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When discussing atoms, ions are a fundamental concept. An ion is simply an atom that has lost or gained electrons, which results in a net charge. There are two types of ions: cations and anions. Cations are positively charged ions that form when an atom loses one or more electrons. For instance,

• Cadmium ion Cd\(^{2+}\) results when Cadmium loses two electrons.
• Silver ion Ag\(^{+}\) is formed by losing one electron.

Anions, on the other hand, are negatively charged ions formed when an atom gains electrons. Examples include:

• Phosphorus ion P\(^{3-}\), which gains three electrons.
• Arsenic ion As\(^{3-}\), also gaining three electrons.

Understanding how atoms become ions is crucial, as it helps explain their electronic configurations and reactions. By calculating the difference between the protons and electrons, we determine the ionic charge of each atom.

Noble-gas configurations refer to the electron configuration that corresponds to that of a noble gas. Noble gases are elements in Group 18 of the periodic table, well known for their stability. This stability arises because their outermost electron shell is full. Atoms frequently gain, lose, or share electrons in chemical reactions to achieve this same stability.

• For example, Cd\(^{2+}\) with its electron configuration \([Kr] 4d^{10}\) matches that of Krypton, a noble gas.
• Similarly, P\(^{3-}\) has the configuration of Neon, \([Ne]\).
• Conversely, Ru\(^{3+}\) does not achieve any noble-gas configuration, indicating it doesn't achieve the same stable electron shell.

When an ion matches the electron configuration of a noble gas, it achieves lower energy and increased stability. This is why such configurations are often the 'goal' of ion formation in atoms.

The periodic table is a powerful tool that organizes all known elements based on their atomic number, electron configurations, and recurring chemical properties.

• The elements are arranged into periods (rows) and groups (columns).
• The recurring pattern of electron configurations shapes the layout of the table.
• Noble gases located in Group 18 play a crucial role in understanding the stability of electron configurations.

To write electron configurations, like those of ions in the exercise, we refer to this table, following the order set by the Aufbau principle. This principle dictates how electrons populate atomic orbitals, sequentially filling lower energy levels before moving to higher ones. Electrons fill orbitals in a specific order, typically observed as:

• 1s, 2s, 2p,
• 3s, 3p, 4s, 3d, 4p,
• 5s, 4d, 5p, etc.

By understanding the periodic table, one can predict the electron configurations of ions and whether they will share configurations with noble gases. This insight allows chemists to foresee the reactivity and bonding habits of various elements.