How many isotopes does the element with atomic number 21 have?

In the reaction between a crushed antacid tablet and vinegar what gas is emitted

Which of the following contributes to the structural rigidity of cellulose? adjacent glucose polymers are stabilized by hydrogen bonding. glucose residues are joined by (α1→4) linkages. cellulose is a highly branched molecule. the conformation of the glucose polymer is a coiled structure.

What is the percentage of hydrogen in nitrogen trihydride