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Introduction
The periodic table of elements is arranged not only in the order of atomic numbers, but is also grouped based on chemical properties. Because of this, you can find many trends on it.
Atomic Radius
Definition: The distance from the nucleus to the outermost electron orbital of an atom.
Horizontal Trend: Atomic radius decreases rightwards across the periodic table because of increasing nuclear charge. Higher number of protons exert a stronger pull on electrons (effective nuclear charge), which reduce the size of the electron cloud.
Vertical Trend: The atomic radius increases downwards across the periodic table. Heavier elements obtain more orbitals as lower energy levels fill up.
Ionic Radius
Definition: The distance from the nucleus to the outermost electron orbital of an ion with a full valence shell.
Horizontal Trend:
Cations (+) lose electrons and often entire subshells from their neutral form. Due to having less electrons, cations have a smaller ionic radius.
Anions (-) gain electrons. The new electrons cause more electrostatic repulsion, resulting in the expansion of the electron cloud. Therefore, they have a larger ionic radius.
Vertical Trend: The ionic radius increases as the as the periodic table progresses down periods. Heavier elements obtain more orbitals as lower energy levels fill up.
Electronegativity
Definition: The strength of attraction that a nucleus exerts on electrons in a chemical bond. It is associated with the effective nuclear charge.
Horizontal Trend: Electronegativity increases rightwards across the periodic table because of increasing nuclear charge.
Vertical Trend: Electronegativity increases upwards across the periodic table. Heavier elements have more energy levels between the nucleus and valence electrons, which weakens the effective nuclear charge (electron shielding).
Electron Affinity
Definition: How much energy is released when an atom accepts an electron.
Group 1 has little electron affinity because they have a half-filled s subshell, yet a full s subshell is more stable.
Group 2 has no electron affinity because those elements have a full s subshell already.
In the p block, electron affinity increases rightwards. Elements that are closer to a full valence shell are more reactive towards gaining electrons. This is due to effective nuclear charge (more protons) and how elements that are closer to a full valence shell have more potential energy, whereas a full valence shell is stable and has low potential energy. A greater drop in potential energy makes a reaction more energetically favorable.
Electron affinity increases upwards across the periodic table. Heavier elements have more energy levels between the nucleus and valence electrons, which weakens the effective nuclear charge (electron shielding).
Ionization Energy
Definition: How much energy is required to eject an electron out of an atom.
Horizontal Trend: Ionization energy decreases leftwards across the periodic table, as those elements are less electronegative. It is also way easier for elements leftward to lose their electrons than to gain a greater number of electrons to fulfill a full valence shell.
Vertical Trend: Ionization energy decrease downwards across the periodic table. Heavier elements have more energy levels between the nucleus and valence electrons, which weakens the attraction between them. This makes it easier to eject electrons from heavier elements.
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