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THINK ABOUT IT

Lithium, Li, and sodium, Na, are both located in Group 1A on the periodic table because they have very similar properties. Both are soft, silvery metals that react with water. Both form compounds with chlorine in a 1:1 ratio—lithium chloride, LiCl, and sodium chloride, NaCl. What do lithium atoms have in common with sodium atoms that make them behave similarly?

Why do elements in the same group in the periodic table have similar properties?

To answer this question, you will explore

EXPLORING THE TOPIC

Electron Shells

Recall that elements that are grouped together in columns on the periodic table share similar properties.

Consider lithium, Li, and sodium, Na, from Group 1A on the periodic table. You can get the following information from these two element squares on the periodic table:

This basic information doesn’t provide any evidence of similarities between atoms of lithium and atoms of sodium. However, examining the structures of these atoms in more detail reveals some patterns.

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In these models, the electrons are shown orbiting in concentric circles around the nucleus. Chemists call these circles electron shells, and these atomic models are called shell models.

Lithium has a total of three electrons in two shells. Sodium, on the other hand, has a total of 11 electrons in three shells. But the atoms of both elements do have one thing in common: a single electron in their outer shells. If this feature is responsible for the similar properties of lithium and sodium, you would expect the atoms of the other elements in the same group to also have a single electron in their outer shells.

The atoms of two more Group 1A elements are shown in the illustration. Compare the models of hydrogen and potassium with those of lithium and sodium. The pattern holds: Each of the Group 1A elements has one electron in the outer shell of its atoms. This pattern also extends to other Group 1A elements not shown here, such as rubidium and cesium.

Patterns in Atomic Structure

Examining the shell models of elements beyond lithium on the periodic table reveals further patterns in the outermost electrons. Shell models for the first 14 elements are shown in this illustration. A repeating pattern emerges from the diagram.

Starting with lithium, notice how the number of electrons in the outer shell repeats in a regular pattern, going from one to eight and then starting over again.

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When these electron shell models are organized according to the periodic table, even more is revealed. Examine this table. The atoms in the first row, or Period 1, have only one shell. The atoms in Period 2 have two shells. The atoms in each new period of the periodic table have an additional electron shell.

It turns out that for main group elements, Groups 1A through 8A, everything you need to know about the placement of an atom’s electrons can be decoded from the periodic table.

In the first column of the table, every element has one electron in its outermost electron shell. In the second column, each has two electrons in the outermost shell, and so on. In the eighth column, each element has eight electrons in its outermost shell. This feature turns out to be the key to many of the properties of the atoms.

Valence and Core Electrons

The outermost shell that electrons occupy is very important in chemistry and is referred to as the valence shell. The electrons in that shell are called the valence electrons. The electrons that are located in all of the inner shells are known as core electrons.

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For main group elements, the number of electrons in the valence shell corresponds to the group number on the periodic table. So, every Group 4A element has four electrons in its outermost shell, while every Group 8A element has eight electrons in its outermost shell.

Elements with the same number of electrons in their outermost shells have similar properties. In other words, elements with the same number of valence electrons have similar properties.

LESSON SUMMARY

Why do elements in the same group in the periodic table have similar properties?

Elements that are grouped together in columns on the periodic table share similar properties. For main group elements, atoms in the same group have the same number of electrons in their outermost shells. The outermost shell is called the valence shell, and the electrons in that shell are called valence electrons. Electrons in the inner shells are called core electrons. The number of valence electrons in an atom of a main group element corresponds to the element’s group number. So, the periodic table is a direct reflection of the atomic structures of the atoms.