, or 1 chemical bonds in their molecules and have covalences of 3, 2, or 1. We do not assign a '+' or '-' for covalence because the electrons are shared. Group 18 elements rarely form chemical bonds. Why do you think this is?

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