Atomic Properties and Periodic Trends
Atomic Properties
Electron Configuration
The electron configuration of an atom describes the distribution of its electrons among the various atomic orbitals. Several principles govern this distribution:
- Pauli Exclusion Principle: No two electrons in the same atom can have the same four quantum numbers. Each orbital can hold a maximum of two electrons with opposite spins (anti-spin).
- Building-Up Principle (Aufbau Principle): Electrons fill orbitals successively in order of increasing energy.
- Hund’s Rule: When filling orbitals within the same sublevel, electrons will individually occupy each orbital before pairing up. Unpaired electrons in a sublevel have parallel spins.
Periodic Properties
Several key properties of atoms exhibit trends across the periodic table:
- Atomic Radius: Represents the distance between the nucleus and the outermost electron shell (valence shell). It helps determine the size of the atom.
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Ionic Radius:
The radius of an atom that has gained or lost electrons, achieving the electron configuration of the nearest noble gas. There are two cases:
- Anion Formation: When an element gains electrons, the added electron(s) occupy empty orbitals, forming an anion.
- Cation Formation: When an element loses electrons, it typically loses its valence electrons, forming a cation.
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Ionization Energy: The minimum energy required to remove an electron from a neutral, gaseous atom in its ground state, forming a positive ion.
- Group Trend: Decreases down a group because the atomic volume increases, and the outermost electron is farther from the nucleus.
- Period Trend: Increases across a period (left to right) because there are more electrons and protons, leading to a stronger attraction.
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Electron Affinity: The energy change that occurs when a gaseous atom in its ground state accepts an electron, forming a negative ion.
- Group Trend: Generally decreases down a group because the added electron is farther from the nucleus and less strongly attracted.
- Period Trend: Generally increases across a period, favoring electron gain to achieve a noble gas configuration.
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Electronegativity: The tendency of an atom to attract electrons in its outermost shell.
- Group Trend: Decreases down a group because the outermost electrons are farther from the nucleus and less strongly attracted.
- Period Trend: Increases across a period, enhancing the tendency to gain electrons to achieve a noble gas configuration.
Chemical Notation: Symbols and Formulas
Chemical nomenclature and notation provide rules for representing chemical substances in an abbreviated form. Formulas express what happens in a chemical change without explanatory words, providing information about the substances involved.
- Symbol: Identifies an element. In a formula, it represents one atom of that element and a specific amount of that element.
- Formula: Represents the number and types of atoms in a molecule of an element or a compound. It’s an abbreviated way of representing the composition of a substance using symbols and numbers.