Question

Identify the elements X and Y from the following information.

(a) X has 17 protons and 18 neutrons

(b) Y has 17 protons and 20 neutrons

Answer 1

Atomic number

Atomic number gives the number of protons in an atom of the element. It does not depend on the number of neutrons. In a neutral molecule, the number of protons will be equal to the number of electrons in the atom. Therefore, atomic number for a neutral molecule is equal to its number of electrons. The atomic number is always a smaller value than the mass number. Each element has a unique atomic number.

Mass number

The sum of number of protons and neutrons in an atom gives the mass number or atomic mass of an element. It depends on the number of neutrons in the atom. Elements with same atomic number but different mass number have a difference in the number of neutrons in its nucleus and are called isotopes.

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(a) The atomic number of X is 17 because it has 17 protons. The element with atomic number 17 is Chlorine (Cl).

The number of neurons is 18. Therefore, mass number can be calculated as:

$\begin{array}{rcl}massnumber& =& n_{proton}+n_{neutron}\\ & =& 17+18\\ & =& 35\end{array}$

Therefore, the element can be represented as ${}_{17}{}^{35}Cl$.

(b) The atomic number of Y is 17 because it has 17 protons. The element with atomic number 17 is Chlorine.

The number of neurons is 20. Therefore, mass number can be calculated as:

$\begin{array}{rcl}massnumber& =& n_{proton}+n_{neutron}\\ & =& 17+20\\ & =& 37\end{array}$

Therefore, the element can be represented as ${}_{17}{}^{37}Cl$.

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The atomic number of X and Y are same while its mass number is different. Therefore, ${}_{17}{}^{35}Cl$ and ${}_{17}{}^{37}Cl$ are isotopes of the same element, Chlorine.