An element x has two isotopes. x–15 has 7 protons and 8 neutrons. determine the atomic number and mass number of the second isotope, which has 10 neutrons in its nucleus. a. atomic number = 8; mass number = 9 b. atomic number = 8; mass number = 17 c. atomic number = 7; mass number = 17 d. atomic number = 7; mass number = 15

c. Atomic number = 7; mass number = 17  

The atomic number of the first isotope is 7.

Isotopes of the same element have the same atomic number, so the atomic number of the second isotope must also be 7.

Mass number = no. of protons + no. of neutrons, or

A = p + n

∴ A = 7 + 10 = 17

Options “a.” and “b.” have the wrong atomic numbers, and option “d.” has the wrong mass number.

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.