Many tools made of metal, from ancient bronze vessels to the rumbling large machines in modern factories, and even the shot put that we struggle to throw far in gym class, seem to be "heavy" and "sturdy" "tough guys". Don't metal materials also have a "lightweight" "posture"?
This is not the case. Many metals are also "light as a swallow", such as aluminum alloys and titanium alloys used in the manufacture of aerospace vehicles like airplanes and satellites, and magnesium alloys used in the manufacture of mobile phone and laptop shells. They are called light alloys, that is, alloys with relatively low density. Their density is much lower than that of traditional metal materials such as copper and iron. After all, no one would like to carry around a mobile phone as heavy as a shot put all day.
Of course, "light" and "heavy" are relative. Under normal circumstances, the density of water is about 1 g/cm³. Even the very light metal magnesium has a density of 1.7 g/cm³, which is heavier than water. So, are there metals that are even lighter than water?
There are nearly 100 kinds of metals in the periodic table, and three of them have a density lower than water, namely lithium, sodium, and potassium. The density of sodium is about 0.97 g/cm³; the density of potassium is about 0.86 g/cm³; and the density of lithium is only 0.53 g/cm³, which is much lighter than water, about one-fifth of that of aluminum, and is truly the king of light metals. If these three metals are thrown into water, they will never sink to the bottom of the water. However, due to the very reactive chemical properties of lithium, sodium, and potassium, they will react violently with water. In the laboratory, sodium and potassium can only be stored in kerosene with a density of only 0.8 g/cm³. Sodium and potassium can sink in kerosene, thus isolating the air; while lithium is too light and even "can't be contained" in kerosene, so it can only be confined in solid paraffin.