High School Chemistry/Chemical Family Members

Combining what we have learned from the previous lessons, we can now determine how the core electrons and the valence electrons relate to the properties of families in the periodic table. We have looked at the trend of the number of valence electrons within each group and now we will expand this to include a look at the trends that exist between the sublevel of valence electrons and the relative ability of an element to react in a chemical reaction. We will explore the possibility of predicting chemical reactivity based on the number of valence electrons and the sublevel to which they belong. Let's begin our lesson with a review of the core electrons and discuss their application in chemical reactions. Following this, we will have a similar discussion about valence electrons.

Lesson Objectives

• Explain the role of the core electrons.
• Explain the role of valence electrons in determining chemical properties.
• Explain how the chemical reactivity trend in a chemical family is related to atomic size.

Core Electrons Can be Ignored in Determining an Element's Chemistry

As learned in an earlier lesson, the core electrons are the inner electrons found in the electron configuration for the element. These electrons fill up the inner sublevels and are thus not responsible for bonding and are not involved in chemical reactions. The core electrons also are not directly responsible for determining the properties of the elements. Remember that the core electrons represent all of the electrons except for the valence electrons.

Look at the electron configuration for beryllium (Be).

Be: 1s²2s²

There are two core electrons (1s²) and two valence electrons (2s²).

When we look at the electron configuration for Selenium (Se), we see the following.

Se: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁴

There are 28 core electrons (1s²2s²2p⁶3s²3p⁶3d¹⁰) and six valence electrons (4s²4p⁴). Again, none of the core electrons from either of these elements, or any element for that matter, will participate in chemical reactions.

Sample Question What are the core electrons in each of the following? (a) O: 1s22s22p4 (b) In: [Kr]5s24d105p1 Solution: (a) O: core electrons = 1s2, therefore there are two of them. (b) In: indium has three valence electrons and all the rest are core electrons. The core electrons include all the electrons in the energy levels below n = 5. (4d10 are core electrons since they are held in a lower energy level.)

Valence Electrons Determine Chemical Properties

The valence electrons, unlike the core electrons, are responsible for all of the chemical reactions that take place between elements. They determine the properties of the elements. We have already learned that all metallic elements in the main group elements are able to lose their valence electrons since the energy requirements to do so are relatively low. The non-metallic elements (other than noble gases) are able to gain electrons readily because of high electron affinities. For elements in Group 3A, the atoms will have to lose three electrons to have electron configurations similar to the previous noble gas. For elements in Group 6A, they will have to gain two electrons to have similar electron configurations as their nearest noble gas. Earlier we determined that all elements in the same group have the same Lewis electron dot diagram. What this means is that all elements in the same group have the same number of valence electrons and will react the same way to gain or lose electrons when participating in reactions. The number of valence electrons determines what types of chemical properties the elements will have.

When you look at Table 9.11, you can see that the Group 1A metals all have the same number of valence electrons and also have the same appearance. These metals also react the same way under similar conditions.

Table 9.11: Electron Configuration for Group 1A Elements

Element	Electron Configuration
Li	$1s^{2}2s^{1}$
Na	$1s^{2}2s^{2}2p^{6}3s^{1}$
K	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}$
Rb	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{1}$
Cs	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{1}$
Fr	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{2}4f^{14}5d^{10}7s^{1}$

We can see that all of the elements in the Group 1A metals all have one valence electron in their outer energy levels. This means that they can lose this one electron and become isoelectronic with a noble gas configuration. Thus, elements in Group 1A will readily lose this electron because it takes very low energy to remove this one outer electron.

As the atomic number increases for the elements in the alkali metal family group, the valence electron is further away from the nucleus. The attraction between the valence electron and the nucleus decreases as the atomic size increases. The further the electrons are away from the nucleus, the less hold the nucleus has on the electrons. Therefore the more readily the electrons can be removed and the faster the reactions can take place. So, the valence electrons will determine what reactions will occur and how fast they will occur based on the number of electrons that are in the valence energy level. All Group 1A elements will lose one electron and form a 1+ cation.

Looking at Table 9.12 for the electron configuration of the Group 2A metals, we see that the outer energy level holds two electrons for each of the metals in Group 2A.

Table 9.12: Electron Configuration for Group 2A Elements

Element	Electron Configuration
Be	$1s^{2}2s^{2}$
Mg	$1s^{2}2s^{2}2p^{6}3s^{2}$
Ca	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}$
Sr	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}$
Ba	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{2}$
Ra	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{2}4f^{14}5d^{10}7s^{2}$

As with the Group 1A metals, the Group 2A metals will form cations by losing the s electrons, but this time they will lose two electrons. And, as with the Group 1A elements, the elements are more reactive as the atomic number increases because the s electrons are held further away from the nucleus.

Consider the electron configurations for the elements in family 7A (the halogens).

Table 9.13: Electron Configuration for Group 7A Elements

Element	Electron Configuration
F	$1s^{2}2s^{2}2p^{5}$
Cl	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}$
Br	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{5}$
I	$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{5}$

The elements in group 7A were placed in the same chemical family because they all have similar chemistry, that is, they react with other substances in similar ways and their compounds with the same metals have similar properties. Now we know why they have similar chemical properties… because chemical properties are controlled by the valence electrons and these elements all have the same configuration of valence electrons.

The trend for chemical reactivity (speed of reaction) for the non-metal families is the reverse of the situation in the metal families. The metallic families lose electrons and the largest atoms lose their electrons most easily so the larger the atom, the faster it reacts. In the case of the non-metals, such as the halogens, the atoms react by taking on electrons. In these families, the smaller atoms have the largest electron affinity and therefore take on electrons more readily. Hence, in the non-metals, the smaller atoms will be more reactive.

Lesson Summary

• Core electrons are the inner electrons that are in filled orbitals and sublevels.
• Core electrons are not responsible for chemical reactivity and do not participate in chemical reactions.
• Valence electrons are the outermost electrons, are responsible for determining the properties, and are the electrons that participate in chemical reactions.
• Because the members of each group in the main group elements has the same number of valence electrons, there are similar properties and similar trends in chemical reactions found in the group.
• For metals, chemical reactivity tends to increase with increases in atomic size because the outermost electrons (or the valence electrons) are further away from the nucleus and therefore have less attraction for the nucleus.

Review Questions

1. What is the difference between valence electrons and core electrons?
2. Why would the valence electrons be responsible for the chemical reactivity?
3. Which of the following pairs of elements would have the greatest similarities in terms of chemical properties?

   (a) Ca and Ca²⁺

   (b) Ca and Mg

   (c) Ca and K

   (d) Ca and Ar

4. What is the correct number of core electrons in the phosphorous atom?

   (a) 3

   (b) 5

   (c) 10

   (d) 15

5. What is the correct number of valence electrons in the iodine atom?

   (a) 4

   (b) 5

   (c) 6

   (d) 7

6. For the valence electrons in Group 6A, what conclusions can you draw about the trend in chemical reactivity?
7. For the valence electrons in Group 7A, what conclusions can you draw about the trend in chemical activity?

Vocabulary

noble gas core

When working with noble gas electronic configurations the core electrons are those housed in the noble gas symbolic notation.

This material was adapted from the original CK-12 book that can be found here. This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License