its professor David let's talk about the periodic table
pretty much everyone knows with this is even if they don't know much about
chemistry. it's the periodic table of the elements which at first seems like a
random arrangement of substances most of which sounds strange and foreign but the
way the elements are arranged reveals many beautiful patterns that tell us
about how nature operates. in the mid 1800's lots of chemists were trying to
come up with a way to depict all the elements in table form and many
different formats were proposed but it was the one by Dmitri Mendeleev that
stuck because of how well it correlates data as well as its predictive powers. he
arranged the elements into rows called periods and columns called groups
elements that had similar behavior were put in groups together which helped to
correlate existing data and it also predicted the existence of elements that
had never been seen before. with the gaps in the table Mendeleev said there must
be elements that go in these spots and he predicted some of their properties
eventually these elements were discovered with the properties precisely
as expected and now we have all the metals, metalloids and nonmetals
organized nicely
it wasn't known at the time but the reason elements in the same group behave
similarly is because they have the same number of valence electrons. look at
group one for example, these elements all have one valence electron or one
electron in their outermost shell. as you go down the table and n increases you
gain a shell each time but whichever is the outermost shell there is only one
electron in it. every element in group 2 has two electrons in its outermost shell
and so forth
this simple fact determines many characteristics about each element in
ways we will continue to see as we learn more chemistry. there are some periodic
trends that we can recognize when we look at the table. the first one is
atomic radius or the size of the atom. as we proceed downward on the table
atomic size increases because we add shells. as we go to the right
atomic radius decreases because we are moving within a shell and each element
to the right has one more proton in the nucleus than the last
so there is a stronger electromagnetic attraction felt by the electrons and the
radius shrinks, that means overall atomic radius increases going this way on the
periodic table
ionic radius is a little different, electrons repel each other so adding an
electron makes an atom bigger. taking one away makes it smaller
ions with the same electron configuration will have their radii
decrease as the atomic number increases
next we look at ionization energy. this is the energy required to remove an
electron from the atom. it will always be an electron in the outermost shell. the
electromagnetic force that attracts the electrons to the protons drops off very
quickly with distance so the farther away an electron is from the nucleus
the easier it is to pull it away. this means the ionization energy trend is
precisely the opposite of the atomic radius trend. francium, a very large
atom with only one valence electron will be easy to ionize because the electron
is so far away from the nucleus and atoms like to have their outermost shell
completely full
losing the electron means this shell is gone and the one below is completely
full so elements in group 1 will easily lose one electron. looking at the
opposite corner with helium there is only one shell so the electrons are very
close to the nucleus, and the shell is full so it is very stable. for this
reason it requires much more energy to ionize helium so the ionization energy
increases going this way on the periodic table. elements can have successive
ionization energies for removing more than one electron. a second ionization
energy will always be greater than the first and continue to increase from
there since the more electrons you remove the less stable the atom becomes
an element will have a huge jump in ionization energy after you take the
last one in a shell because then you jump to the noble gas electron
configuration from the previous shell which is full so it really won't want to
give up any more electrons. there are just a couple exceptions to the
ionization energy trend but we can rationalize them. look for example at the
second row from lithium to neon. the ionization energy should increase each
time we add a proton to the nucleus and the radius contracts a little bit
something like oxygen which dips downwards from nitrogen's ionization
energy does so because of orbital symmetry. here is nitrogens orbital
diagram as well as oxygen's. notice that nitrogen's 2p orbitals are precisely
half full
this gives nitrogen a special stability
just like elements that have a full outermost shell. if nitrogen loses an
electron it loses that special stability but if oxygen loses an electron it will
gain that special stability, that's why oxygen's ionization energy is a little
bit lower than nitrogen's even though oxygen has one additional proton. all
deviations from the ionization energy trend can be explained by discrepancies
in orbital symmetry like this one
next we will look at electron affinity. this is exactly the opposite of
ionization energy since ionization energy is how much energy you need to
remove an electron and electron affinity tells us how much an atom wants to gain
an electron. disregarding the noble gases as their shells are full
electron affinity increases this way. fluorine has the highest electron
affinity because if it gains one electron it will have a full shell
or noble gas electron configuration. looking at the opposite corner these
elements don't want to gain electrons they would rather lose them
exceptions to this trend happen for exactly the same reasons as the
exceptions to the ionization energy trend. lastly we want to look at
electronegativity, this is the ability of an atom to hold electrons tightly. it
will increase this way because a smaller atom like fluorine with more protons for
its energy level or higher effective nuclear charge will hold electrons best
again we will disregard the noble gases for this trend. electronegativity will be
important in the next clip where we learn about chemical bonds. so the
trends to remember are atomic radius which goes this way as well as
ionization energy, electron affinity, and electronegativity which all go this way
let's check comprehension
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