Lithium be cut with a knife. When cut, it possesses a silvery-white color that quickly changes to gray as it oxidizes to lithium oxide. While it has one of the lowest melting points among all metals (180 °C), it has the highest melting and boiling points of the alkali metals. Lithium has a very low density (0.534 g/cm), comparable with pine wood. It is the least dense of all elements that are solids at room temperature; the next lightest solid element (potassium, at 0.862 g/cm) is more than 60% denser. Furthermore, apart from helium and hydrogen, it is less dense than
Mendeleev's predicted elements place of eka-tantalum in what is now called Group 5. Eka-tantalum is actually dubnium. Mendeleev's 1869 table had implicitly predicted a heavier analog of titanium and zirconium, but in 1871 he placed lanthanum in that spot. The 1923 discovery of hafnium validated Mendeleev's original 1869 prediction. In 1902, having accepted the evidence for elements helium and argon, Mendeleev placed these noble gases in Group 0 in his arrangement of the elements. As Mendeleev was doubtful of atomic theory to explain the law of definite proportions, he had no "a priori" reason to believe hydrogen was the lightest of elements, and