Concept: Noble Gases and Their Atomic Numbers

Noble gases are elements in Group 18 of the periodic table. They are characterized by having a complete valence shell, making them very stable and unreactive. To determine how many noble gases exist up to atomic number 20, we need to list all noble gases with atomic numbers less than or equal to 20.

Step 1: Identify the Noble Gases

The noble gases are: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn).

Step 2: List Their Atomic Numbers

• Helium (He): Atomic number = 2
• Neon (Ne): Atomic number = 10
• Argon (Ar): Atomic number = 18
• Krypton (Kr): Atomic number = 36 (which is greater than 20)
• Xenon (Xe): Atomic number = 54 (>20)
• Radon (Rn): Atomic number = 86 (>20)

Step 3: Count Noble Gases with Atomic Number ≤ 20

From the list, only Helium (2), Neon (10), and Argon (18) have atomic numbers less than or equal to 20. So, there are 3 noble gases.

Final Answer

The number of noble gases up to atomic number 20 is $3$.

Related Topics

• Periodic Table Groups: Understanding how elements are grouped based on similar properties.
• Atomic Number and Mass Number: The atomic number defines the element, and noble gases have full electron shells.
• Chemical Reactivity: Noble gases are inert due to their stable electron configuration.

Important Formulae and Theory

Electron Configuration for Noble Gases:

• Helium: $1^2$
• Neon: $1^2{2}^6$ or [He] $2^6$
• Argon: $1^2{2}^6{3}^6$ or [Ne] $3^6$

Theory: Noble gases have the general electron configuration of ${\mathrm{ns}}^2{\mathrm{np}}^6$ (except Helium, which is $1^2$), representing a full valence shell. This makes them highly stable and non-reactive.