The long-standing model of pigment cell origins is implicitly based upon a progressive fate restriction model of neural crest development and postulates the existence of a partially restricted progenitor common to all pigment cell types (i.e., a chromatoblast) [38] .
Sodium is the fourth most abundant element on earth, comprising about 2.6% of the earth's crust; it is the most abundant of the alkali group of metals. It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride.
#Na_3N#. Sodium is an element in group 1 of the periodic table. As such, it will form an electron by losing a single electron to form a #1+# ion: #Na^+#. Nitrogen is an element in group 5 of the periodic table. When it forms the nitride ion, it gains three electrons to form a #3-# ion: #N^(3-)#. In ionic compounds, the charges of constituent ions must balance. This can be achieved by having three sodium ions per nitride ion. Therefore, the formula of sodium nitride is #Na_3N#.