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Chemistry

What are exceptions to electron configurations?

Chromium (Cr) and copper (Cu) are the two main exceptions to expected electron configurations that IB Chemistry students need to know. Instead of following the normal filling pattern, chromium has the configuration $\ce{[Ar]} 4s^1 3d^5 $ rather than the expected $\ce{[Ar]} 4s^2 3d^4. $ Similarly, copper has $\ce{[Ar]} 4s^1 3d^{10}$ instead of $\ce{[Ar]} 4s^2 3d^{9}.$ In both cases, an electron from the $4s$ orbital is promoted to the $3d$ orbital, leaving only one electron in the $4s$ subshell rather than the expected two. This occurs because half$-$filled $(d^5)$ and fully$-$filled $(d^{10})$ $d$ subshells have special stability. The key principle behind these exceptions is that atoms adopt the electron configuration that gives them the lowest overall energy. While there are other exceptions in the periodic table (particularly among heavier transition metals), chromium and copper are the only ones IB chemistry students are expected to know and explain.

History

What were some new technologies used in the Hundred Years' War?

The Hundred Years’ War took place intermittently between 1337 and 1453 CE, and it is commonly divided into three phases, which were separated by long truces: the Edwardian War (1337–1360), the Caroline War (1369–1389), and the Lancastrian War (1415–1453). The war included a variety of factions from across Western Europe, but was predominantly fought between the kingdoms of England and France. Due to the war’s prolonged duration, a range of significant military technologies emerged and gained widespread adoption. The English invented their famous Longbow, which allowed archers to fire with relative accuracy at ranges over 200 yards, piercing armor and undermining the use of heavy cavalry. They were well documented for their effectiveness during the battles of Crécy (1346), Poitiers (1356), and Agincourt (1415). Early gunpowder weapons also gained greater use during the 13th and 14th centuries in Europe, though they were invented hundreds of years earlier in China. “Hand cannons” existed, but were generally less inaccurate and therefore less significant than bombards and full-sized cannons, which were used to break sieges and castle walls, thus making medieval fortifications more obsolete. Later in the war, the French army under Charles VII became known for its use of artillery. Improved warships and naval innovations were also a notable advance during the Hundred Years’ War. Specialized warships designed to transport troops rapidly and to allow for shipborne longbow volleys became decisive factors in various battles, such as the Battle of Sluys (1340). Finally, while not a “technology” in the traditional sense, the Hundred Years’ War saw the birth of an organizational innovation, the professional “standing” army. Charles VII of France established the first permanent standing army in Europe since the Roman Empire. Standardized equipment and training for an army under direct royal control enhanced military capabilities compared to the traditional feudal and mercenary armies, giving the French military a significant advantage.

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.

Psychology

Why can't causation be inferred from correlational studies?

Causation cannot be inferred from correlational studies because they do not control or manipulate variables. When conducting a correlational study, the researchers investigate the potential relationship between two or more pre-existing variables. Without manipulating and controlling variables, it is impossible for the researchers to rule out other explanations for any observed relationships. It is possible that a third unmeasured variable could be responsible for the relationship. For example, some correlational studies have indicated a relationship between coffee consumption and lung cancer. However it is entirely possible that people who drink more coffee might also smoke more. In short, correlational studies investigate relationships between variables, but because they lack the rigorous manipulation and control found in true experiments, cause-and-effect relationships cannot be established.

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.’

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