# **Atomic Mass Unit**

The atomic mass unit is the measurement of the mass of atoms. Students will learn about the atom definition, the atomic model, atomic number and atomic mass, atomic mass formula, etc. In this discussion, we will relate atomic mass units with mathematical operations, addition sums for class 3, and subtraction sums for class 3.

### What Is an Atom?

According to the English physicist and chemist John Dalton, every matter is made up of tiny indivisible particles known as atoms.

### Atomic Model:

An atom comprises three small subatomic particles: Protons, Neutrons, and electrons. The protons and neutrons make up the centre of the atom, called the nucleus, and the electron roams around the nucleus in a small cloud. Proton is positively charged, and the electron is negatively charged. The number of protons and electrons in an ordinary atom is the same.

### What Is Atomic Number?

The total number of protons in an atom is called the atomic number. We represent the atomic number as ‘Z’. Atomic numbers are different for different elements. The atomic number of any atom is written as the subscript of the notation of an atom.

### Define Atomic Mass Unit.

The elements’ atomic mass is the average mass of the elements’ atoms measured in the atomic mass unit (AMU, also known as Daltons, D).

### What Is Atomic Mass?

The atomic mass is the weighted average of all the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. We signify the atomic mass as ‘A’. Isotopes are atoms with different atomic masses. The atomic mass is written as a superscript of the notation of an atom.

We can easily find the number of neutrons from the notation of any atom. If the number of neutrons is ‘N’ of any atom is, then the atomic mass formula is given below.

### Atomic Mass Formula

### Example:

Praseodymium Has an Atomic Number of 59 and an Atomic Mass of 141. How Many Neutrons Does It Have?

Answer:

Atomic number = Z = 59 and

Atomic mass = A = 141

So, the number of neutrons is

141 – 59 = 82

**Subtraction Word Problem**

Question 1:

Here Is a List of Isotopes of Plutonium. Find Their Number of Neutrons.

Isotopes | Number of neutrons (N) |
---|---|

^{228}Pu_{94} | |

^{229}Pu_{94} | |

^{232}Pu_{94} | |

^{233}Pu_{94} | |

^{234}Pu_{94} | |

^{240}Pu_{94} | |

^{241}Pu_{94} | |

^{242}Pu_{94} | |

^{243}Pu_{94} | |

^{244}Pu_{94} | |

^{245}Pu_{94} | |

^{246}Pu_{94} | |

^{247}Pu_{94} |

Answer:

Isotopes | Number of neutrons (N) |
---|---|

^{228}Pu_{94} | N = 228 – 94 = 134 |

^{229}Pu_{94} | N = 229 – 94 = 135 |

^{232}Pu_{94} | N = 232 – 94 = 138 |

^{233}Pu_{94} | N = 233 – 94 = 139 |

^{234}Pu_{94} | N = 234 – 94 = 140 |

^{240}Pu_{94} | N = 240 – 94 = 146 |

^{241}Pu_{94} | N = 241 – 94 = 147 |

^{242}Pu_{94} | N = 242 – 94 = 148 |

^{243}Pu_{94} | N = 243 – 94 = 149 |

^{244}Pu_{94} | N = 244 – 94 = 150 |

^{245}Pu_{94} | N = 245 – 94 = 151 |

^{246}Pu_{94} | N = 246 – 94 = 152 |

^{247}Pu_{94} | N = 247 – 94 = 153 |

Question 2:

Here Is a List of Elements With Their Atomic Mass And Atomic Numbers. Find the Number of Neutrons of the Elements.

