Atomic Radii • Definition: The size of the atom from the center of the nucleus to the valence shell of electrons.
As we move from left to right across the PERIOD, we see an ever increasingly positive nucleus. If the atom remains neutral, the number of electrons must increase also. This would cause us to assume the atoms size also increases as we move from left to right across the PERIOD.
The atomic radii actually decreases as we move across the PERIOD. This is caused by the ever increasingly positive nucleus pull on the valence shell electrons with more and more force. Even though we are adding more electrons, we are adding them in the same shell as the previous element.
Lithium Fluorine e- e- e- e- e- e- 9+ e- e- 3+ e- e- e- e-
Other TRENDS: Ionization Energy – How much energy it takes to remove a valence electron. Electronegativity – The likelihood that an atom will GAIN an electron(s) to fill its outside shell.
For the SAME reason the atom becomes smaller, BOTH these trends increase in value as wemove across the PERIOD.
GROUP TRENDS • Atomic radii increases as we move down the GROUP. This is due to adding a level of electrons. • Both the IONIZATION ENERGY and the ELECTRONEGATIVITY decrease as we move down the GROUP. This is due to the added level of electrons. Even though the nucleus is more positive, the added level of electrons diminish the pull on the VALENCE ELECTRONS.