period 4

K Potassium Alkali metal · Z 19 · 39.098 u A metal so reactive it must be kept away from air, and so essential that no cell in your body works without it. Ca Calcium Alkaline earth metal · Z 20 · 40.078 u Structural element of bone, shell and cement: calcium is how life learned to build hard things. Sc Scandium Transition metal · Z 21 · 44.956 u A rare-earth-adjacent metal that makes aluminium alloys far stronger, at a price that keeps it exotic. Ti Titanium Transition metal · Z 22 · 47.867 u As strong as steel, 45% lighter, and immune to seawater — the metal of implants, jets and deep submersibles. V Vanadium Transition metal · Z 23 · 50.942 u A transition metal that turns four different colours in solution depending on its oxidation state. Cr Chromium Transition metal · Z 24 · 51.996 u The element that puts the shine in chrome and the "stainless" in stainless steel. Mn Manganese Transition metal · Z 25 · 54.938 u Essential in trace amounts, brittle in bulk, and the reason your steel does not shatter in the cold. Fe Iron Transition metal · Z 26 · 55.845 u The most common element on Earth by mass, the endpoint of stellar fusion, and the metal that named an age. Co Cobalt Transition metal · Z 27 · 58.933 u The blue in medieval stained glass and Ming porcelain, and half of the magnets that hold hard drives together. Ni Nickel Transition metal · Z 28 · 58.693 u Tough, corrosion-resistant and magnetic — and the second-biggest ingredient in stainless steel. Cu Copper Transition metal · Z 29 · 63.546 u The first metal humans smelted, and still the nervous system of the electrified world. Zn Zinc Transition metal · Z 30 · 65.38 u The sacrificial metal: zinc corrodes so that the steel it coats does not. Ga Gallium Post-transition metal · Z 31 · 69.723 u A metal that melts in your hand at 29.8 degrees C and is the foundation of blue LEDs and 5G radios. Ge Germanium Metalloid · Z 32 · 72.64 u The semiconductor that came first: the very first transistor was made of germanium, not silicon. As Arsenic Metalloid · Z 33 · 74.922 u The classic poison of history — tasteless, undetectable for centuries, and still a groundwater crisis for millions. Se Selenium Nonmetal · Z 34 · 78.96 u The element your eyes need to work and photocopiers needed to exist. Br Bromine Halogen · Z 35 · 79.904 u One of only two elements that are liquid at room temperature, and the one that smells like a swimming pool from hell. Kr Krypton Noble gas · Z 36 · 83.798 u Named "hidden", used to define the metre, and briefly the fictional weakness of a superhero.