Atomic Structure & the Periodic Table p2

Periodic Trends

Covalent Radius Van Der Waals Radius

Atomic radius is half the distance between the nuclei in a molecule consisting of 2 identical atoms.

Generally, the atomic radius decreases going from left to right across a period and increases down a given group. Therefore because Francium is located at the bottom of the Periodic Table on the far left side, it has the greatest atomic radius. Likewise because Fluorine is located in the top right area of the Periodic Table, it has one of the smallest atomic radii.

Moving from left to right across a period, electrons are added one at a time to the outer energy shell. As electrons are added, there becomes more attraction between the protons in the nucleus and electrons in the electron shells. This attraction pulls the electron shells towards the nucleus, reducing atomic radii.

Moving down a group in the periodic table, the number of electrons and the number of electron shells increase. With more electron shells but the same number of valence electrons, the electrons in inner shells shield the valence electrons from the proton, leaving less attraction. With more shells, atomic radii increases.

Ionization Energy

The ionization energy, or ionization potential, is the energy necessary to remove an electron from an atom. When the electrons and electron shells are closer to the nucleus and there is more attraction between the electrons and the protons, the ionization energy is higher. The first ionization energy is the energy required to remove the first valence electron from the atom. The second ionization energy is the energy required to remove a second valence electron from the ion. The second ionization energy is always larger than the first ionization energy.

Ionization energies increase moving from left to right across a period. Ionization energy decreases moving down a group. Group I elements have low ionization energies because the loss of an electron is forms a stable octet.

Electron Affinity

Electron affinity is the ability for an atom to accept an electron (opposite of ionization energy). Electron affinity is the energy given off when an electron is added to an atom.

Moving left to right accross the period, electron affinity becomes more negative, because the elements on the left side of the period want to accept protons so they are closer to the stable octet. The elements on the right side of the table would rather lose an electron than gain 6 or 7 electrons.

Moving down a group, electron affinity becomes smaller. As more electron energy levels are added, atoms care less about getting more electrons when their electrons are so far away from the nucleus.

Electronegativity

Electronegativity is a measure of the attraction of an atom for the electrons in a chemical bond. The higher the electronegativity of an atom, the greater its attraction for bonding electrons. Electronegativity is related to ionization energy.

Low ionization energies = low electronegativities

High ionization energies = high electronegativities

This is because with higher ionization engergies, atoms have more protons and electrons and a stronger attractive force on its electrons.

As can be seen in the diagram below, moving left to right accross the period electronegativity increases but moving down a group electronegativity decreases.

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