Periodic table

Sodium is an alkali metal with one loosely held outer electron, so it reacts very quickly with water to form sodium hydroxide and hydrogen gas. The reaction is exothermic enough to make the hydrogen ignite.

Noble gases are mostly unreactive because their outer electron shells are already full. That stable arrangement makes them much less likely to form bonds.

They become more reactive because the outer electron is farther from the nucleus and easier to remove. As you go down Group 1, atoms are larger and hold that electron less tightly.

Fluorine is extremely reactive because it is very eager to gain one electron and complete its outer shell. Its small size and high electronegativity make it one of the most aggressive elements chemically.

Halogens react readily because they are one electron short of a full outer shell. That makes them highly likely to form salts and other compounds.

Noble gases are very unreactive because their outer electron shells are already full. That stable electron arrangement makes them reluctant to gain, lose, or share electrons.

Transition metals often have partially filled d-orbitals that absorb specific wavelengths of light. The remaining light is what gives their compounds vivid colors.

Fluorine is the most reactive halogen because it strongly attracts electrons and is very small, so its nucleus pulls in bonding electrons intensely. It has the highest electronegativity of any element, about 3.98 on the Pauling scale.

Gold resists corrosion because its electrons are arranged in a way that makes it hard to oxidize. Relativistic effects also stabilize gold’s 6s electrons, helping keep it shiny and chemically stubborn.

Sodium reacts explosively because it loses its outer electron very easily, producing heat and hydrogen gas. The heat can ignite the hydrogen, causing flames or even an explosion.

Iron rusts because it reacts with oxygen and water to form iron oxides. Moisture speeds up the process, which is why rusting is faster in damp air or saltwater.

Aluminum forms a thin, tough oxide layer almost immediately when exposed to air. That coating protects the metal underneath from further corrosion.

Chlorine disinfects because it forms reactive compounds in water that attack cell membranes, proteins and DNA. That makes it effective against many bacteria and viruses.

Carbon can form four strong covalent bonds, letting it build chains, rings and complex structures. Its bonding versatility is the basis of organic chemistry and life as we know it.

They are different because carbon atoms bond in different patterns. Diamond forms a 3D lattice of strong bonds, while graphite makes flat layers that slide easily.

Helium is used because it is light and nonflammable, unlike hydrogen. It provides lift without the fire risk that made hydrogen dangerous in early airships.

Transition metals often make colored compounds because their partially filled d orbitals absorb specific wavelengths of visible light. The remaining light we see gives the compound its color.

Iodine sublimes because its molecules are held together by relatively weak intermolecular forces. When warmed, it can go directly from solid to gas without becoming liquid first.

Stainless steel resists rust because it contains chromium, which forms a thin protective oxide layer. That layer blocks oxygen and water from reaching the iron inside.

Phosphorus is essential for plant growth because it is needed for DNA, ATP and cell membranes. Plants cannot make it from nothing, so soils must supply it or fertilizers must replace it.

Sodium is stored in oil because it reacts quickly with air and moisture. The oil keeps water and oxygen away from the metal.

Different nuclei are stable for different proton-neutron combinations, so some elements can exist in many isotope forms. Stability depends on nuclear forces and the balance between protons and neutrons.

Helium was identified in the Sun’s spectrum in 1868 before scientists isolated it on Earth. Its name comes from Helios, the Greek word for Sun.

Salt dissolves because water molecules are polar and can pull sodium and chloride ions apart. The ions become surrounded by water molecules in a process called hydration.

They glow because heat excites their electrons to higher energy levels, and the electrons release light as they fall back down. Different elements emit different colors.

Bromine sits in between chlorine and iodine, so its molecules have intermolecular forces strong enough to make it a liquid, but not strong enough to make a solid at room temperature. Its molecular size puts it between the two extremes.