Atomic Number and Mass Number

The periodic table is a systematic arrangement of elements based on their atomic number, electron configurations, and recurring chemical properties. Understanding the concept of atomic numbers and atomic mass is essential to understanding the organization of the periodic table.

1.0Atomic Number Definition

As we know an atom is the smallest unit of an element that retains the chemical properties of that element. It consists of a nucleus containing protons and neutrons, surrounded by electrons in various energy levels or orbitals.

The atomic number is the number of protons in the nucleus of an atom. It uniquely identifies an element and determines its position on the periodic table.

Key Features of Atomic Number:

2.0Representation of Atomic Number and Mass Number

• The atomic number is usually written as a subscript to the left of the chemical symbol of the element.
• The mass number is written as a superscript to the left of the chemical symbol of the element.

For Example:

In this example, the mass number of carbon (C) is 12, indicating that the nucleus contains 6 protons and 6 neutrons (12 = 6 protons + 6 neutrons).

3.0Mass Number Definition

The mass number is the total number of protons and neutrons in an atom's nucleus.

Key Features of Mass Number:

• Symbol: A
• Calculation: Mass number (A) = Number of protons (Z) + Number of neutrons (N).
• Isotopes: Atoms of the same element can have different mass numbers due to varying numbers of neutrons. These different forms are called isotopes. For example, carbon-12 and carbon-14 are isotopes of carbon, with mass numbers of 12 and 14, respectively.
• Representation: Isotopes are often represented as ^A_ZElement, for example, ¹²₆C for carbon-12.

4.0Examples of Atomic number and Mass number

1. Hydrogen:

• Atomic Number (Z) = 1
• Mass Number (A) for common isotope (Protium) = 1 (1 proton + 0 neutrons)
• Isotopes: Protium (¹₁H), Deuterium (²₁H), Tritium (³₁H)

2. Carbon:

• Atomic Number (Z) = 6
• Common Mass Numbers (A): 12,
• Isotopes: Carbon-12 (¹²₆C), Carbon-13 (¹³₆C), Carbon-14 (¹⁴₆C)

3. Oxygen:

• Atomic Number (Z) = 8
• Common Mass Numbers (A): 16, 17, 18
• Isotopes: Oxygen-16(¹⁶₈O), Oxygen-17 (¹⁷₈O), Oxygen-18 (¹⁸₈O)

5.0Difference between Atomic Number and Mass Number

6.0Importance of Atomic Number and Atomic Mass

• Chemical Behavior: The atomic number determines the chemical properties and behavior of an element, as it defines the arrangement of electrons around the nucleus.
• Stability and Radioactivity: The number of neutrons, and thus the mass number, affects the stability of the nucleus. Isotopes with certain neutron-to-proton ratios are more stable, while others may be radioactive.
• Atomic Mass: The atomic mass (or atomic weight) of an element is the weighted average mass of the isotopes, considering their natural abundance. It is usually close to the mass number of the most stable isotope.

7.0Solved Examples of Atomic Number and Atomic Mass

1. Find the number of neutrons in an isotope of Uranium-238 (⁹²²₃₈U).

• Atomic Number (Z): 92
• Mass Number (A): 238
• Number of Neutrons (N): A−Z = 238−92 =146
• Protons: 92
• Neutrons: 146
• Electrons: 92 (in a neutral atom)

2. Determine the mass number of an isotope with 17 protons and 20 neutrons.

• Atomic Number (Z): 17 (this identifies the element as chlorine)
• Neutrons (N): 20
• Mass Number (A): Z +N =17+20 =37
• The isotope is ³⁷₁₇​Cl.

3. Calculate the number of protons, neutrons, and electrons in an isotope of Sulfur-32 (³²​₁₆S).

• Atomic Number (Z): 16
• Mass Number (A): 32
• Protons: 16
• Neutrons: A−Z=32−16=16
• Electrons: 16 (in a neutral atom)