Grade 9
  1. Matter and its nature  1.1. Introduction to matter  1.2. Physical and chemical properties of matter  1.3. States of matter  1.3.1. Solid state  1.3.2. Fluid state  1.3.3. Gaseous state  1.3.4. Plasma and Bose–Einstein condensate  1.4. Changes in the states of matter  1.4.1. Melting and freezing  1.4.2. Evaporation and Condensation  1.4.3. Sublimation and deposition  1.5. Classification of matter  1.6. Elements, Compounds, and Mixtures  1.7. Pure substances and impurities  1.8. Separation Techniques  1.8.1. Filtration  1.8.2. Distillation  1.8.3. Crystallization  1.8.4. Chromatography  1.8.5. Centrifugation  1.8.6. Sublimation  1.8.7. Decantation and sedimentation
  2. Atomic Structure  2.1. Discovery of atoms  2.2. Dalton's atomic theory  2.3. Structure of the atom  2.4. Subatomic particles  2.5. Atomic number and mass number  2.6. Isotopes and Isobars  2.7. Electronic configuration  2.8. Bohr's atomic model  2.9. Modern Atomic Model (Quantum Mechanical Model - Introduction)  2.10. Valency and chemical bonding
  3. C hemical reactions and equations  3.1. Introduction to Chemical Reactions  3.2. Chemical Equation Formulation  3.3. Balancing Chemical Equations  3.4. Types of Chemical Reactions  3.4.1. Combination Reactions  3.4.2. Decomposition reactions  3.4.3. Displacement reactions  3.4.4. Double displacement reactions  3.4.5. Redox reactions  3.4.6. Endothermic and Exothermic Reactions  3.5. Effects of chemical reactions  3.6. Energy Changes in Chemical Reactions  3.7. Catalysts and their role in reactions
  4. Periodic table and periodicity  4.1. History of Periodic Classification  4.2. Mendeleev's periodic table  4.3. Modern Periodic Table  4.4. Periods and groups in the periodic table and periodicity  4.5. Trends in the Periodic Table  4.5.1. Atomic size  4.5.2. Ionization Energy  4.5.3. Electron affinity  4.5.4. Electronegativity  4.5.5. Metallic and Non-Metallic Properties  4.6. Noble gases and their properties
  5. Chemical bond  5.1. Introduction to Chemical Bonding  5.2. Types of chemical bonds  5.2.1. Ionic bond  5.2.2. Covalent bond  5.2.3. Metallic bond  5.2.4. Hydrogen bonding  5.2.5. Van der Waals force  5.3. Properties of Ionic and Covalent Compounds  5.4. Molecular Structure and Geometry (VSEPR Theory - Basics)
  6. Acids, Bases and Salts  6.1. Introduction to Acids and Bases  6.2. Properties of Acids  6.3. Properties of bases  6.4. pH scale and its importance  6.5. Indicators and their uses  6.6. Neutralization reactions  6.7. Strength of Acids and Bases (Strong vs. Weak)  6.8. Preparation of salts  6.9. Uses of acids, bases and salts in daily life
  7. Metals and Nonmetals  7.1. Physical Properties of Metals  7.2. Physical Properties of Nonmetals  7.3. Chemical properties of metals  7.4. Chemical Properties of Nonmetals  7.5. Reactivity Series of Metals  7.6. Extraction of metals  7.7. Corrosion and its prevention  7.8. uses of metals and nonmetals
  8. Carbon and its compounds  8.1. Introduction to Carbon  8.2. Allotropes of carbon (diamond, graphite, fullerene)  8.3. Hydrocarbons  8.3.1. Hydrocarbons  8.3.2. Alkene  8.3.3. Alkynes  8.4. Functional groups in organic chemistry  8.5. Isomerism in organic compounds  8.6. Alcohols and Carboxylic Acids  8.7. Soaps and detergents  8.8. Polymers (Introduction to Natural and Synthetic Polymers)
  9. Solutions and Mixtures  9.1. Types of mixtures  9.2. Homogeneous and Heterogeneous Mixtures  9.3. Concentration of solutions  9.4. Solubility and factors affecting solubility  9.5. Suspensions and Colloids  9.6. Saturated, unsaturated and supersaturated solutions
  10. Air and atmosphere  10.1. Composition of air  10.2. Role of gases in the atmosphere  10.4. Acid rain and its effects  10.5. Greenhouse effect and global warming  10.6. Ozone layer and its depletion  10.7. Air pollution control measures
  11. Water and its importance  11.1. Composition and properties of water  11.2. Hardness of water (temporary and permanent hardness)  11.3. Causes of water pollution  11.4. Effects of Water Pollution  11.5. Water purification methods (filtration, boiling, chlorination)
  12. Industrial Chemistry  12.1. Introduction to Industrial Chemistry  12.2. Important industrial chemicals (sulfuric acid, ammonia, sodium hydroxide)  12.3. Chemical processes in industry  12.4. Uses of Industrial Chemistry in Daily Life  12.5. Environmental impact of industrial chemical processes

Grade 9 → Atomic Structure ↓


Atomic number and mass number


Introduction

In the study of chemistry, atoms are considered the basic building blocks of matter. Understanding the properties of atoms, such as their atomic number and mass number, is important for understanding the nature of different elements and how they interact in chemical reactions. This lesson will provide a detailed explanation of atomic number and mass number, and their importance in atomic structure.

