JEE Main · 2019 · 10 Jan 2019 · Shift 1 · s-Block Elements

X-ray tube windows are made of beryllium because its very low atomic number, $Z=4$, means few electrons to absorb the beam, so the X-rays pass through with little loss. Beryllium also has enough mechanical strength to seal a tube and good thermal conductivity to shed heat. Heavier light metals like magnesium, sodium or calcium would either soak up too much of the beam or lack the strength, so beryllium is the standard choice.