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ssss1 Trends in variation of atomic radii (in angstroms; 1 Å = 10⁻¹⁰ m) with their position on the periodic table, illustrated by rows 3 and 4. With few exceptions, radii tend to decrease from left to right and from bottom to top.

Atomic radii

ssss1 Radii (in angstroms) of some common cations in relationship to the atomic radius of the neutral atoms.

valence electrons

ionic radii

ssss1 Radii (in angstroms) of some common anions in relationship to the atomic radius of the neutral atoms.

The ionic radii of anions are significantly larger than the atomic radii of the same neutral (uncharged) element (ssss1). When electrons are added to the electron cloud during anion formation, the positively charged protons in the nucleus exert a smaller force on each of the electrons. This allows the electrons to move farther away from the nucleus, which causes the electron cloud to expand, increasing the effective radius of the anion. The larger the charge on the anion, the more its effective radius is increased.