In depth:  Periodic trends

Periodic Trends:

• Atomic radius:  Simply put, this is a measurement of the size of an atom (it’s determined by finding ½ the bond distance between two atoms of the same element).
• As you move from left to right across a period, the atomic radius of an element decreases.  This is because the energy of the valence electrons is roughly the same (because each period represents an energy level) but the nucleus gains more protons – this causes the electrons to be pulled more closely toward the nucleus.
• As you move from top to bottom down a group, the atomic radius increases because you’re adding a new energy level (and new valence electrons to go with them).
• Ionic radius:  Because cations have fewer electrons than neutral atoms, they’re smaller than neutral atoms.  Because anions have more electrons than neutral atoms, they’re bigger.
• Ionization energy:  This is the amount of energy needed to pull an electron off of an atom in a gas.
• As you move from left to right across a period, the ionization energy increases because the nucleus has more protons and holds the electrons more tightly.
• As you move from higher to lower in a group, the ionization energy decreases because of the shielding effect (which says that the farther an electron is from the nucleus, the less tightly the atom grabs it).
• Multiple ionization energies:  If you want to pull off more than one electron from an atom, more energy is required for each additional one you want to grab.  Once you’ve reached a noble gas configuration (for example, once magnesium has lost two electrons to become like neon), any further electrons you pull off will require a huge amount of energy.
• Electronegativity:  This is a measurement of how much an atom wants to pull electrons away from atoms that it has bonded to.
• As you move from left to right across a period, the electronegativity increases because elements on the left side want to lose electrons to be like the nearest noble gas (octet rule) so don’t want to grab electrons, while elements on the right side want to gain electrons to be like the nearest noble gas so grab electrons a lot.  The exception to this is the noble gases – they have no electronegativity at all because they are noble gases, and don’t need any more electrons.
• As you move from higher to lower in a group, the electronegativity decreases due to the shielding effect (which states that electrons in outer energy levels are held less tightly than those in lower energy levels).  Generally, if an atom doesn’t hold the electrons it has very much, it won’t grab electrons from other atoms much, either.