# **10.3 POSITION OF AN ELEMENT IN THE PERIODIC TABLE** The electronic configuration of an element is the key to determining its position in the periodic table. By analyzing the valence electron configuration, specifically the principal quantum number ($n$) and the sub-shell type, you can identify an element's period, group, and block. ### **Determining the Position of an Element** The location of an element is defined by its period (horizontal row) and group (vertical column). #### **1. Period Number** The period number corresponds to the principal quantum number (*n*) of the outermost or valence shell. - If the valence electrons of an element are in the L shell ($n=2$), the element belongs to the **2nd period**. - If the valence electrons are in the M shell ($n=3$), the element belongs to the **3rd period**. #### **2. Group Number** The group number is determined by the number of electrons in the valence shell. * **For s-block elements:** The group number is equal to the total number of electrons in the valence shell. * **For p-block elements:** The group number is found by adding 10 to the total number of valence electrons. * For example, if the valence shell configuration is $2s^2 2p^3$, the total number of valence electrons is $2+3=5$. The element belongs to **Group 15** (or VA). The calculation is: $$ \text{Group Number} = (\text{Valence electrons}) + 10 = 5 + 10 = 15 $$ --- ### **Characteristic Properties of Elements in a Group** *Chemical periodicity* refers to the repeating pattern of elemental properties in the periodic table. Elements within the same group share similar chemical properties because they have analogous valence electron configurations. * **Atomic Size:** Gradually **increases** from top to bottom in a group. Elements lower in the group have larger atomic radii. * **Ionization Energy:** Generally **decreases** from top to bottom in a group. It is easier to remove an electron from elements lower in the group. * **Electronegativity:** **Decreases** down the groups. Elements further down a group are less electronegative. * **Metallic Properties:** **Increase** as you move down a group. Elements at the bottom of a group are more metallic than those at the top. * **Chemical Reactivity:** Elements in the same group exhibit similar chemical reactivity due to having the same number of valence electrons. --- ### **Example 10.1: Find the position of the following elements in the periodic table.** **(a) Nitrogen (atomic number 7)** **(b) Oxygen (atomic number 8)** #### **Problem Solving Strategy:** 1. Write the full electron configuration of the element. 2. Identify the valence shell and its configuration. 3. The principal quantum number ($n$) of the valence shell gives the **period number**. 4. The total number of electrons in the valence shell (plus 10 for p-block) gives the **group number**. #### **Solution:** **(a) Nitrogen (N)** 1. **Electronic Configuration:** $1s^2 2s^2 2p^3$ 2. **Valence Shell Configuration:** $2s^2 2p^3$ 3. **Period Number:** The highest $n$ value is **2**. 4. **Group Number:** * Total valence electrons = $2 + 3 = 5$. * Since it is a p-block element, Group = $5 + 10 = 15$. * **Conclusion:** Nitrogen is in the **2nd period** and **Group 15 (VA)**. **(b) Oxygen (O)** 1. **Electronic Configuration:** $1s^2 2s^2 2p^4$ 2. **Valence Shell Configuration:** $2s^2 2p^4$ 3. **Period Number:** The highest $n$ value is **2**. 4. **Group Number:** * Total valence electrons = $2 + 4 = 6$. * Since it is a p-block element, Group = $6 + 10 = 16$. * **Conclusion:** Oxygen is in the **2nd period** and **Group 16 (VIA)**. --- ### **Example 10.2: Determine the valence shell electronic configuration.** **(a) Phosphorus (P)** **(b) Neon (Ne)** #### **Problem Solving Strategy:** 1. Identify the period number to find the principal quantum number ($n$) of the valence shell. 2. Identify the group number to find the total number of valence electrons. 3. Distribute the valence electrons into the sub-shells ($s$ and $p$) of the valence shell. #### **Solution:** **(a) Phosphorus (P)** 1. **Position:** Phosphorus is in **Period 3** and **Group 15 (VA)**. 2. **Valence Shell:** Since it is in Period 3, $n=3$. The valence sub-shells are $3s$ and $3p$. 3. **Valence Electrons:** Since it is in Group 15, it has **5** valence electrons. 4. **Configuration:** The 5 electrons are distributed as follows: 2 in the $3s$ sub-shell and the remaining 3 in the $3p$ sub-shell. * **Valence shell electronic configuration:** $$3s^2 3p^3$$ **(b) Neon (Ne)** 1. **Position:** Neon is in **Period 2** and **Group 18 (VIIIA)**. 2. **Valence Shell:** Since it is in Period 2, $n=2$. The valence sub-shells are $2s$ and $2p$. 3. **Valence Electrons:** Since it is in Group 18, it has **8** valence electrons. 4. **Configuration:** The 8 electrons are distributed as follows: 2 in the $2s$ sub-shell and the remaining 6 in the $2p$ sub-shell. * **Valence shell electronic configuration:** $$2s^2 2p^6$$ *** > **Do You Know?** > > **What Makes Up >99% of the Normal Matter in the Universe?** > > Hydrogen and Helium are the most abundant elements, accounting for 98% of all matter (approximately 73% hydrogen and 25% helium). All other elements make up the remaining 2%. The next most abundant element is oxygen, at a tiny 0.05%, followed by neon, nitrogen, carbon, and silicon. *** ### **Possible Questions/Answers** - **Q:** Find the valence shell electronic configuration of Mg and Cl from their position in the periodic table. **A:** * **Mg:** Located in Period 3, Group 2. This means $n=3$ and it has 2 valence electrons. Configuration: **$3s^2$**. * **Cl:** Located in Period 3, Group 17. This means $n=3$ and it has 7 valence electrons. Configuration: **$3s^2 3p^5$**. - **Q:** Find the position of K (atomic no. 19) and S (atomic no. 16) in the periodic table. **A:** * **K (19):** Electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$. * The valence shell is $n=4$, so it is in **Period 4**. * It has 1 valence electron, so it is in **Group 1**. * **S (16):** Electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^4$. * The valence shell is $n=3$, so it is in **Period 3**. * It has $2+4=6$ valence electrons. Since it's a p-block element, its group is $6+10=16$. It is in **Group 16**.