Question

Refer to the periodic table and write the predicted electron configuration for each of the following negative ions using core notation: (a) \(\mathrm{Br}^{-}\) (b) \(\mathrm{Te}^{2-}\) (c) \(\mathrm{As}^{3-}\) (d) \(\mathrm{O}^{2-}\)

Short answer

(a) [Ar] 3d^{10} 4s^{2} 4p^{6}; (b) [Kr] 4d^{10} 5s^{2} 5p^{6}; (c) [Ar] 3d^{10} 4s^{2} 4p^{6}; (d) [He] 2s^{2} 2p^{6}.

Step-by-step solution

Locate the Element on the Periodic Table

For each ion, start by identifying the element on the periodic table. Determine its atomic number, which tells you the number of protons and electrons in the neutral atom.

Neutral Electron Configuration

Write the electron configuration for the neutral atom of the element using core notation, where the configuration starts with the previous noble gas in brackets, followed by the remaining electron configuration beyond that noble gas.

Adjust for the Ion Charge

Modify the electron configuration for the ion by adding the appropriate number of electrons. For negative ions, add electrons based on the charge of the ion to the normal electron configuration.

Example Ion: Br^{-}

Bromine (Br) is element 35. Its neutral electron configuration is [Ar] 3d^{10} 4s^{2} 4p^{5}. For Br^{-}, add 1 electron to get [Ar] 3d^{10} 4s^{2} 4p^{6}.

Example Ion: Te^{2-}

Tellurium (Te) is element 52. Its neutral electron configuration is [Kr] 4d^{10} 5s^{2} 5p^{4}. Add 2 electrons for Te^{2-} to make it [Kr] 4d^{10} 5s^{2} 5p^{6}.

Example Ion: As^{3-}

Arsenic (As) is element 33. Its neutral electron configuration is [Ar] 3d^{10} 4s^{2} 4p^{3}. Add 3 electrons for As^{3-}, resulting in [Ar] 3d^{10} 4s^{2} 4p^{6}.

Example Ion: O^{2-}

Oxygen (O) is element 8. Its neutral electron configuration is [He] 2s^{2} 2p^{4}. Add 2 electrons for O^{2-} to get [He] 2s^{2} 2p^{6}.

Key concepts

Understanding Negative Ions

Negative ions, also known as anions, form when an atom gains one or more electrons. This process results in a net negative charge.
Atoms strive to achieve a stable electron configuration, often resembling that of the nearest noble gas.
For example:

• When bromine becomes a negative ion (\( \mathrm{Br}^{-} \)), it gains one electron.
• Tellurium, as \( \mathrm{Te}^{2-} \), gains two electrons, while arsenic, as \( \mathrm{As}^{3-} \), gains three.
• Similarly, oxygen in its negative form (\( \mathrm{O}^{2-} \)) gains two electrons.

This gain of electrons completes the outer electron shell of the atom, making it more stable.

Exploring Core Notation

Core notation simplifies the representation of electron configurations. Instead of listing all orbitals, we use a noble gas to represent filled electron shells.
This method reduces complexity and highlights valence electrons more clearly.
For instance:

• Bromine's electron configuration in core notation is \([\text{Ar}] 3d^{10} 4s^{2} 4p^{5}\).
• For negative ions, additional electrons are added after the noble gas core, as seen in \( \mathrm{Br}^{-} \) becoming \([\text{Ar}] 3d^{10} 4s^{2} 4p^{6}\).

This method aids in quickly understanding chemical properties related to valence electrons.

Position Matters: The Periodic Table

The periodic table is a map that helps us locate elements and predict their behavior when they form ions.
It organizes elements by increasing atomic number and similar chemical properties.
Understanding its structure is crucial for identifying electron configurations.

• Elements like bromine and tellurium are within groups that commonly form anions.
• Their positions hint at the number of electrons they gain to become stable.

For example, halogens (such as bromine) tend to gain one electron, while elements in the same period as tellurium often gain two.

Ion Charge Adjustment in Electron Configurations

Ion charge adjustment involves adding electrons to an atom's electron configuration to reflect its ion form.
This is especially relevant for negative ions, which gain electrons.

• Bromine as \( \mathrm{Br}^{-} \) gains one electron to fill its outer shell, reflecting a more stable state.
• Oxygen gains two electrons in forming \( \mathrm{O}^{2-} \), achieving the same configuration as neon.

The adjusted electron configuration provides insights into the reactivity and stability of ions, linking electron gain with increased stability.