The arrangement of electrons dictates how an element interacts with others. Electrons exist in distinct energy levels surrounding the nucleus. For a neutral atom of Strontium, with an atomic number of 38, there are 38 electrons that must be accounted for. Determining the number of electrons in the inner shells, known as core electrons, is a fundamental step in predicting Strontium’s role in forming compounds.
Locating Strontium on the Periodic Table
Strontium (Sr) is found on the periodic table by its atomic number, 38. This number indicates that a neutral Strontium atom contains 38 protons and 38 electrons. Strontium resides in Group 2, the Alkaline Earth Metals, characterized by metallic properties and reactivity. Its position in Period 5 signifies that Strontium’s electrons are distributed across five main energy shells. This placement provides the necessary framework for differentiating between the electrons involved in chemical reactions and those held securely close to the nucleus.
Defining Core and Valence Electrons
The 38 electrons in a Strontium atom are divided into two distinct categories: valence electrons and core electrons. Valence electrons are those located in the outermost occupied shell, which is the highest principal energy level. These outermost electrons are primarily responsible for chemical bonding and reactivity, as they are the most loosely held by the nucleus.
In contrast, core electrons are all the remaining electrons that occupy the inner, lower-energy shells. These electrons are tightly bound to the nucleus and are inert in standard chemical transformations. Core electrons function to shield the valence electrons from the full attractive force of the nucleus, influencing the atom’s size and ionization energy. The chemical properties of an element are largely determined by the number of valence electrons, while the core electrons establish a stable, inner electronic configuration. For any atom, the total number of electrons is the sum of the core electrons and the valence electrons. Identifying the number of valence electrons is thus a direct path to finding the core electron total.
Determining Strontium’s Electron Shell Arrangement
The specific arrangement of Strontium’s 38 electrons is detailed by its electron configuration, which follows the Aufbau principle. Electrons sequentially fill the lowest available energy levels first. Strontium’s full electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2\). This configuration shows that the electrons occupy orbitals across five principal energy levels, consistent with Strontium’s position in Period 5.
The final two electrons occupy the \(5s\) orbital, which represents the highest principal quantum number (\(n=5\)). These \(5s^2\) electrons are in the outermost shell and are identified as the valence electrons.
The condensed notation simplifies this sequence by replacing the inner, filled shells with the symbol for the preceding noble gas. The configuration \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\) is identical to the electron configuration of Krypton (Kr). This allows Strontium’s arrangement to be written concisely as \([\text{Kr}] 5s^2\). The two electrons in the \(5s\) orbital are the atom’s two valence electrons, a count characteristic of all elements in Group 2. Strontium typically loses these two outermost electrons during chemical reactions to achieve the stable, filled-shell configuration of a noble gas.
Calculating the Core Electron Total
Calculating the number of core electrons requires subtracting the valence electrons from the total electron count. A neutral Strontium atom has 38 total electrons and two valence electrons, which reside in the outermost \(5s\) orbital. The core electrons constitute the rest of the atom’s electronic structure. To find the total, the number of valence electrons is subtracted from the total number of electrons.
Performing the calculation: 38 total electrons minus 2 valence electrons equals 36 core electrons.
These 36 core electrons are distributed across the first four energy shells, making up the stable internal electron structure equivalent to the noble gas Krypton. Specifically, the core electrons completely fill the \(1s\), \(2s\), \(2p\), \(3s\), \(3p\), \(4s\), \(3d\), and \(4p\) orbitals.