Question

Define the following terms:
$1$. Atom
$2$. Isotope
$3$. Atomic number
$4$. Atomic mass number
$5$. Moderator in nuclear reactor

Answer 1

The given terms can be defined using the atomic structure model. The existence of an element in nature gives information about isotope. The moderator is used to moderate or control a nuclear reaction.

Complete answer:
$1$. Atom: It is the basic and smallest unit of a matter. Combination of atoms forms an element. The components of an atom are nucleus and electrons. The nucleus is composed of neutrons and protons. The electron in an atom is a negatively charged species. The proton carries positive charge and neutrons are neutral species. The neutron is heavier than protons and electrons. The mass of a nucleus is responsible for the mass of an atom.
$2$. Isotope: The isotope is an element which has the same atomic number but different mass number. The atomic number is the same because both the elements have the same number of protons and electrons but differ in their number of neutrons. Example element carbon exists as two isotopes carbon-$12$ and carbon-$13$. Both have the same atomic number 6 but the mass number of $C - 12$ is $6$ and the mass number of $C - 13$ is $7$.
$3$. Atomic number: It is defined as the number of protons present in the nucleus of an atom. For a neutral atom the number of protons and electrons are equal. The modern periodic table is prepared by arranging the elements in increasing order of atomic number. Knowledge of atomic number helps in identifying the number of electrons and the corresponding electron configuration. It is denoted as the subscript. For example for the carbon element, $C_6^{12}$, where $6$ is the atomic number.
$4$. Atomic mass number: It is defined as the number of nucleons present in an atom. The nucleons are the total number of protons and neutrons in the nucleus of the atom. The number of protons and neutrons can be the same or different. An element having a different number of neutrons leads to an isotope of the particular element with the same number of protons and neutrons.
$5$. Moderator in a nuclear reactor: As the name suggests a moderator is the substance which is used to slow down a chain reaction. In nuclear reactors fast moving neutrons are present which helps in propagating the nuclear reaction by producing a large number of collisions with the nucleus of the reactants. In order to control the speed of neutrons, slow down the process and prolong the reaction moderator are used. Light water is used as a moderator in nuclear reactors.

Note: Most of the space occupied by an atom is composed of the nucleus. The mass of the proton is around $1850$ times the mass of electrons. Electrons are the lightest particle present in an atom. A molecule of an element is composed of $6.023 \times {10^{23}}$ numbers of atoms.