An element A decays into an element C by a two-step process:\$A\rightarrow B+He_2^4\$ and \$B\rightarrow C+{ 2e }_{ -1}^{ 0 }\$.Then

Conservation of mass no. and charge holds true\$A \rightarrow B\$\$Z_B = Z_A -2\$\$M_B = M_A -4\$\$B \rightarrow C + 2e\$\$Z_C = Z_B +2 = Z_A\$\$M_C = M_B = M_A-4\$Different mass, but same atomic No.Therefore, A and C are isotopes.

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Atomic Masses and Composition of Nucleus

Q: The value of 1 amu is equal to:

One an amu is \$\dfrac{1}{12}\$ of the mass of one carbon-12 atom. It is equal to \$ \dfrac{1}{N_A} \$\$= \dfrac{1}{6.022\times10^{23}} = 1.66 \times 10^{-24} g\$.

Q: What are isotopes? Give one example.

Isotope \$\rightarrow\$ Isotopes are atoms with the same number of protons, but differ in numbers of neutrons. Isotopes are different forms of a single element.Example - Carbon 12 and Carbon 14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons.

Q: What is the mass of one atom of \$C-12\$ in grams?

Mass of \$1\$ mole = \$12\,gm\$Mass of \$6.022 \times 10^{23}\,atom = 12\,gm\$mass of \$1\$ atom = \$\dfrac{12}{6.023 \times 10^{23}} = 1.993 \times 10^{-23}\,gm\$

Q: \$1\$ amu is equal to \$1.66\times 10^{-24}\$ g.

The atomic mass unit (amu) is equal to \$\frac{1}{12}\$ of the mass of one atom of the carbon-12 isotope.1 mole carbon-12 atoms weigh 12 g.\$6.022 \times 10^{-23}\$ carbon -12 atoms weigh \$12g\$.1 carbon-12 atom weighs \$\dfrac {12}{6.022 \times 10^{-23}} \ g \$\$\therefore 1\ amu = \dfrac{1}{12} \times\$ mass of one C-12 atom \$= \dfrac {1}{12}\$ \$\times \dfrac {12}{6.022 \times 10^{-23}}\$ \$= 1.66 \times 10^{-24} g\$ or \$1.66 \times 10^{-27} kg\$.

Q: Composition of the nuclei of two atomic species \$X\$ and \$Y\$ are given as under \$X\$ \$Y\$Protons \$=\$ \$6\$ \$6\$Neutrons\$=\$ \$6\$ \$8\$Given the mass numbers of \$X\$ and \$Y\$. What is the relation between the two species?

Atomic mass=number of protons+number of neutronsAtomic mass of X = 6+6=12Atomic mass of Y =6+8=14Both X and Y have same atomic number but different atomic mass number so they are isotopes.

Q: Which of the following isotopes is used for treatment of cancer?

\$Co^{60}\$Cobalt therapy or cobalt -60 therapy is the medical use of gamma rays from the radioisotope cobalt 60 to treat conditions such as cancer.

Q: An atom of an element contains \$13\$ electrons. Its nucleus has \$14\$ neutrons. Find out its atomic number and approximate atomic mass. An isotope has atomic mass \$2\$ units higher. What will be the number of protons, neutrons and electrons in the isotope?

Atomic number=No. of protons=No. of electrons in ground state(unchanged atom)Atomic number=\$13\$=No. of protonsAtomic mass=no. of protons+no. of neutrons=\$13+14=27\$For isotope no. of proton=\$13\$(same atomic number but different mass number are isotopes)no. of electrons=\$13\$no. of neutrons=\$14+2=16\$

Q: \$4.6\times 10^{22}\$ atoms of an element weight \$13.8\$ g. What is the atomic mass of the element?

\$1\$ mole of any substance contains \$6.02 \times 10^{23}\$ atoms. Thus, \$4.6 \times 10^{22}\$ atoms corresponds to \$\dfrac{4.6 \times 10^{22}}{6.022 \times 10 ^{23}}=0.0764\$ moles.\$0.0764\$ moles weighs \$13.8\$ g. Thus, \$1\$ mole will weigh \$\dfrac{13.8}{0.0764}= 180.6 \ g\$. Hence, the atomic mass of the element will be 180.6 u.

