hey it's professor Dave, let's talk about the periodic table. pretty much everyone knows what 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 sound strange and foreign.
but the way the elements are arranged reveals many beautiful patterns that tell us about how nature operates. in the mid-1800s 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 non-metals organized nicely. it wasn't known at the time but the reason elements in the same group behave similarly 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 two has two electrons in its outermost shell and so forth. this simple fact determines many characteristics about each element 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 downwards 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 one 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, but something like oxygen which dips downwards from nitrogen's ionization energy does so because of orbital symmetry.
here is nitrogen's orbital diagram as well as oxygen's. notice that nitrogen's two p 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 a 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 channel for more tutorials, and as always feel free to email me