Atomic Number, Mass Number, Isotopes and Isobars Questions in English

(N/A) The atomic number is considered a more fundamental attribute because it represents the number of protons in the nucleus of an atom,which uniquely defines the identity of an element.
Unlike the mass number,which can vary due to the presence of different numbers of neutrons (isotopes),the number of protons remains constant for all atoms of a specific element.
Furthermore,the chemical properties of an element are primarily determined by its electronic configuration,which is directly dictated by the atomic number (number of protons/electrons in a neutral atom).

(N/A) 'He' has atomic number $2$,so it has $2$ electrons in the $K$-shell,which is the first energy level and has a maximum capacity of $2$ electrons. Thus,its valence shell is completely filled,making it a noble gas. Conversely,'Be' has an atomic number of $4$ and an electronic configuration of $2, 2$. It has $2$ electrons in the $L$-shell,which has a capacity of $8$ electrons. Since its valence shell is incomplete,'Be' acts as a metal with a valency of $2$.
$(b)$ Hydrogen exists in three isotopic forms: protium $({ }_{1}^{1} H)$,deuterium (${ }_{1}^{2} H$ or $D$),and tritium (${ }_{1}^{3} H$ or $T$). All three isotopes have the same atomic number $(1)$,meaning they possess the same electronic configuration ($1$ electron in the $K$-shell). Since chemical properties of an element are determined by its electronic configuration,all isotopes of hydrogen exhibit identical chemical behavior.

(A) The mass number of an atom is the sum of the number of protons and neutrons in its nucleus.
$M: 3 + 4 = 7$
$N: 9 + 10 = 19$
$O: 8 + 8 = 16$
$P: 8 + 10 = 18$
$(b)$ $A$ cation is a positively charged ion formed by the loss of electrons (number of protons > number of electrons). In particle $M$,protons = $3$ and electrons = $2$,so it is a cation.
An anion is a negatively charged ion formed by the gain of electrons (number of electrons > number of protons). In particle $N$,electrons = $10$ and protons = $9$,so it is an anion.

(A) An ion $M^{2+}$ is formed by the loss of $2$ electrons. Since the ion has $10$ electrons,the neutral atom $M$ must have $10 + 2 = 12$ electrons. In a neutral atom,the number of electrons equals the number of protons,so the atomic number is $12$. The mass number is the sum of protons and neutrons: $12 + 12 = 24$.
$(b)$ No,it is not possible. An atom is electrically neutral because the number of positively charged protons is equal to the number of negatively charged electrons. If an atom had $12$ protons and $13$ electrons,it would be an ion,not a neutral atom.
$(c)$ Helium gas is inert because its outermost shell (the $K$ shell) is completely filled with $2$ electrons,making it stable and chemically unreactive.

(N/A) Some applications of isotopes are:
$(i)$ An isotope of uranium $(U-235)$ is used as a fuel in nuclear reactors.
$(ii)$ An isotope of cobalt $(Co-60)$ is used in the treatment of cancer.
$(iii)$ An isotope of iodine $(I-131)$ is used in the treatment of goitre.
$(iv)$ An isotope of sodium $(Na-24)$ is used to detect constrictions in the circulatory system and to detect blood clots.
$(v)$ An isotope of chromium $(Cr-51)$ is used to study blood diseases.
$(vi)$ The uptake of phosphorus by plants is studied using an isotope of phosphorus $(P-32)$.

(N/A) The number of electrons in the $X^{2+}$ ion is $18$.
Since a positive ion is formed by the loss of electrons from a neutral atom,the number of electrons lost is equal to the number of units of positive charge on the ion.
Therefore,the number of electrons in the neutral atom $= 18 + 2 = 20$.
For a neutral atom,the atomic number is equal to the number of protons,which is also equal to the number of electrons.
Thus,the atomic number of element $X = 20$.
The mass number of element $X$ is the sum of the number of protons and the number of neutrons,which is $20 + 20 = 40$.
The element with atomic number $20$ is calcium $(Ca)$.

(N/A) The number of electrons in the $Y^{3-}$ ion is $18$.
$A$ negative charge is formed by the gain of electrons by a neutral atom,and the number of electrons gained is equal to the magnitude of the negative charge.
Therefore,the number of electrons in the neutral atom $Y$ is $18 - 3 = 15$.
For a neutral atom,the atomic number is equal to the number of protons,which is also equal to the number of electrons.
Therefore,the atomic number of the element $Y$ is $15$.
The mass number of an element is the sum of the number of protons and the number of neutrons.
Mass number $= 15 + 16 = 31$.
The element $Y$ with atomic number $15$ is Phosphorus $(P)$.

