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.