Across the periods of the modern periodic table, valency first increases and then decreases as you move from left to right. This fundamental trend is crucial for understanding how elements combine to form compounds.
Understanding Valency
Valency refers to the combining capacity of an element. It is determined by the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration, typically an octet (eight electrons in its outermost shell). These electrons are known as valence electrons.
The Trend Across a Period
As we traverse from left to right across a period, new electrons are added to the same electron shell. The trend for valency can be broadly observed as follows:
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Initial Increase: For the first few elements in a period (Groups 1, 2, 13, and 14), valency generally increases with the number of valence electrons.
- Group 1 (Alkali Metals): Valency is 1, as they have one valence electron they readily lose.
- Group 2 (Alkaline Earth Metals): Valency is 2, as they have two valence electrons they readily lose.
- Group 13 (Boron Family): Valency is 3, as they typically lose three valence electrons.
- Group 14 (Carbon Family): Valency is 4, as these elements can either lose, gain, or share four electrons to achieve stability.
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Subsequent Decrease: After reaching a valency of 4 (in Group 14), the valency generally decreases for the subsequent non-metal groups (Groups 15, 16, 17, and 18). For these elements, it's often easier to gain electrons to complete their octet. The valency can be calculated as
8 - (number of valence electrons).- Group 15 (Nitrogen Family): Has five valence electrons. Their common valency is 3 (8 - 5 = 3), as they tend to gain three electrons.
- Group 16 (Oxygen Family): Has six valence electrons. Their common valency is 2 (8 - 6 = 2), as they tend to gain two electrons.
- Group 17 (Halogens): Has seven valence electrons. Their common valency is 1 (8 - 7 = 1), as they tend to gain one electron.
- Group 18 (Noble Gases): Has a complete octet (eight valence electrons, except Helium with two). Their valency is 0, as they are already stable and do not readily participate in chemical bonding under normal conditions.
This progressive change is directly linked to the atomic structure and the drive of atoms to achieve a stable electron configuration, often resembling that of the nearest noble gas.
Examples from Period 3
Let's illustrate this trend with the elements of Period 3 in the modern periodic table:
| Element | Symbol | Group No. | Valence Electrons | Valency (Common) | How Stability is Achieved |
|---|---|---|---|---|---|
| Sodium | Na | 1 | 1 | 1 | Loses 1 electron |
| Magnesium | Mg | 2 | 2 | 2 | Loses 2 electrons |
| Aluminum | Al | 13 | 3 | 3 | Loses 3 electrons |
| Silicon | Si | 14 | 4 | 4 | Shares 4 electrons |
| Phosphorus | P | 15 | 5 | 3 | Gains 3 electrons |
| Sulfur | S | 16 | 6 | 2 | Gains 2 electrons |
| Chlorine | Cl | 17 | 7 | 1 | Gains 1 electron |
| Argon | Ar | 18 | 8 | 0 | Already stable |
As shown in the table, the valency for Period 3 elements follows the pattern: 1, 2, 3, 4, 3, 2, 1, 0. This clearly demonstrates the initial increase followed by a decrease.
Key Takeaways
- Valency is determined by an element's desire to achieve a stable electron configuration.
- The trend of valency across a period is increasing up to Group 14 and then decreasing towards Group 18.
- This trend is predictable and helps explain the diverse chemical behaviors of elements across the periodic table.
