Question

Look for selenium in the periodic table. (a) Write the complete electron configuration for an atom of selenium. (b) Write the shorthand electron configuration for an atom of selenium. (c) How many valence electrons does selenium have? (d) Underline the valence electrons in the shorthand electron configuration.

Short answer

The complete electron configuration for selenium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴. The shorthand configuration is [Ar] 4s² 3d¹⁰ 4p⁴, and selenium has 6 valence electrons. The valence electrons in the shorthand configuration are underlined: [Ar] 4s² 3d¹⁰ \textbf{4p⁴}.

Step-by-step solution

Identify Selenium in the Periodic Table

Firstly, locate selenium in the periodic table. Selenium has the atomic number 34, which means that a neutral selenium atom has 34 electrons.

Determine the Electron Configuration

Next, write out the complete electron configuration for selenium. Start from the lowest energy level, filling each subshell according to the Aufbau principle, the Pauli exclusion principle, and Hund's rule until all 34 electrons are accounted for. The orbitals are filled in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Therefore, the complete electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.

Write the Shorthand Electron Configuration

For the shorthand electron configuration, find the previous noble gas, which is argon (Ar) with atomic number 18, and use its symbol to represent the core electrons. The configuration beyond argon will be added after the noble gas symbol. Thus, the shorthand electron configuration for selenium writes as [Ar] 4s² 3d¹⁰ 4p⁴.

Determine the Number of Valence Electrons

Valence electrons are the electrons in the outermost energy level of an atom. For selenium, the electrons in the 4s and 4p orbitals are considered valence electrons. By adding them up we have 2 (from 4s) + 4 (from 4p) resulting in a total of 6 valence electrons.

Identify and Underline the Valence Electrons in the Shorthand Configuration

In the shorthand configuration [Ar] 4s² 3d¹⁰ 4p⁴, the valence electrons are the ones included after the noble gas notation [Ar]. Therefore, underline the 4s² 3d¹⁰ 4p⁴ portion of the configuration to signify that these are the valence electrons in selenium's shorthand electron configuration.

Key concepts

Understanding the Periodic Table

The periodic table is a fundamental tool in the study of chemistry, organizing all known elements in order of increasing atomic number. Elements are arranged into rows called periods and columns known as groups. Similar properties recur periodically when elements are arranged by atomic number, primarily because elements in the same group have the same number of electrons in their outermost shell, known as valence electrons.

When looking at the arrangement, one can notice that elements are also divided into blocks, corresponding to the subshell that is being filled with electrons during the electron configuration: s, p, d, and f blocks. The properties of each element, including reactivity and the type of bonds it can form, are vastly influenced by its electron configuration. Thus, understanding the layout of the periodic table aids immensely in predicting how elements will behave in chemical reactions.

Valence Electrons and Their Importance

Valence electrons are the electrons that reside in the outermost shell of an atom. These electrons play a pivotal role in chemical reactions, as they are the ones involved in forming chemical bonds. For selenium, an element located in group 16 of the periodic table, it is the electrons in the 4s and 4p orbitals that are the valence electrons.

Selenium, like other elements in its group, typically has six valence electrons. This configuration influences selenium's chemical properties, including its ability to gain or share electrons during reactions. Recognizing the number of valence electrons helps predict the types of bonds selenium will form and its reactivity with other elements.

The Aufbau Principle Explained

The Aufbau Principle is a fundamental concept in quantum chemistry, providing a guideline for predicting the ground state electron configuration of an element. According to this principle, electrons are added to an atom's orbitals in order of increasing atomic energy level. The order in which orbitals are filled is based on their relative energies, starting with the lowest energy orbital, 1s, and proceeding to higher energy levels in a defined sequence.

The sequence, often remembered with the help of a diagram or mnemonic, is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on, following the pattern implied by the Madelung rule. In the case of selenium, this principle dictates the electron configuration process, filling the lower energy orbitals before the higher ones, resulting in its ground state electron configuration.

Noble Gas Shorthand Notation

The noble gas shorthand notation is a simplified way of representing an element's electron configuration. Instead of writing out the entire configuration from the 1s orbital, the nearest noble gas preceding the element in the periodic table is used to represent the filled electron shells. This method simplifies the electron configuration, making it shorter and easier to read.

For selenium, the noble gas prior to it in the periodic table is argon (Ar), with an atomic number of 18. The shorthand notation starts with the symbol [Ar], followed by the electron configuration from that point forward. In selenium's case, the shorthand is [Ar] 4s² 3d¹⁰ 4p⁴. This notation accentuates the valence electrons because they determine the chemical behavior of the atom and are typically the only electrons involved in chemical bonding.