Question

If atom A has $18$ protons and $21$ neutrons and atom B has $19$ protons and $21$ neutrons, are these isotopes of the same elements?
A) There is not enough information to determine this.
B) No, because they have different numbers of protons.
C) No, because they have the same number of neutrons.
D) Yes, because they have different numbers of protons.
E) Yes, because they have different numbers of neutrons.

Answer 1

Atomic number= The number of unit positive charges carried by the nucleus of an atom is called the atomic number of the element.
Since, an atom as a whole is electrically neutral, the number of planetary electrons present in an atom is equal to the number of positive charges present in the nucleus. Thus,
Atomic number of an element $=$ Number of unit positive charges present in the nucleus
Atomic number of an element $=$ Number of protons present in the nucleus
Atomic number of an element $=$ Number of electrons present outside the nucleus of the same atom

Complete step by step answer:
-Atomic number= The atomic number of each element has certain characteristic values and two elements have the same atomic number. Therefore, the atomic number of an element is a fundamental property.
-All atoms of the same element have the same number of protons in their nuclei. Therefore, atoms of the same elements have the same atomic number. However, atoms of different elements have different atomic numbers because their nuclei have different numbers of protons.
-It must be remembered that the atomic number of an element is equal to the number of electrons only in a neutral atom, not in an ion.
-This is because the number of protons is equal to the number of electrons only in a neutral atom. On the other hand, an ion is formed by the removal of electrons from a neutral atom or by the addition of electrons to a neutral atom and therefore contains either less or more electrons than protons.
Mass number= Atom is composed of protons, neutrons and electrons. Since the mass of electrons is negligible, the real mass of an atom is due to the protons and neutrons present in it. The total sum of the protons and neutrons present in the nucleus of an atom is called its mass number. i.e.
Mass number $=$No. of protons$+$ No. of neutrons
-The mass number of an element is usually denoted by the letter $A$.
-Since, the number of protons in an atom is equal to the atomic number of the element, therefore
Mass number $A =$ Atomic number $Z +$ Number of neutrons
-So, Number of neutrons $= A - Z$
-Isotopes of the same element must have the same number of protons but different number of neutrons.
-As atoms A and B have different numbers of protons, thus they are not isotopes of each element.
Hence, option B is correct.

Additional information:
Isotopes= Isotopes are the atoms of the same element having same atomic number but different mass numbers.

Note:
We have to keep in mind that isotopes of an element have the same number of electrons and protons but have different numbers of neutrons. Since, chemical properties of the elements depend upon the atomic number, isotopes of an element have similar chemical properties but are different in their physical properties. Atomic mass of an element is the average of the mass number of the different isotopes.