# Periodic Table Trends: Step-by-Step Explanations ## 1. Trend for Atomic Radius **Atomic radius** refers to the size of an atom, typically measured from the nucleus to the outermost electron shell. - **Across a Period (Left → Right):** Atomic radius decreases. - Reason: Electrons are added to the same shell, but the number of protons increases. This stronger positive charge pulls electrons closer to the nucleus. - **Down a Group (Top → Bottom):** Atomic radius increases. - Reason: Additional electron shells are added, making atoms larger despite increased nuclear charge. --- ## 2. Trend for Ionization Energy **Ionization energy** is the energy required to remove an electron from an atom in the gas phase. - **Across a Period (Left → Right):** Ionization energy increases. - Reason: Atoms have more protons and a stronger pull on electrons, so it's harder to remove one. - **Down a Group (Top → Bottom):** Ionization energy decreases. - Reason: Outer electrons are farther from the nucleus and more shielded, so they are easier to remove. --- ## 3. Order of Increasing Atomic Radius: Be, Te, Se Let's locate the elements: - **Be (Beryllium):** Period 2, Group 2 (alkaline earth metal) - **Se (Selenium):** Period 4, Group 16 - **Te (Tellurium):** Period 5, Group 16 **Step-by-Step Reasoning:** - **Compare periods:** Atomic radius increases as you go down a group (higher period number). - **Be** is in period 2 (smallest), **Se** in period 4, **Te** in period 5 (largest). - **Se** and **Te** are in the same group, so **Te** is larger. - **Be** is far left in the table; **Se** is several periods below and to the right. **Order (smallest to largest):** - **Be < Se < Te** --- ## 4. Order of Increasing Ionization Energy: As, Sr, Ca Let's locate the elements: - **As (Arsenic):** Period 4, Group 15 (nonmetal) - **Ca (Calcium):** Period 4, Group 2 (alkaline earth metal) - **Sr (Strontium):** Period 5, Group 2 **Step-by-Step Reasoning:** - **Ionization energy increases across a period** (left to right), so As (right) has a higher ionization energy than Ca (left). - **Down a group, ionization energy decreases**, so Sr (below Ca) has a lower ionization energy than Ca. - Therefore, **Sr < Ca < As**. **Order (smallest to largest):** - **Sr < Ca < As** --- ## **Summary Table** | Question | Elements | Increasing Order | |----------|-------------|------------------------------------| | 3 | Be, Se, Te | Be < Se < Te | | 4 | Sr, Ca, As | Sr < Ca < As | --- **Key takeaways**: - Atomic radius increases down a group and decreases across a period. - Ionization energy decreases down a group and increases across a period.