Atomic Structure

To understand atomic number and mass number, we must first discuss the basic atomic structure. An atom is made up of three elementary particles: protons, neutrons, and electrons.

Components of an atom

  • Proton: Positively charged particle found in the nucleus of an atom.
  • Neutrons: Neutral particles (having no charge) that are located in the nucleus along with the protons.
  • Electrons: Negatively charged particles that orbit the nucleus in different energy levels or shells.

The arrangement of these particles defines the identity and properties of the atom. Let us find out how this is related to atomic number and mass number.

Atomic number

The atomic number of an element is the number of protons in the nucleus of an atom of that element. It is represented by the symbol Z and is important as it defines the identity of the element.

For example, all atoms with an atomic number of 6 are carbon atoms. This is because carbon has 6 protons in its nucleus. Similarly, any atom with an atomic number of 8 is oxygen, which has 8 protons.

Visual example of atomic number

6 Carbon (C) 8 Oxygen (O)

In the visual example above, the atomic number inside the nucleus circle represents the number of protons specific to each element.

Importance of atomic number

The atomic number is important for several reasons:

  • Element Identification: The atomic number uniquely identifies a chemical element.
  • Position in the Periodic Table: Elements are arranged in the periodic table in the order of increasing atomic number.
  • Electron configuration: The atomic number determines the number of electrons and also the arrangement of these electrons in energy levels around the nucleus.

Examples of atomic number

  • Hydrogen: Atomic number is 1. Hydrogen has 1 proton.
  • Helium: Atomic number is 2. Helium has 2 protons.
  • Aluminum: The atomic number is 13. Aluminum has 13 protons.

Mass number

The mass number of an atom is the sum of the number of protons and neutrons present in its nucleus. It is represented by the symbol A

Unlike atomic number, mass number is not fixed for all atoms of an element due to the presence of isotopes. Isotopes are different forms of the same element, where atoms have the same atomic number but different mass numbers due to different numbers of neutrons.

The formula for calculating the mass number is: 

Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)

Visual example of mass number

Protons: 6 Neutrons: 6 Mass number: 12

In the visual example above, the mass number of carbon is shown as 12. This is calculated by adding up the 6 protons and 6 neutrons found in the nucleus of a typical carbon atom.

Importance of mass number

The mass number is important for a few reasons:

  • Determination of Isotopes: Isotopes are identified by their different mass numbers.
  • Atomic mass: The mass number is closely related to the atomic mass, which is an average that reflects all isotopes and their abundances.
  • Nuclear Properties: The properties of the nucleus such as stability and radioactive properties are affected by the distribution of protons and neutrons.

Examples of mass number

  • Carbon-12: 6 protons and 6 neutrons; mass number = 12.
  • Carbon-14: 6 protons and 8 neutrons; mass number = 14. This isotope is famous for its use in radiocarbon dating.
  • Oxygen-16: 8 protons and 8 neutrons; mass number = 16.

Difference between atomic number and mass number

It is important to make a clear distinction between atomic number and mass number because they provide different information about an atom:

  • Atomic number is a fundamental property that remains constant for each atom of an element and determines the chemical behaviour of the element.
  • The mass number may vary between atoms of the same element if different isotopes are considered due to differences in the neutron number.
  • The atomic number determines the number of electrons and protons and has a direct impact on the chemical identity and properties.
  • The mass number defines the relative isotopic mass and is important when looking at nuclear reactions and decays in physics.

Conclusion

Understanding atomic number and mass number is important in the study of atoms in chemistry. While the atomic number (protons) defines what element an atom belongs to, the mass number (protons + neutrons) provides more detailed information about the specific isotope of that element. These numbers are essential for working with chemical symbols, understanding the periodic table, predicting chemical reactions, and studying atomic mass and isotopes.

Through atomic number and mass number, scientists and students alike can determine the essential properties of elements and learn more about the atomic world.


Grade 9 → 2.5


U
username
0%
completed in Grade 9

← Prev (2.4)
Subatomic particles
Next (2.6) →
Isotopes and Isobars

Comments 



Atomic number and mass number

https://www.chemistryelite.com/g9/2.5?ceal=en