We know that the periodic table is one of the most predictive tools in science. We're
gonna be looking at the periodic trends based on the periodic table so that we can have
a better understanding of how all the elements react together. We're gonna look at atomic
radii, ionization energy, and electron affinity. If we think about the atomic radii we're gonna
look at the periodic table in two directions the group going down as we talk about families
and groups such as the main group element 1A and then period. A period will be using
the same energy level and going from left to right on the periodic table. If we look
at going down a group the atomic radii is going to increase. This makes obvious sense
we're adding more and more electrons and as we add electrons we add more and more shells
and these shells are gonna increase the size. If we look across the period from left to
right we see a decrease in atomic radii. This doesn't make sense. We have increased the
number of electrons but the size is getting smaller. To understand this we need to look
at a phenomenon. Our Z effective our effective nuclear charge. It turns out as our nucleus
is increasing in the number of protons so as we go from left to right in the periodic
table we are adding the number of protons in the nucleus. As this occurs we're having
a higher effective attractive force on the outer electrons. This is the Z effective.
So as we go from lithium to fluorine our Z effective is increasing therefore the attractive
force pulling in those electrons has increased. That explains why when we go across the period
from lithium to fluorine we go from a larger atomic radius to a smaller atomic radius.
It's important to understand when we learn our periodic trends the bases. The base is
going down the group for atomic radii is a fact that we're adding more electrons in getting
more shells the basis for why we decrease atomic radius as we go across the periodic
table is the fact that we're actually increasing the Z effective, the attractive force that
pulls in those electrons. When we look at ionization energy ionization energy is defined
as the energy required to remove an electron from an atom to make an ion, a cation. When
we look at ionization energy the general trend is that it's gonna be increasing across the
period, going from left to right. It's gonna be decreasing in energy as we go down the
group, that makes perfect sense as there's more and more electrons it should be easier
to take an electron out of the system. Going across the period that also makes sense because
as we go from one electron to two in those outer shells it should be harder to remove
the next electron. There are a couple of hiccups in this elect ionization periodic trends and
that is when we get to fully filled orbitals so as the S2 and the half filled such as the
P3. At these points we see a small deviation in our energies. We see the same type of trends
when we get to electron affinity. Electron affinity is the amount of energy released
when electron is given to an atom. So if we look at chlorine if I give an electron to
chlorine how much energy will be released? If we think about this one we should see a
greater electron affinity for halogens because they're only one electron away from our next
noble gases. This is the trend we see but like the ionization energies we see small
variations when we get to fully filled, fully filled S and half-filled, half-filled P's
and half-filled D's. Using the periodic trends can help you understand the reactivity of
the elements understand their physical property, size, size of the ions, the amount of energy
required to either add or remove an electron. It's important for you to understand the periodic
trends because using the periodic table to its fullest amount will make your journey
through chemistry much simpler.