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Environmental Systems and Societies

What factors contribute to different biomes around the globe?

Insolation (sunlight), temperature, and precipitation are the main abiotic factors that influence the distribution of biomes on land. These factors together determine an area's climate. For example, a climate that is consistently sunny, hot, and wet will support a different ecosystem compared to one with seasonal changes in daylight, temperature fluctuations, and little rain. Similar ecosystems form in similar climatic conditions, and these groups are called biomes. Higher **insolation** boosts photosynthesis, which converts more sunlight energy into chemical energy. This leads to more growth of plants, increasing primary productivity and supporting a more diverse ecosystem. Warmer **temperatures** also raise productivity. Chemical reactions in organisms, such as photosynthesis and respiration, happen faster in higher temperatures. This results in more plant food for herbivores, more energy for herbivores to escape predators, and faster decomposition, which speeds up the cycling of important nutrients like carbon, nitrogen, and phosphorus. More **precipitation** provides a steady water supply for ecosystems. Water’s unique properties, like its ability to dissolve chemicals for reactions and its high heat capacity, which allows it to absorb and transport heat, are essential for life. Each of these abiotic factors has an optimal range, known as the "Goldilocks zone" which is just right for life to thrive. This is named after a girl in a story. She wants her porridge to be ‘not too hot, not too cold, but just right.’

Chemistry

Why is helium placed in group 18 on the periodic table?

Helium is placed in Group 18 with the other noble gases because of its chemical properties, even though its electron configuration might suggest it belongs in Group 2. With only two electrons in the $1s$ orbital $(1s^2),$ helium has a completely filled valence shell, making it extraordinarily stable and chemically inert. This complete valence shell gives helium the same fundamental characteristic as all other noble gases: it has no tendency to gain, lose, or share electrons under normal conditions. Like neon, argon, krypton, and xenon, helium exists as monatomic gas and forms no known stable compounds under standard conditions, making its chemical behavior identical to the other Group 18 elements. The periodic table is organized primarily by chemical properties rather than strict electron configuration patterns. If helium were placed in Group 2 above beryllium based solely on having two valence electrons, it would be grouped with the alkaline earth metals, which are highly reactive metals that readily lose two electrons to form $2+$ cations. This would be completely misleading since helium shares none of these properties. Instead, helium's placement in Group 18 correctly indicates that it is an unreactive noble gas. The key insight is that for helium, two electrons represent a full valence shell (the first shell only holds two electrons), while for beryllium, two electrons represent an incomplete second shell that can hold eight. This distinction between a full shell and simply having two electrons is crucial, and it is why helium rightfully belongs with the noble gases despite its unique electron configuration.

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