Which of the following elements has a total of 3 shells with 8 electrons in the last orbit?
Ar
The question asks us to identify the element that has a specific electron arrangement: a total of 3 electron shells and exactly 8 electrons in its outermost shell. This arrangement is key to understanding the properties and position of the element in the periodic table.
Understanding Electron Shells and Valence Electrons
Electrons in an atom occupy different energy levels, often referred to as electron shells. These shells are organized around the nucleus. The first shell (n=1) is the closest to the nucleus, followed by the second shell (n=2), the third shell (n=3), and so on.
The outermost shell is called the valence shell, and the electrons in this shell are called valence electrons. The number of valence electrons largely determines how an element will interact chemically with other elements.
The question specifies two criteria:
- Total number of shells = 3
- Number of electrons in the last (outermost) shell = 8
Analyzing the Options
Let's examine the electron configuration for each given element to determine the number of shells and the number of valence electrons.
| Element Symbol | Atomic Number (Z) | Electron Configuration | Number of Shells | Electrons in Last Shell (Valence Electrons) |
|---|---|---|---|---|
| S | 16 | $1s^2 2s^2 2p^6 3s^2 3p^4$ | 3 (n=1, n=2, n=3) | 6 (from $3s^2 3p^4$) |
| Al | 13 | $1s^2 2s^2 2p^6 3s^2 3p^1$ | 3 (n=1, n=2, n=3) | 3 (from $3s^2 3p^1$) |
| Ar | 18 | $1s^2 2s^2 2p^6 3s^2 3p^6$ | 3 (n=1, n=2, n=3) | 8 (from $3s^2 3p^6$) |
| P | 15 | $1s^2 2s^2 2p^6 3s^2 3p^3$ | 3 (n=1, n=2, n=3) | 5 (from $3s^2 3p^3$) |
Evaluating Against the Criteria
We need to find the element that satisfies both conditions: 3 shells and 8 electrons in the last shell.
- Sulfur (S): Has 3 shells but only 6 valence electrons. Does not meet the criteria.
- Aluminum (Al): Has 3 shells but only 3 valence electrons. Does not meet the criteria.
- Argon (Ar): Has 3 shells and 8 valence electrons ($3s^2 3p^6$). Meets both criteria.
- Phosphorus (P): Has 3 shells but only 5 valence electrons. Does not meet the criteria.
Based on this analysis, Argon (Ar) is the only element among the options that has a total of 3 shells with 8 electrons in its last orbit (valence shell). Elements with 8 valence electrons (except Helium with 2) are noble gases, which are known for their stability due to having a full valence shell.
Revision Table: Element Electron Structure
| Element | Atomic Number | Number of Shells | Valence Electrons | Matches Criteria (3 shells, 8 valence e-) |
|---|---|---|---|---|
| S | 16 | 3 | 6 | No |
| Al | 13 | 3 | 3 | No |
| Ar | 18 | 3 | 8 | Yes |
| P | 15 | 3 | 5 | No |
Additional Information: Periodic Table and Electron Configuration
The periodic table is organized based on the electron configurations of the elements. The period number corresponds to the highest principal quantum number (n), which indicates the number of electron shells containing electrons. The group number often relates to the number of valence electrons.
- Elements in the same period have the same number of electron shells.
- Elements in Group 18 (Noble Gases) typically have a full valence shell, which is 8 electrons (octet rule), except for Helium (2 electrons).
- Argon is in Period 3 and Group 18, consistent with having 3 shells and 8 valence electrons.
Knowing how to determine electron configuration is crucial for understanding an element's position and properties in the periodic table and answering questions about electron shells and valence electrons.
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