Elements | Number of Neutrons |
---|---|

_{98}Tc^{43} | N = |

_{145}Pm^{61} | N = |

_{209}Po^{84} | N = |

_{210}At^{85} | N = |

_{222}Pn^{86} | N = |

_{227}Ac^{89} | N = |

_{237}Np^{93} | N = |

_{243}Am^{95} | N = |

_{247}Cm^{96} | N = |

_{247}Bk^{97} | N = |

_{251}Cf^{98} | N = |

_{257}Fm^{100} | N = |

_{252}Es^{99} | N = |

_{258}Md^{101} | N = |

_{261}Rf^{104} | N = |

_{259}Tc^{102} | N = |

_{277}Hs^{108} | N = |

_{266}Sg^{106} | N = |

Answer:

Elements | Number of Neutrons |
---|---|

_{98}Tc^{43} | N = 98 – 43 = 55 |

_{145}Pm^{61} | N = 145 – 61 = 84 |

_{209}Po^{84} | N = 209 – 84 = 121 |

_{210}At^{85} | N = 210 – 85 = 125 |

_{222}Pn^{86} | N = 222 – 86 = 132 |

_{227}Ac^{89} | N = 227 – 89 = 138 |

_{237}Np^{93} | N = 237 – 93 = 144 |

_{243}Am^{95} | N = 243 – 95 = 148 |

_{247}Cm^{96} | N = 247 – 96 = 151 |

_{247}Bk^{97} | N = 247 – 97 = 150 |

_{251}Cf^{98} | N = 251 – 98 = 153 |

_{257}Fm^{100} | N = 257 – 100 = 157 |

_{252}Es^{99} | N = 252 – 99 = 153 |

_{258}Md^{101} | N = 258 – 101 = 157 |

_{261}Rf^{104} | N = 261 – 104 = 157 |

_{259}Tc^{102} | N = 259 – 102 = 157 |

_{277}Hs^{108} | N = 277 – 108 = 169 |

_{266}Sg^{106} | N = 266 – 106 = 160 |

Question 3:

Here Is a Picture of the Three Isotopes of Carbon. Find Their Number of Neutrons.

Answer:

The number of neutrons in the C-12 atom is

12 – 6 = 6

The number of neutrons in the C-13 atom is

13 – 6 = 7

The number of neutrons in the C-13 atom is

14 – 6 = 8

Question 4:

Nickel (Z = 28, a = 58) Has 30 Isotopes That Have the Same Atomic

Number. But Their Atomic Masses Range From a = 48 to a = 78. a) How Many Neutrons Does the Lightest Isotope of Nickel Have?

b) How Many Neutrons Does the Heaviest Isotope of Nickel Have?

Answer:

- The lightest isotope of nickel has A = 48.

so, the number of neutrons = 48 – 28 = 20 - The heaviest isotope of nickel has A = 78

So, the number of neutrons = 48 – 78 = 30

**Addition Word Problem:**

Question 5:

The Class Teacher Has Given the Students the Number of Neutrons and the Atomic Number of the Following Element. He Also Taught Them That-

Here Is the List of the Elements. Read the List Carefully and Find Their Atomic Mass. Fill In the Blank Rows. One Is Done for You as an Example.

Element | Atomic Number (Z) | Number of Protons (N) | Atomic Mass (A) |

Technetium | Z = 43 | N = 55 | A = N + Z = 55 + 43 = 98 |

Promethium | Z = 61 | N = 84 | |

Polonium | Z = 84 | N = 121 | |

Radon | Z = 86 | N = 132 | |

Astatine | Z = 84 | N = 125 | |

Curium | Z = 96 | N = 151 | |

Fermium | Z = 104 | N = 157 | |

Rutherfordium | Z = 104 | N = 157 |

Answer:

Element | Atomic Number (Z) | Number of Protons (N) | Atomic Mass (A) |

Technetium | Z = 43 | N = 55 | A = N + Z = 55 + 43 = 98 |

Promethium | Z = 61 | N = 84 | A = N + Z = 61 + 84 = 145 |

Polonium | Z = 84 | N = 121 | A = N + Z = 84 + 121 = 205 |

Radon | Z = 86 | N = 132 | A = N + Z = 86 + 132 = 218 |

Astatine | Z = 84 | N = 125 | A = N + Z = 85 + 125 = 210 |

Curium | Z = 96 | N = 151 | A = N + Z = 96 + 151 = 247 |

Fermium | Z = 104 | N = 157 | A = N + Z = 100 + 157 = 257 |

Rutherfordium | Z = 104 | N = 157 | A = N + Z = 104 + 157 = 261 |