How many neutrons does element X have if its atomic number is 29 and its mass number is 86?

Element x will have 57 nutrons.

Atomic number: is the total number of protons.

Atomic mass: is the number of nutron + number of protons.

(57+29=86)

Calculation of nutrons: The mass number of an atom equals the number of protons plus the number of neutrons. Rearranging the equation, the number of neutrons equals the mass number minus the atomic number.

Number or nutrons = mass number - atomic number

86-29 = 57

The correct answer would be 57.

Atomic number is defined as the total number of protons in an atom. Thus, the number of protons in element X would be 29.

In addition, the mass number is defined as the sum of neutrons and protons present in an atom.

Therefore, the sum of the number of neutrons and protons in an element X would be 86.

Thus, the number of neutrons = Mass number - the number of protons

⇒86-29

⇒57

Hence, the number of neutrons would be equal to 57.