Question

Regarding the representative elements: (a) What does knowing the group number tell you about an element's valence electrons? (b) What does knowing the period number tell you about an element's valence electrons?

Short answer

(a) Knowing the group number of a representative element tells us the number of valence electrons the element possesses, as all elements in a group have the same number of valence electrons. (b) Knowing the period number tells us the energy level where the valence electrons are present in an element, as elements in the same period have the same number of electron shells.

Step-by-step solution

Understanding Periodic Table and Valence Electrons

Before diving into the exercise's questions, let's briefly understand the concept of valence electrons and the periodic table. The valence electrons are the electrons in an atom's outermost shell, which contribute to the chemical properties of the element. The periodic table is a tabular arrangement of the chemical elements organized based on their atomic number, electron configurations, and recurring chemical properties. Now, let's discuss how the group number and period number relate to an element's valence electrons.

(a) Group Number and Valence Electrons

The group number in the periodic table represents the vertical columns. All elements in a group of representative elements or main groups have the same number of valence electrons. The group number of representative elements (1A to 8A or 1 to 18) provides information about the number of valence electrons in those elements. For example, for Group 1A (1) or alkali metals, all elements in this group have one valence electron in their outermost shell. Similarly, elements in Group 2A (2) or alkaline earth metals have two valence electrons, and so on until Group 8A (18) or noble gases have eight valence electrons (except helium, which has only two valence electrons). So, knowing the group number tells us about the number of valence electrons an element possesses.

(b) Period Number and Valence Electrons

The period number in the periodic table represents the horizontal rows. Elements in the same period have the same number of electron shells, where valence electrons reside. As the period number increases from 1 to 7, elements in the same period have the same number of electron shells (or energy levels). Valence electrons always occupy the outermost shell of the atom. So, knowing the period number tells us about the energy level in which valence electrons are present. In summary, the period number informs us about which electron shell the valence electrons are found in an atom.

Key concepts

Understanding the Periodic Table

The periodic table is a powerful tool used by chemists to understand and predict the properties of elements. It displays all known chemical elements in an organized manner.
The table is arranged in order of increasing atomic number, which tells us the number of protons in an atom's nucleus. This arrangement helps highlight periodic trends in various properties, like reactivity and atomic radius, allowing us to see the patterns in element behavior.
The structure of the periodic table can be divided into vertical columns and horizontal rows:

• **Groups (Columns):** Indicate elements with similar properties and the same number of valence electrons.
• **Periods (Rows):** Show the trend in properties and energy levels as you move across the table.

In this way, the periodic table isn't just a list of elements; it's a roadmap that connects their structure and properties.

Decoding Group Number

In the periodic table, the group number holds vital clues about an element's valence electrons. As you glance through the columns or groups, each group's elements share a similar number of valence electrons. This consistency within a group explains the similar chemical behavior of its elements.
For example:

• **Group 1 (1A):** Elements here, like lithium and sodium, have one valence electron. They are known as alkali metals and are highly reactive.
• **Group 18 (8A):** The noble gases have eight valence electrons (or a full outer shell) making them mostly inert.

The group number can be directly used to predict the number of valence electrons for main group elements (1A to 8A). This explains why elements in the same group exhibit similar chemical characteristics, aiding chemists in predicting reactions and bonding behavior.

Role of the Period Number

The period number on the periodic table offers insight into the energy levels of an element's electrons. As you move across a period, the number of electron shells or energy levels increases.
Elements in the same period have their valence electrons in the same principal energy level. This commonality plays a significant role in determining their size and how they react with other elements.
Key points about periods:

• **Period 1:** Contains only two elements, hydrogen and helium, which have electrons only in the first shell.
• **Period 2 and beyond:** As periods increase, more electrons and energy levels are involved, affecting atomic size and other properties.

Knowing the period number allows us to understand the layer (or shell) where valence electrons are located, contributing to an element’s behavior in terms of energy and reactivity.