Video Transcript
In this video, we will learn how to determine the valence of different elements by using their electronic configurations and locations on the periodic table.
Atoms of different elements make up the matter that is all around us. Atoms form molecules and compounds when they undergo chemical reactions. An atom tends to react in a particular way with other atoms because of how its electrons are arranged. Electrons in atoms are arranged in energy levels. The first four energy levels are K, L, M, and N. Each energy level can hold a maximum number of electrons. For example, the energy level closest to the nucleus of an atom, which is the K energy level, can hold a maximum of two electrons. The next energy level, which is the L level, can hold a maximum of eight electrons.
Many atoms tend to end up with full outer energy levels during chemical reactions. For this to happen, some atoms tend to lose electrons from their outer energy level, while other atoms tend to gain electrons in their outer energy level. Some atoms may also get a full outer energy level by sharing electrons with other atoms. Usually, when the outer energy level of an atom is full, the atom is in a stable state. We know the K energy level can only hold a maximum of two electrons. So, the K level is full when it contains two electrons.
When the K level is the outermost energy level of the atom and it is full, the atom is in a stable duplet state. If the K level is not the outermost energy level in an atom, then the atom tends to react so that it ends up with eight electrons instead of two in its outermost energy level. For example, let’s say the outermost energy level of an atom is the L energy level. The L level is full when it contains eight electrons. The atom we are showing here has eight electrons in its outermost energy level, and this level is not the K level. So, this atom is in a stable octet state.
Energy levels M and N can hold greater than eight electrons. Atoms in which the M or N level is the outermost energy level still tend to end up with eight electrons in the outermost energy level after they react.
Now, we know that atoms tend to lose, gain, or share electrons during a chemical reaction and end up in the stable duplet or octet state. The number of electrons an atom starts with in its outermost energy level affects how it will react with other atoms. The valence of an atom is the number of electrons that an atom gains, loses, or shares when forming a bond during a chemical reaction.
Let’s take a look at the electronic configurations of a couple of atoms to help us understand how to determine the valence. An oxygen atom has an electronic configuration of 2,6. We can see that the outermost energy level of an oxygen atom is the L level. The L level in an oxygen atom contains six electrons. The oxygen atom would be in a stable octet state if the L level contained eight electrons. One way that an oxygen atom could get two more electrons in the L level is by gaining two electrons from another atom. When an oxygen atom gains two electrons, it becomes an oxide ion, which has a charge of two minus. We can see that the L energy level in the oxide ion now contains eight electrons. So, the oxygen atom has attained a stable octet state.
Another way that an oxygen atom could get two more electrons in its L energy level is by sharing two electrons with another atom. An oxygen molecule forms when two oxygen atoms each share two electrons and form a covalent bond. By looking closely at the diagram of the oxygen molecule, we can see that each oxygen atom is in a stable octet state. Each oxygen atom has four electrons in the L energy level that are not being shared and two electrons that are being shared for a total of eight electrons in the L energy level. Because an oxygen atom tends to gain or share two electrons when forming a bond during a chemical reaction, oxygen has a valence of two.
Now let’s consider a magnesium atom. A magnesium atom has an electronic configuration of 2,8,2. The outermost energy level of a magnesium atom is the M energy level. In a magnesium atom, the M energy level contains two electrons. The atom could be in a stable state if the M energy level had eight electrons. A magnesium atom could get eight electrons in the M energy level if it gained six electrons from another atom. But this is pretty unlikely. Instead, a magnesium atom tends to lose two electrons in a chemical reaction and form a magnesium ion, which has a charge of two plus.
We can see that the M energy level in the magnesium ion is empty, which is why it is not shown. But the L energy level is now the outermost energy level, and it contains eight electrons. So, the magnesium atom has attained a stable octet state. Because a magnesium atom tends to lose two electrons when forming a bond during a chemical reaction, magnesium has a valence of two.
Magnesium is a metal. In general, metal atoms tend to attain stable states as they lose electrons. On the other hand, nonmetal atoms like oxygen tend to gain or share electrons to attain stable states. But what about noble gases?
The noble gases are found in the very last column, or group, on the periodic table. They include helium, neon, argon, krypton, xenon, radon, and oganesson. Let’s take a look at the electronic configurations of the first three noble gases.
In a helium atom, there are two electrons in the K energy level. And since the K level is the outermost energy level of a helium atom, it is already in the duplet state. Now let’s look at a neon atom. The electronic configuration of a neon atom is 2,8. The outermost energy level is the L level. In a neon atom, the L level contains eight electrons. So a neon atom is already in a stable octet state. Finally, the electronic configuration of an argon atom is 2,8,8. The outermost energy level in an argon atom is the M level. In an argon atom, the M energy level contains eight electrons. So, just like a neon atom, an argon atom is already in a stable octet state. In fact, the atoms of the remaining four noble gases are also in stable octet states, although not much is known about oganesson.
