Question

Using only a periodic table, write the full electron configuration for each of the following elements. Circle the valence electrons. (a) Mg (b) \(P\) (c) \(\mathrm{K}\) (d) \(\mathrm{F}\) (e) Se (f) \(\mathrm{N}\)

Short answer

(a) Mg: [Ne]3s^2 (2 valence electrons) (b) P: [Ne]3s^23p^3 (5 valence electrons) (c) K: [Ar]4s^1 (1 valence electron) (d) F: [He]2s^22p^5 (7 valence electrons) (e) Se: [Ar]3d^{10}4s^24p^4 (6 valence electrons) (f) N: [He]2s^22p^3 (5 valence electrons)

Step-by-step solution

Identify the Atomic Number

To begin writing the electron configuration for an element, identify the atomic number of the element from the periodic table. The atomic number indicates the number of protons and, neutrally charged atom, the number of electrons in the atom.

Write the Electron Configuration

Write down the electron configuration starting from the 1s orbital and moving outwards to higher energy levels. Follow the order of 's', 'p', 'd', and 'f' orbitals, and use the periodic table to determine the filling sequence of the electrons across the periods and groups, adhering to the Aufbau principle, Pauli exclusion principle, and Hund's rule.

Circle the Valence Electrons

The valence electrons are the electrons in the outermost energy level of an atom that can participate in chemical bonding. After writing out the electron configuration, identify and circle the valence electrons. For main group elements, these are the electrons in the outermost 's' and 'p' orbitals.

Key concepts

Valence Electrons

Understanding the role of valence electrons is crucial in grasping the basics of electron configuration. Valence electrons are the electrons located in the outermost shell, or energy level, of an atom. They play a pivotal role in chemical reactions, as they are the electrons most likely to be involved in bonding with other atoms.

For example, in the periodic table, the group number for main group elements (not transition metals) often gives away the number of valence electrons. So, for group 1 elements (alkali metals), there is typically 1 valence electron; for group 2 (alkaline earth metals), there are 2, and this pattern continues up to group 18 (noble gases), which have 8 valence electrons (except for Helium, which has 2).

In an exercise where students are asked to circle the valence electrons, they would first deduce the total electron configuration and then identify the electrons in the outermost 's' and 'p' sublevels. Valence electrons are significant because they dictate the chemical properties of an element and are often a primary focus in understanding how atoms interact with one another.

Aufbau Principle

The Aufbau principle, a fundamental concept in quantum chemistry, guides the proper ordering of electron placement in an atom's orbitals. The name 'Aufbau' is derived from German, meaning 'building up' or 'construction.' The principle posits that electrons fill atomic orbitals starting with the lowest energy levels before moving to higher levels.

The order can be remembered using mnemonic devices or by referring to diagrams such as the Aufbau chart. This principle works in conjunction with the Pauli exclusion principle, which states that no two electrons can have the same set of quantum numbers, and Hund's rule, which dictates that electrons must fill degenerate orbitals singly before pairing up.

An understanding of the Aufbau principle is critical when determining the electron configuration of an element. For instance, when finding the electron configuration for magnesium (Mg), you would start filling from the 1s orbital up to the 3s orbital, where the last two valence electrons would reside. If we start with 1s, the configuration would be 1s² 2s² 2p⁶ 3s², where the 3s² electrons are the valence electrons.

Periodic Table

The layperson might see the periodic table simply as an organized chart of elements, but in the hands of a chemist, it is a powerful tool for predicting a multitude of elemental properties and behaviors. The arrangement of the elements in the periodic table is based on increasing atomic number, where similar chemical and physical properties recur periodically.

When working with electron configurations, the periodic table becomes an invaluable map. It delineates the order of orbital filling and can provide insights into the number of valence electrons, element categories, and even the possible types of chemical bonds an element can form. Elements are often grouped based on filled electron subshells; for example, the 's-block' includes groups 1 and 2, the 'd-block' contains transition metals, the 'p-block' encompasses groups 13 to 18, and the 'f-block' contains the lanthanides and actinides.

The periodic table's structure assists in predicting the electron configuration patterns of elements, making it a cornerstone of chemical education and understanding. Referring to the periodic table while circling valence electrons, as in the textbook exercise, allows students to connect the abstract concept of electron configuration with the visually structured representation of elements.