How do atomic number and electron configuration determine an element's properties?

Definition

The atomic number ($Z$) of an element is the number of protons in its nucleus. This uniquely identifies the element and determines its position in the periodic table.

The electron configuration describes the arrangement of electrons in an atom's shells and subshells. It follows the Pauli exclusion principle, Hund's rule, and the Aufbau principle.

These two factors dictate an element's chemical properties (such as reactivity, valence, and bonding behavior) because chemical reactions involve the gain, loss, or sharing of electrons, especially those in the outermost shell (valence electrons).

Worked Example

Example: Compare sodium ($Z = 11$) and chlorine ($Z = 17$).

1. Atomic Number

• Sodium: $Z = 11$ (11 protons)
• Chlorine: $Z = 17$ (17 protons)

2. Electron Configuration

• Sodium: $1s^2,2s^2,2p^6,3s^1$ - Chlorine: $1s^2,2s^2,2p^6,3s^2,3p^5$ ### 3. Properties - **Sodium** has one electron in its outermost shell ($3s^1$), making it highly reactive and likely \to lose one electron \to form $text{Na}^+$. - **Chlorine** has seven valence electrons ($3s^2,3p^5$), making it highly reactive and likely \to gain one electron \to form $text{Cl}^-$. **Result:** Sodium and chlorine react \to form sodium chloride ($text{NaCl}$), with sodium donating an electron to chlorine.

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Key Takeaways

• The atomic number determines the element's identity and its number of electrons in a neutral atom.
• The electron configuration reveals the distribution of electrons, especially valence electrons, which govern chemical behavior.
• Elements with similar valence electron configurations (same group) exhibit similar chemical properties.