Because atoms of noble gases are already in stable states, they generally do not gain, lose, or share electrons during chemical reactions. So, noble gases are considered inert, which means they tend to be unreactive. So, the valence of a noble gas atom is zero. Now that we know the valence of noble gas atoms, let’s take a look at the valence of atoms in other groups on the periodic table.
Elements in the same column or group on the periodic table tend to have the same valence. For example, we learned that the atoms of noble gases have a valence of zero. The noble gases are found in group 18. The element at the very bottom of this group is oganesson, and scientists do not know much about its properties yet. So, the valence of oganesson is not known. The halogens are found in group 17. The atoms of halogens have a valence of one. The last element in this group is tennessine, and not much is known about its properties. So, like oganesson, its valence in unknown.
The atoms of elements in group one also have a valence of one, and the atoms of group two elements have a valence of two. The atoms of some elements have multiple valences. So, these atoms can react in multiple ways and form different molecules and compounds. For example, nitrogen and phosphorus atoms can have a valence of three or five. A valence of three means these atoms can gain or share three electrons when chemically reacting to form new substances. A valence of five means that these atoms can also share five electrons.
Atoms of sulfur have three possible valences: two, four, and six. Many metals found in the middle section of the periodic table have multiple valences. Iron and copper are two examples. Copper has valences of one and two, while iron has valences of two and three. Special names are given to the different valences of metals like iron and copper. Iron with a valence of two is known as ferrous, while iron with a valence of three is known as ferric.
Now, we know that individual atoms may have one or more valences, but groups of atoms bonded together also have valences. Many chemical reactions involve atomic groups. An atomic group is a set of atoms joined together that have a combined valence and behave as one unit during a chemical reaction. Atomic groups are also known as polyatomic ions.
An example of an atomic group is the carbonate group. The formula of the carbonate is CO3 2−. C is the chemical symbol of the element carbon, and O is the chemical symbol of the element oxygen. We can see in the formula, that there is a subscript of three after the symbol for oxygen. This means that the carbonate group contains three oxygen atoms. There is an assumed subscript of one after the symbol of carbon. So, the carbonate group contains one carbon atom. The combined valence of carbonate is two.
Another atomic group that has a valence of two is the sulfate group. The formula of sulfate is SO4 2−. The phosphate group has the chemical formula PO4 3− and has a valence of three. Atomic groups that have a valence of one include ammonium, bicarbonate, hydroxide, nitrate, and nitrite. The valence of an atom or atomic group can be used to predict the chemical formula of a compound. For example, a sodium atom has a valence of one and the carbonate group has a valence of two. The valence of an atom or atomic group can be used to predict the chemical formula of a compound. However, we will not discuss the process in this video.
Now let’s take a look at a question.
A nonmetal on the right-hand side of the periodic table has an electronic structure of 2,6. What is the valence of this element?
First of all, valence is the number of electrons that an atom gains, loses, or shares when forming a bond during a chemical reaction. Atoms sometimes end up in a stable state when they react. A stable state may result from an atom gaining, losing, or sharing outermost energy level electrons. There are two types of stable states: the duplet state and the octet state.
An atom is in the duplet state if the outermost energy level is K and this level contains two electrons. An atom is in the octet state if the outermost energy level is not K and this level contains eight electrons. We are told that a nonmetal atom has an electronic structure of 2,6. Having an electronic structure of 2,6 means that the atom has two electrons in the K energy level and six electrons in the L energy level. This atom has a total of eight electrons, so its atomic number is eight. The element with an atomic number of eight is oxygen. The outermost energy level of an oxygen atom is the L level, and this level contains six electrons. This means that an oxygen atom needs two more electrons in the L level to attain a stable octet state.
Oxygen is a nonmetal. Nonmetal atoms tend to reach a stable state when they gain or share electrons with other atoms. An oxygen atom tends to gain or share two electrons during a chemical reaction. Therefore, the valence of an element with an electronic structure of 2,6 is two.
Now let’s summarize what we’ve learned with a few key points. Atoms sometimes attain a stable state as they gain, lose, or share electrons. An atom is in the duplet state if it has two electrons in the K energy level, and K is the outermost energy level. An atom is in the octet state if it has eight electrons in the outermost energy level, and the outermost energy level is not K.
Valence is the number of electrons an atom gains, loses, or shares in a chemical reaction. Atoms in the same group on the periodic table tend to have the same valence. The atoms of noble gases have a valence of zero, while the atoms of halogens have a valence of one. The atoms of elements in group one have a valence of one. And the atoms of elements in group two have a valence of two. Some atoms have more than one valence. For example, iron has valences of two and three, phosphorus and nitrogen have valences of three and five, and sulfur has valences of two, four, and six. Atomic groups are a set of atoms with a combined valence that act as a single unit during chemical reactions.
