Which of these statements is incorrect regarding an atom of the element scandium?

a. its chemical symbol is sc.
b. it has 21 positively charged protons in its nucleus.
c. it has 21 neutrons in its nucleus.
d. it has 21 negatively charged electrons.

The answer is C.

Scandium has 24 neutrons, not 21.

Scandium is a metal that is silvery in color. When exposed to air, it tends to tarnish.  It burns quite easily and will react  with water. Scandium was discovered by Lars Fredrik Nilson in the year 1879 in the country of Sweden and its name is from the Latin word Scandia, meaning Scandinavian.

Other properties of Scandium include:

Melting point: 1539.0 °C

Boiling point: 2832.0 °C

Density: 2.989 grams/cm³

Normal phase: Solid

Family: Transition metals 

Period:  4

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.