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 thanks for watching guys subscribe to my
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