Elements, compounds and mixtures Study Guide

The way an element reacts chemically is directly linked to the arrangement of electrons in its atoms, particularly the electrons in the outermost occupied shell, known as valence electrons. Atoms strive to achieve a stable electron configuration, typically a full outer electron shell, which usually contains eight electrons (the octet rule, though the first shell is full with two electrons). Elements with incomplete outer shells tend to react to gain, lose, or share electrons to attain this stability. The number of protons in the nucleus defines an element and is called its atomic number. In a neutral atom, the atomic number also equals the number of electrons. Therefore, the atomic number dictates the total number of electrons, which in turn determines the electron shell configuration and the number of valence electrons. Elements in the same group of the Periodic Table have the same number of valence electrons and thus exhibit similar chemical properties. For example, Group 1 metals (alkali metals) all have one valence electron, which they readily lose to form a positive ion with a +1 charge. This loss of an electron makes them highly reactive. Conversely, Group 17 non-metals (halogens) all have seven valence electrons and readily gain one electron to form a negative ion with a -1 charge, also making them highly reactive. Group 0 elements (noble gases) have a full outer shell (eight valence electrons, or two for helium), which makes them very stable and unreactive under normal conditions. This fundamental relationship between atomic number, electron arrangement, and reactivity underpins all chemical reactions.

Nanoparticulate materials are substances with at least one dimension between 1 and 100 nanometres. Due to their extremely small size, they exhibit unique properties compared to their bulk counterparts, which makes them useful in various applications like sunscreens, cosmetics, catalysts, and drug delivery systems. However, this small size also leads to potential risks that are not fully understood. Nanoparticles have a very high surface area to volume ratio, making them highly reactive. Their small size allows them to easily enter the body through inhalation, ingestion, or skin absorption, potentially bypassing the body's natural defence mechanisms. Once inside, they can travel through the bloodstream, accumulating in organs like the lungs, liver, and brain, where their chemical reactivity could cause cellular damage, inflammation, or interfere with biological processes. For example, some nanoparticles have been shown to induce oxidative stress, leading to DNA damage or cell death. The long-term effects of exposure are currently unknown and require further research. Environmental concerns also exist, as nanoparticles could accumulate in water systems or soil, impacting ecosystems. Therefore, careful risk assessment and regulation are crucial for the safe development and use of nanoparticulate materials.