Q: Which of the following pairs are isotopes?

Isotopes are elements with same atomic number but different mass number. Hydrogen and Deuterium are isotopes with same atomic number but different mass number. Hydrogen have atomic number 1 and mass number 1 where as deuterium have atomic number 1 but mass number 2.

Physics

definition

Atomic mass unit

An atomic mass unit (a.m.u) is defined as precisely

\frac{1}{1 2}

the mass of carbon-12 atom. It is a unit of mass of elements e.g the atomic mass of Helium is 4 a.m.u.

diagram

Atomic masses of various elements

The atomic masses of various elements are:

definition

Atomic mass unit

Atomic mass unit (u) is defined as

\frac{1}{1 2}^{t h}

of the mass of the carbon

(^{12} C)

atom. According to this definition

1 u = \frac{m a s s o f o n e ^{12} C a t o m}{1 2} = \frac{1 . 9 9 2 6 4 7 \times 1 0 ^{- 26} k g}{1 2} = 1.660539 \times 1 0^{- 27} k g

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Isotopes, Isobars, Isotones

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Important Questions

Composition of the nuclei of two atomic species $X$ and $Y$ are given as under
$X$ $Y$
Protons $=$ $6$ $6$
Neutrons $=$ $6$ $8$

Given the mass numbers of $X$ and $Y$ . What is the relation between the two species?

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Which of the following isotopes is used for treatment of cancer?

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An atom of an element contains $13$ electrons. Its nucleus has $14$ neutrons. Find out its atomic number and approximate atomic mass. An isotope has atomic mass $2$ units higher. What will be the number of protons, neutrons and electrons in the isotope?

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$4.6 \times 1 0^{22}$ atoms of an element weight $13.8$ g. What is the atomic mass of the element?

Medium

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Which of the following pairs are isotopes?

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An element A decays into an element C by a two-step process:
$A \to B + H e_{2}^{4}$ and $B \to C + 2 e_{- 1}^{0}$ .Then

Medium

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definition

Atomic mass unit

diagram

Atomic masses of various elements

definition

Atomic mass unit

REVISE WITH CONCEPTS

Isotopes, Isobars, Isotones

Quick Summary With Stories

Atomic Mass and Composition of Nucleus

The value of 1 amu is equal to:

What are isotopes? Give one example.

What is the mass of one atom of C−12 in grams?

1 amu is equal to 1.66×10−24 g.

Composition of the nuclei of two atomic species X and Y are given as under X Y Protons = 6 6 Neutrons= 6 8 Given the mass numbers of X and Y. What is the relation between the two species?

Which of the following isotopes is used for treatment of cancer?

An atom of an element contains 13 electrons. Its nucleus has 14 neutrons. Find out its atomic number and approximate atomic mass. An isotope has atomic mass 2 units higher. What will be the number of protons, neutrons and electrons in the isotope?

4.6×1022 atoms of an element weight 13.8 g. What is the atomic mass of the element?

Which of the following pairs are isotopes?

An element A decays into an element C by a two-step process: A→B+He24​ and B→C+2e−10​.Then

What is the mass of one atom of $C - 12$ in grams?

$1$ amu is equal to $1.66 \times 1 0^{- 24}$ g.

Composition of the nuclei of two atomic species $X$ and $Y$ are given as under
$X$ $Y$
Protons $=$ $6$ $6$
Neutrons $=$ $6$ $8$

Given the mass numbers of $X$ and $Y$ . What is the relation between the two species?

An atom of an element contains $13$ electrons. Its nucleus has $14$ neutrons. Find out its atomic number and approximate atomic mass. An isotope has atomic mass $2$ units higher. What will be the number of protons, neutrons and electrons in the isotope?

$4.6 \times 1 0^{22}$ atoms of an element weight $13.8$ g. What is the atomic mass of the element?

An element A decays into an element C by a two-step process:
$A \to B + H e_{2}^{4}$ and $B \to C + 2 e_{- 1}^{0}$ .Then