Question

How many valence electrons does each of the following atoms possess? a. cesium, \(Z=55\) b. boron, \(Z=5\) c. krypton, \(Z=36\) d. magnesium, \(Z=12\)

Short answer

a. Cesium: 1 valence electron b. Boron: 3 valence electrons c. Krypton: 2 valence electrons d. Magnesium: 2 valence electrons

Step-by-step solution

Write the electron configuration

An element's electron configuration shows the distribution of electrons in its atomic orbitals. We can use the periodic table to determine the electron configuration by following the Aufbau principle: filling orbitals from lowest to highest energy levels (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p). a. Cesium: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^1\) b. Boron: \(1s^2 2s^2 2p^1\) c. Krypton: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10\) d. Magnesium: \(1s^2 2s^2 2p^6 3s^2\)

Identify the outermost energy level

After determining the electron configuration for each element, identify the outermost energy level, which contains the valence electrons. This is usually the highest principal quantum number (n) in the electron configuration. a. Cesium: 6 b. Boron: 2 c. Krypton: 5 d. Magnesium: 3

Count the valence electrons

Count the number of electrons in the outermost energy level to find the number of valence electrons for each atom. a. Cesium: One electron in the 6s level (\(6s^1\)): 1 valence electron b. Boron: Two electrons in the 2s level (\(2s^2\)) and one electron in the 2p level (\(2p^1\)): 2 + 1 = 3 valence electrons c. Krypton: Two electrons in the 5s level (\(5s^2\)) and six electrons in the 4d level (\(4d^{10}\)): 2 + 0 = 2 valence electrons d. Magnesium: Two electrons in the 3s level (\(3s^2\)): 2 valence electrons So, the number of valence electrons for each atom is: a. Cesium: 1 b. Boron: 3 c. Krypton: 2 d. Magnesium: 2

Key concepts

Electron Configuration

Understanding the electron configuration of an atom is crucial for predicting its chemical properties, such as reactivity and bonding. The electron configuration is a shorthand notation that shows the distribution and arrangement of electrons in an atom's orbitals. For example, the electron configuration for boron, with an atomic number of 5, is written as 1s^2 2s^2 2p^1. Here, the 1s^2 indicates two electrons in the first energy level 's' orbital, 2s^2 signals two electrons in the second energy level 's' orbital, and 2p^1 shows one electron in the second energy level 'p' orbital.

These configurations reflect how electrons fill orbitals based on the increasing energy level, which is crucial to discerning the behavior of atoms in chemical reactions. To offer a clear path to mastery, it's vital to emphasize the counting of electrons from the lowest to the highest energy level and the concept of 'shells' and 'subshells', which are designated by numbers and letters, respectively.

The practice of reading and interpreting electron configurations enables students to identify valence electrons—which are those occupying the outermost shell and participate in bonding—streamlining the study of periodic trends and chemical reactions.

Periodic Table

The periodic table is a key tool in chemistry, categorizing elements according to atomic number and electronic structure. It provides a visual framework to grasp the conceptual flow from lighter to heavier elements, along with trends in properties. Each row (period) on the periodic table signifies a new energy level filling with electrons, while each column (group) indicates elements with the same number of electrons in their outermost shell, sharing similar chemical characteristics.

For instance, the group including boron and aluminum contains elements that typically have three valence electrons. It's crucial when teaching this concept to walk through the periodic trends, such as atomic radius, electronegativity, and ionization energy—all of which are related to the configuration of valence electrons. An interactive approach, using color-coding or mnemonic devices, can greatly assist learners in developing a robust intuitive sense of the periodic table's structure and functionality.

Aufbau Principle

The Aufbau principle is an essential rule in understanding how electrons are structured within an atom. This principle states that electrons occupy the lowest energy orbital available. An analogy often used is that of a hotel where electrons are guests checking into rooms (orbitals) starting from the lowest floor (lowest energy level). Beginning with '1s', the lowest energy level, and moving upwards, this principle guides the electron configuration process.

Remembering this order can be tricky; thus, visualization tools such as diagrams or arrows indicating the sequence can be highly effective. Moreover, explaining exceptions to the rule where orbitals of similar energies are involved helps to deepen understanding. The use of the Aufbau principle to write electron configurations reinforces the order of fill and also brings clarity to the concept of valence electrons by spotlighting the outermost occupied orbitals. Exercising with a range of elements familiarizes students with patterns that appear in configurations, facilitating predictive insight into chemical behavior.