Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions:
How many electrons and protons does element X have?
Given Mass number = 35 and Number of neutrons = 18. Using the formula Mass number = Number of protons + Number of neutrons, the Number of protons = 35 - 18 = 17. In a neutral atom, the Number of electrons equals the number of protons, so it has 17 electrons.
What is its atomic number?
The atomic number is equal to the number of protons, which is 17.
Identify the element X.
Element X is the element with atomic number 17.
Write its electronic configuration.
The electronic configuration of element X, distributing 17 electrons across shells, is 2, 8, 7.
How many valence electrons does it have?
The outermost shell contains 7 electrons, so element X has 7 valence electrons.
What will be the mass number if two neutrons are added to its nucleus?
If two neutrons are added, the new number of neutrons becomes 18 + 2 = 20. The new mass number will be 17 (protons) + 20 (neutrons) = 37.
What will be the relation of X with the new atom?
The new atom has the same atomic number (17) as element X but a different mass number (37). As defined, atoms of the same element with the same atomic number but different mass numbers are called isotopes. Therefore, the new atom is an isotope of X.
Explanation
The solution applies the formula Mass number = Number of protons + Number of neutrons to find the number of protons. For a neutral atom, the number of protons equals the number of electrons, and the atomic number is defined by the number of protons. The electronic configuration follows the 2n² rule for the first two shells (2, 8) and the remaining electrons in the next (7). Adding neutrons changes the mass number but not the atomic number, which directly leads to the definition of isotopes provided in the text: atoms with the same atomic number but different mass numbers.