Chemistry: Elements and Reactions

Explanation: The symbol Au comes from the Latin "aurum". Gold (atomic number 79) is a precious metal known since antiquity for its resistance to corrosion.

Explanation: H₂O means that each water molecule contains 2 hydrogen atoms and 1 oxygen atom bonded by covalent bonds.

Explanation: Dmitri Mendeleev published his periodic table in 1869, classifying elements by atomic mass and predicting the existence of elements not yet discovered.

Explanation: The periodic table has 118 elements, the latest being Oganesson (Og, number 118), officially recognized in 2016.

Explanation: Nitrogen (N₂) makes up 78% of the atmosphere, followed by oxygen (21%), argon (0.9%), and CO₂ (0.04%). It is an inert gas essential to the nitrogen cycle in nature.

Explanation: Sulfuric acid (H₂SO₄) is used in lead-acid batteries. It is one of the most industrially produced acids in the world.

Explanation: Hydrogen is the lightest and most abundant element in the universe. Its atom contains only one proton and one electron.

Explanation: An exothermic reaction releases energy in the form of heat. Conversely, an endothermic reaction absorbs heat.

Explanation: A pH of 7 is neutral (neither acidic nor basic). Pure water at 25°C has a pH of 7. Below is acidic, above is basic.

Explanation: Carbon dioxide (CO2) is the main human-caused greenhouse gas, primarily from burning fossil fuels. Methane and water vapor also contribute.

Explanation: Mercury (Hg) is the only metal that is liquid at room temperature. Its use in thermometers is now banned due to its toxicity.

Explanation: Sodium chloride (NaCl) is composed of sodium ions (Na⁺) and chloride ions (Cl⁻). It is the most widely used salt in cooking and industry.

Explanation: Carbon can form 4 covalent bonds, allowing for complex chains and rings. It is the basis of all organic chemistry and life.

Explanation: Marie Curie discovered these two radioactive elements with her husband Pierre. She remains the only person to have received two Nobel Prizes in two different sciences.

Explanation: Covalent bonds involve the sharing of electrons between atoms. In H₂O, oxygen shares electrons with both hydrogens.