Trends in the periodic table


Periodic Trend Across (right) Down
Atomic Number Increase Increase
Valence Electrons Increase Increase
Mass Increase Increase
Atomic radius Decrease Increase
Ionisation Energy Increase Decreases
Electronegativity Increase Decreases
Ionic radius Decreasesbut if new shell, increase Increases
Metallic character Decrease Increases

Atomic Radii

  • Dependent on number of electrons, number of shells and number of protons.
  • Going down a group, the number of protons increase. However, so does the number of electrons and the number of shells. Since there are more shells shielding the valence electrons from the nucleus, the outer electrons are held more weakly. Thus, the atom is larger.
  • Going across a period, the number of protons increase but the number of shells remain the same. Any additional electrons are added to the outermost shell and do not contribute to the shielding of the valence electrons. Hence, the core charge increases and the outer electrons are held more strongly, making the atom smaller.

First Ionisation Energies

  • Going down, the number of protons increase, and so does the number of shells. Thus, the outermost electrons have a smaller core charge since they are farther away from the nucleus, making them easier to remove.
  • Going across, the number of shells remains the same. New electrons are added to the outermost shell, but protons are being added to the nucleus. Therefore, the nucleus attracts the outer electrons more strongly, resulting in higher IEs.


  • Electronegativity is the tendency of atoms to attract electrons (almost opposite to ionisation energy).
  • Going down, electronegativity decreases as the distance between the nucleus and any incoming electrons increases. Thus, these electrons are not as attracted as strongly.
  • Going across, atoms become smaller, so any nearby electrons are greatly attracted to it. Core charge increases, so electrons feel a greater force.
  •  You can predict where an electron will spend most of its time in a bond.

For NaCl

Since Cl is much more electronegative than Na, the electrons spend most of its time near the chlorine atom.

Since you can’t fully get rid of the electron, it still has an attractive force upon the Na.

Thus, pure ionic substances do not exist.

The difference between the electronegativities can be used to determine the ionic character, how ionic a substance is. NaCl is only 70% ionic, the other 30% of the time, the electron is shared as if in a covalent molecule.

However, pure covalent bonds can exist, if the difference between electronegativities is exactly 0.

Consider O2: since the ENs are the same, the electrons are shared equally; the covalent character of O2 is 100%.

A substance is termed ionic if it conducts electricity in a liquid state.




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