Problem 3 - Chemistry The Molecular Nature of Matter

Question

Using their positions in the periodic table, explain why is it not surprising that strontium-90, a radioactive isotope, replaces calcium in newly formed bones.

Step-by-Step Solution

Answer

Strontium (Sr) and Calcium (Ca) are in the same group on the periodic table, indicating they have similar chemical properties. Strontium's larger atomic size compared to calcium does not significantly hinder it from replacing calcium in bone formation, leading to the incorporation of strontium-90.

Step 1: Understand the Periodic Table Position

Strontium (Sr) and Calcium (Ca) are both alkaline earth metals and are located in the same group on the periodic table, Group 2. This means they have similar chemical properties due to having the same number of electrons in their outermost shell.

Step 2: Compare Atomic Size

Since Sr is positioned below Ca on the periodic table, Sr atoms are larger than Ca atoms. However, the size difference is not significant enough to prevent Sr from mimicking Ca in biological processes.

Step 3: Explain Isotope Behavior

Strontium-90 is a radioactive isotope, but its chemical behavior is similar to that of stable strontium, and thus similar to calcium. This makes it able to replace calcium in the body, including in the bone formation process.

Key Concepts

Alkaline Earth Metals

The alkaline earth metals, including beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), comprise the second group of the periodic table. These elements are characterized by having two electrons in their outermost energy level, which leads to similar chemical reactivity among them. For instance, they typically form +2 cations by losing these two valence electrons. Consequently, these metals are commonly found in compounds rather than in their pure elemental forms.

When it comes to human biology, calcium plays a crucial role in maintaining healthy bones and teeth. However, strontium can exhibit similar behaviors due to its comparable chemical properties. This mimicry is why substances like strontium-90 can find their way into biological systems and replace calcium. Despite the health risks associated with radioactive isotopes, their chemical likeness to calcium allows them to be inadvertently integrated into biological processes.

Chemical Properties of Elements

Elements within the same group in the periodic table possess similar chemical properties because of their analogous electron configurations. This is particularly true for elements in Group 2, or the alkaline earth metals, as they all have two electrons in their outer shell. The similar electron configurations mean that these elements tend to react in predictable ways, often forming similar types of compounds. For example, their typical reaction with water releases hydrogen gas and results in the formation of a hydroxide.

Moreover, the reactivity of Group 2 elements increases as you move down the group, with beryllium being the least reactive and radium the most. This trend occurs due to the increasing ease with which these elements can lose their two valence electrons, a key factor in their chemical behavior. Applying this understanding helps explain why strontium-90 can perform analogously to calcium in living organisms - the underlying principle being their shared chemical properties.

Radioactive Isotopes in Biology

Radioactive isotopes, or radioisotopes, are variants of elements that have unstable atomic nuclei. These isotopes decay over time, releasing radiation in the form of alpha particles, beta particles, or gamma rays. The biological implications of radioactive isotopes can be profound, as their radioactivity can cause damage to living tissue and DNA, leading to various health consequences.

However, radioisotopes are not only associated with risk. They are also valuable tools in medical diagnosis and treatment. For instance, iodine-131 is used to treat thyroid disorders, and technetium-99m is crucial in nuclear medicine imaging. When examining strontium-90, we see that its behavior as a radioisotope is particularly concerning because it directly replaces calcium in bone tissue, due to the way biological systems are often 'tricked' by chemical similarity. This replacement can lead to harmful effects, as the radioactive decay of strontium-90 can cause damage to the bone marrow and potentially lead to disorders such as leukemia.