Atomic Size refers to the physical size of an atom, typically represented by the distance from the nucleus to the outermost electrons. It is often measured in terms of atomic radius, which can be defined in various ways, such as covalent radius, metallic radius, or van der Waals radius, depending on the type of bonding or interactions being considered.
Trends in Atomic Size in the Periodic Table
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Down a Group:
- Atomic size increases as you move down a group in the periodic table. This is primarily due to the addition of electron shells (energy levels). Each successive element in a group has its valence electrons in a higher energy level (further from the nucleus), which increases the overall size of the atom despite the increase in nuclear charge. The effect of increased shielding by inner electrons also plays a role, as it reduces the effective nuclear charge experienced by the outermost electrons, allowing them to be further away.
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Across a Period:
- Atomic size decreases as you move from left to right across a period. As you advance across a period, protons are added to the nucleus, increasing the positive charge and attracting the electrons more strongly. Although electrons are also being added, they enter the same energy level and do not significantly increase shielding. Consequently, this stronger nuclear charge pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.
Key Factors Influencing Atomic Size
- Effective Nuclear Charge (Z_eff): The net positive charge experienced by valence electrons, influenced by shielding from inner electron layers.
- Electron Shells: The greater the number of electron shells, the larger the atom; each added shell increases the distance of the outermost electrons from the nucleus.
- Shielding Effect: Inner electrons repulse outer shell electrons, reducing the effective nuclear charge felt by valence electrons.
Understanding atomic size is critical for predicting other chemical properties, such as ionization energy and electronegativity, as they are all interrelated within the context of an element's position in the periodic table.