Sodium (Na) and Magnesium (Mg) are positioned in the same period on the periodic table because their outermost valence electrons occupy the third electron shell. This fundamental characteristic dictates their placement in Period 3.
Understanding Periods on the Periodic Table
On the periodic table, elements are arranged in rows called periods and columns called groups. A period number directly corresponds to the principal quantum number (n) of the highest energy electron shell that contains electrons for that atom. In simpler terms, it indicates the number of electron shells an atom has that are occupied by electrons.
- Periods run horizontally across the table.
- Each new period signifies the filling of a new principal electron shell.
- Elements within the same period have the same number of occupied electron shells.
Electron Configuration: The Key to Periodicity
To understand why Sodium and Magnesium are in Period 3, we look at their electron configurations, which describe the distribution of electrons in an atom's atomic orbitals.
Sodium (Na)
Sodium has an atomic number of 11, meaning it has 11 protons and 11 electrons. Its electron configuration is:
1s² 2s² 2p⁶ 3s¹
- The '1s²' indicates 2 electrons in the first shell.
- The '2s² 2p⁶' indicates 8 electrons in the second shell.
- The
**3s¹**indicates 1 electron in the third electron shell.
Since the highest occupied principal electron shell for Sodium is the third shell (n=3), it belongs to Period 3.
Magnesium (Mg)
Magnesium has an atomic number of 12, with 12 protons and 12 electrons. Its electron configuration is:
1s² 2s² 2p⁶ 3s²
- The '1s²' indicates 2 electrons in the first shell.
- The '2s² 2p⁶' indicates 8 electrons in the second shell.
- The
**3s²**indicates 2 electrons in the third electron shell.
Similarly, the highest occupied principal electron shell for Magnesium is also the third shell (n=3), placing it in Period 3.
Comparing Sodium and Magnesium
Both Sodium and Magnesium share the crucial characteristic of having their outermost valence electrons in the third electron shell. This is the defining factor for their placement in Period 3 of the periodic table.
Here's a quick comparison:
| Element | Atomic Number | Electron Configuration | Outermost Electron Shell | Period |
|---|---|---|---|---|
| Sodium | 11 | 1s² 2s² 2p⁶ 3s¹ | 3rd | 3 |
| Magnesium | 12 | 1s² 2s² 2p⁶ 3s² | 3rd | 3 |
Implications of Being in the Same Period
While Sodium and Magnesium are in the same period, they are in different groups (Group 1 for Sodium and Group 2 for Magnesium). This difference in group number signifies a different number of valence electrons (1 for Na, 2 for Mg), leading to distinct chemical properties and reactivity. However, their shared period number indicates that their outermost electrons are roughly the same distance from the nucleus and experience similar levels of electron shielding from inner shells.
For a detailed view of the periodic table and electron configurations, you can explore resources like the Periodic Table on Wikipedia or Khan Academy's explanation of electron configurations.
