Atomic numbers of four elements are as follows: F (9), P (15), Cl (17), Ar (18). Which of the following two elements would be chemically similar?
F and CI
Understanding Chemical Similarity of Elements
Chemical similarity between elements is primarily determined by their electron configurations, specifically the number of electrons in their outermost shell, known as valence electrons. Elements with the same number of valence electrons are located in the same group in the periodic table and tend to exhibit similar chemical properties and reactivity.
We are given four elements with their atomic numbers:
- Fluorine (F): Atomic number 9
- Phosphorus (P): Atomic number 15
- Chlorine (Cl): Atomic number 17
- Argon (Ar): Atomic number 18
To determine which elements are chemically similar, we need to find their electron configurations and identify the number of valence electrons.
Electron Configurations and Valence Electrons
Let's write the electron configuration for each element based on its atomic number (which equals the number of electrons in a neutral atom):
- Fluorine (F), Atomic Number 9: The electron configuration is $\text{1s}^2 \text{2s}^2 \text{2p}^5$. The outermost shell is n=2, which contains $2 + 5 = 7$ valence electrons.
- Phosphorus (P), Atomic Number 15: The electron configuration is $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^3$. The outermost shell is n=3, which contains $2 + 3 = 5$ valence electrons.
- Chlorine (Cl), Atomic Number 17: The electron configuration is $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^5$. The outermost shell is n=3, which contains $2 + 5 = 7$ valence electrons.
- Argon (Ar), Atomic Number 18: The electron configuration is $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^6$. The outermost shell is n=3, which contains $2 + 6 = 8$ valence electrons. Argon has a complete valence shell.
| Element | Atomic Number | Electron Configuration | Valence Electrons | Group |
|---|---|---|---|---|
| F | 9 | $\text{1s}^2 \text{2s}^2 \text{2p}^5$ | 7 | 17 (Halogens) |
| P | 15 | $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^3$ | 5 | 15 (Pnictogens) |
| Cl | 17 | $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^5$ | 7 | 17 (Halogens) |
| Ar | 18 | $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^6$ | 8 | 18 (Noble Gases) |
Identifying Chemically Similar Elements
By comparing the number of valence electrons, we can see which elements are in the same group and are therefore chemically similar:
- Fluorine (F) has 7 valence electrons.
- Phosphorus (P) has 5 valence electrons.
- Chlorine (Cl) has 7 valence electrons.
- Argon (Ar) has 8 valence electrons.
Both Fluorine (F) and Chlorine (Cl) have 7 valence electrons. This means they are in the same group (Group 17, the halogens) and are chemically similar. They tend to gain one electron to achieve a stable electron configuration like the noble gases.
Phosphorus (P) is in Group 15 with 5 valence electrons. Argon (Ar) is in Group 18 with 8 valence electrons and is a noble gas, which is generally unreactive.
Therefore, the two elements that would be chemically similar are Fluorine (F) and Chlorine (Cl).
Revision Table: Key Information on Element Similarity
| Element | Atomic Number | Valence Electrons | Group | Chemical Similarity Status |
|---|---|---|---|---|
| F | 9 | 7 | 17 | Similar to Cl (same group/valence electrons) |
| P | 15 | 5 | 15 | Not similar to F, Cl, or Ar based on valence electrons |
| Cl | 17 | 7 | 17 | Similar to F (same group/valence electrons) |
| Ar | 18 | 8 | 18 | Noble gas, generally unreactive, different from others |
Additional Information: Periodicity and Group Properties
The periodic table is arranged based on the principle of periodicity, where elements with similar chemical properties recur at regular intervals. This recurrence is a direct consequence of the repeating pattern of electron configurations, particularly the number of valence electrons.
- Elements within the same vertical column (group) have the same number of valence electrons (for main group elements) and thus share similar chemical behaviours. For example, all elements in Group 1 are alkali metals (except hydrogen) and are highly reactive, losing one electron to form a +1 ion.
- Elements within the same horizontal row (period) have the same highest principal energy level (shell) containing electrons. Properties change gradually across a period.
- Noble gases (Group 18) have a full valence shell, making them very stable and largely unreactive, hence their distinct chemical behaviour compared to other elements.
Understanding electron configuration and valence electrons is fundamental to predicting the chemical properties and reactivity of elements.
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