Strontium (Sr) is a soft, silver-white alkaline earth metal identified by the atomic number 38. Found in Group 2 of the periodic table, its chemical behavior is rooted in its internal atomic structure. To understand how strontium interacts with other substances, we explore the arrangement of its 38 electrons within discrete energy levels, or shells.
Understanding Atomic Energy Levels
Electrons orbit the central nucleus within specific, fixed regions known as energy levels or electron shells. These shells represent discrete, quantized amounts of energy an electron can possess. Electrons cannot exist in the space between these levels, only at the allowed energy states.
These energy levels are sequentially numbered using the principal quantum number, \(n\), starting closest to the nucleus with \(n=1\). As the number increases, the shell is located farther from the nucleus and the electrons possess greater energy. Electrons naturally fill these shells starting from the lowest energy level until they reach the maximum capacity.
Strontium’s Electron Shell Structure
Strontium is located in the fifth period of the periodic table, which indicates it possesses five principal energy levels. The neutral strontium atom has 38 electrons distributed across these five distinct energy shells, designated as \(n=1\) through \(n=5\).
The distribution of the 38 electrons across the five shells is \(2, 8, 18, 8, 2\). The innermost shell (\(n=1\)) holds two electrons. The electron configuration terminates with the \(5s^2\) orbital, confirming that the highest occupied shell is the fifth energy level.
How Strontium’s Outer Shell Determines Reactivity
The chemical behavior of an element is governed by the electrons in its outermost energy level, referred to as the valence shell. For strontium, this valence shell is the fifth energy level (\(n=5\)), containing two electrons in the \(5s\) orbital. This configuration classifies strontium as an alkaline earth metal in Group 2.
Strontium is highly reactive because it can easily lose its two valence electrons to achieve the stable configuration of the noble gas Krypton. Upon losing these two electrons, the strontium atom forms a positively charged ion, \(\text{Sr}^{2+}\), which has a stable outer shell containing eight electrons. This propensity to readily donate two electrons dictates strontium’s chemical reactions, such as its tendency to form ionic compounds.