(N/A) Isotopes of an element have identical electronic configurations,meaning they possess the same number of valence electrons. Since chemical properties are determined by valence electrons,all isotopes of an element exhibit identical chemical properties.
$(b)$ The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes. Because these isotopes have different masses and exist in specific natural abundances,the resulting average atomic mass is often a fraction.
$(c)$ Atoms combine with other atoms to attain the stable electronic configuration of the nearest noble gas (octet or duplet rule),thereby reducing their potential energy and becoming more stable.

(A) The atomic number $(Z)$ of an atom is equal to the number of protons $(p)$ in its nucleus. Here,$Z = 9$,so the number of protons is $9$.
In a neutral atom,the number of electrons $(e)$ is equal to the number of protons. Therefore,the number of electrons is $9$.
The mass number $(A)$ is the sum of the number of protons and neutrons $(n)$.
$A = p + n$
$19 = 9 + n$
$n = 19 - 9 = 10$.
Thus,the atom has $9$ protons,$10$ neutrons,and $9$ electrons.

(B) An alpha particle ($\alpha$-particle) consists of $2$ protons and $2$ neutrons.
It is essentially the nucleus of a helium atom.
Since it lacks electrons, it carries a positive charge of $+2e$.
Therefore, an alpha particle is defined as a doubly-charged helium ion $(He^{2+})$.

(A) Isotopes are atoms of the same element that have the same atomic number (number of protons and electrons) but different mass numbers due to a different number of neutrons in their nuclei.
Since the two oxygen atoms in the sample have the same number of electrons (which defines the element as oxygen) but a different number of neutrons,they are isotopes of oxygen.
Therefore,the correct reason is that the two oxygen atoms are isotopes.

(A) The isotope of cobalt,$Co-60$,is widely used in radiotherapy for the treatment of cancer because it emits gamma rays that can destroy cancer cells.
Iodine-$131$ $(I-131)$ is a radioactive isotope of iodine used in the treatment of goitre (a condition related to the thyroid gland) and other thyroid disorders.

(B) In nuclear reactors,the isotope of uranium that is commonly used as a fuel is $U-235$ because it is fissile and can sustain a chain reaction.
Radiocarbon dating is a method used to determine the age of organic materials by measuring the amount of the radioactive isotope $C-14$ present in the sample.
Therefore,the correct pair is $U-235$ and $C-14$.

(D) hydrogen atom consists of one proton in its nucleus and one electron revolving around it.
Since the hydrogen nucleus contains only a single proton and no neutrons,it is simply referred to as a proton.

(B) For any neutral atom:
$1$. The atomic number $(Z)$ is defined as the number of protons in the nucleus of an atom. In a neutral atom,the number of protons equals the number of electrons. Thus,Atomic number $=$ number of protons $=$ number of electrons. Statement $(iv)$ is true.
$2$. The mass number $(A)$ is defined as the sum of the number of protons and the number of neutrons present in the nucleus. Thus,Mass number $=$ number of protons $+$ number of neutrons. Statement $(ii)$ is true.
$3$. Statement $(i)$ is incorrect because the atomic number is not the sum of protons and electrons.
$4$. Statement $(iii)$ is incorrect because atomic mass is not defined as the equality of protons and neutrons.
Therefore,statements $(ii)$ and $(iv)$ are correct.

(B) In a neutral atom, the number of electrons is equal to the number of protons, which is the atomic number $(Z)$.
Given: Number of electrons = $15$, so Atomic number $(Z)$ = $15$.
The mass number $(A)$ is the sum of the number of protons and neutrons.
$A = Z + \text{Number of neutrons} = 15 + 16 = 31$.
The standard representation of an element is given by $_{Z}^{A}X$, where $A$ is the mass number and $Z$ is the atomic number.
Therefore, the correct representation is $_{15}^{31}X$.

(D) $1$. The mass number $(A)$ of an atom is the sum of protons $(p)$ and neutrons $(n)$. Given $A = 27$ and $n = 14$,we can find the number of protons: $p = A - n = 27 - 14 = 13$.
$2$. $A$ neutral atom has an equal number of protons and electrons. Thus,a neutral atom of this element has $13$ electrons.
$3$. The ion has $3$ positive charges,which means it has lost $3$ electrons.
$4$. Therefore,the number of electrons in the ion = $13 - 3 = 10$.

(FALSE) False.
An isotope of an element consists of atoms that have the same number of protons but a different number of neutrons. Since all atoms in a sample of a pure isotope have the same atomic number and the same chemical properties,it is considered a pure substance. Isotopes are different forms of the same element,not impurities.