Transcript for:
Understanding Periodic Trends in Chemistry

in this lecture we're going to talk about four important periodic trends atomic radius ionization energy electro negativity and electron affinity of atoms now let's begin with the atomic radius so what is an atomic radius well it's exactly what you think it is if you think of our atom as being a sphere then the radius begins at the center of our nucleus and ends at the outermost electron shell so for this atom our radius is the black line so I want to ask the question what happens to our atomic radius as we go from left to right across a period on our periodic table for example let's take the following period let's begin with lithium and go all the way up to floride what happens to our atomic radius well we see that atomic radius decreases as we go from lithium to Florine why is that well it's because of two things first the number of protons or number of protons found in our nucleus increases as we go from lithium to Florine and second the number of electrons found on our outermost electron shell also increases and this means according to kum's law the force also increases in other words the force with which the protons p pull the outermost electrons increases and this means that our effective nuclear charge on our atom increases and if the force is stronger so the protons are pulling our outermost electrons with a greater force that means our radius will decrease the distance between the center the nucleus and the outermost electron will decrease as we go across the period so that means our lithium will have the highest radius the largest radius and the smallest effect of nuclear charge while Al fline will have the highest effective nuclear charge and the smallest atomic radius so now let's talk about a group what happens as we go from top of the group to the bottom of the group so let's look at the following group let's begin with lithium and go to sodium then potassium and so on well as we go down the group our atomic radius tends to increase and this is because with which atom we add a new energy shell so let's look at the following two atoms let's look at lithium and let's look at sodium sodium is right below lithium on the same group on the periodic table so notice that we have two energy levels 1 s and 2 s for the lithium while the sodium has not two but three energy levels 1 s 2s and 3s now this addition of the 3s means that our atom will grow in size will enlarge where this guy the outermost guy is the 3s shell so that means when we move one down to potassium potassium will have a 4S so potassium will be even larger than sodium and definitely larger than lithium and that's exactly what we see in other words as we go down a group our atomic radius tends to grow in size while as we go across the period our atomic radius tends to decrease because our effective nuclear charge of our atom tends to increase so that's atomic radius now let's look at the ionization energy of our atoms so what is ionization energy well electrons don't simply come off the atoms by themselves remember electrons are held together by electrostatic force that comes from the positively charged nucleus and the negatively charge electrons so something must pull those electrons away in other words work or energy must be inputed into our system to pull that electron off so therefore we can Define our ionization energy to be the energy required to pull off that electron that outermost electron now more than one an electron can be pulled off for example calcium calcium in its neutral State can take away two electrons to become calcium plus two so that means some atoms can pull away or uh can give off more than one electron now the energy required to pull away that first electron is known as the first ionization energy while the energy required to pull away that second electron is known as the second ionization energy and so on now let's look at the following now the less likely an atom gives up the electron the more energy is required to pull that electron off and we see that as we go across a period our ionization energy of our atom tends to increase and to explain that let's look at kum's law now kum's law once again states that the force is equal to a constant K time charge q1 * charge Q2 divided by the distance between them squared where this guy is the charge due to the nucleus and Q2 is the charge due to the electrons now what happens as we move for example from lithium to Florine we already said that our effective nuclear charge tends to increase as we go from left to right so Florine has the highest force and in other words the the protons found in the nucleus pull those electrons on the outermost electron shell with a lot of force much more Force than lithium or brillium or Boron or carbon and that means it's going to require much more energy to pull those outermost electrons off and that's exactly why as we go across a period from lithium to Florine our ionization energy tends to increase because as we go this way we have a higher effective nuclear charge which means we have a greater force and also take this into account as we go across our atomic radius decreases and that means our denominator our R also increases so we see that our Q's increase our RS decrease and whenever the denominator decreases that means our for Force tends to increase so not only does this increase in charge cause the force to go up but also the decrease in this in the r or the atomic radius it tends to increase our Force so therefore as we go from left to right our ionization energy also increases how bad when we go down a group well when we go down the group our atomic radius increases and that means if we go back to kum's law if our atomic radius increases that means the distance between q1 and Q2 or the protons and electrons increases so our R also increases and if our R increases our denominated is increased and that means our force is less so as we go down a group our ionization energy tends to decrease so to wrap up basically the high your ionization energy is the less likely you are to give up electrons and we'll see that this directly translates into something called electr negativity so let's look at the third periodic trend called electro negativity now electro negativity is simply the ability of atoms to accept or attract other electrons and we see that as we go from left to right across the period from lithium to Florine our electro negativity increases so let's examine why let's look at the atomic structure and the electron configuration of lithium and compared to that of Florine now lithium has three electrons and three protons so its nucleus is composed of only three protons while its inner shell is composed of two electrons and it's outer shell it's composed only of a single electron now let's look at the Florine Florine has nine protons in its nucleus while two electrons are found in the inner shell seven electrons are found on the outer shell and that means because we have a higher nuclear or effective nuclear charge we have a higher force in other words because we have nine protons in our nucleus and seven electrons on our outer shell our force with which our nucleus pulls those electrons is much greater than the force with which the these three protons are pulling the single El and so that means if we place some arbitrary electron equidistant between these two atoms what we see is that this Florine will pull this electron with much more Force than this guy and that means as we go from lithium to burum to Boron to Chlor to a carbon to nitrogen to oxygen finally to Florine our electr negativity increases and in fact Florine is the most electr negative atom and electr negativity is actually measured on a scale called the polling scale and it's given a highest value of 4.0 now as we go across the period from left to right we see that our electro negativity increases how about as we go from top to bottom well as we go from top to bottom our atomic radius increases so our force with which our protons in the nucleus nucleus pull those electrons decreases because remember Force according to kum's law is equal to K * q1 * Q2 over R2 so our denominator increases decreasing our force and so therefore as we go from top to bottom our electro negativity decreases now for noble gases electr negativity is undefine and that's because in noble gas structure the electron configuration is per perfect noble gases can't accept any more electrons because notice that this 2p orbital can accept one more electron and that's exactly why Florine can accept one more electron but the next atom the noble gas after this guy can't accept any more electrons because it has a 2p6 electron configuration so let's look at our final periodic trend called electron affinity now electron affinity is amount of energy released when an atom gains an electron remember the only way to take an electron away from the outer shell of an atom is to apply work is to input energy because work must be done against the force of the protons in the nucleus attracting those electrons those outer electrons so that means the reverse must be the following whenever an electron or whenever an atom gains an electron energy must be released and that's exactly what happens when Florine for example gains electrons when Florine goes from a neutral molecule gains an electron to form an anion it loses energy the energy level of the outer shell is lower and therefore this molecule this anion is more stable than the neutral counterpart so this reaction going this way is exothermic now um that means whenever we go from lithium to brillium to Boron and so on whenever we go from left to right our electron affinity increases and that means this guy this reaction is more exothermic for Florine than for lithium in other words whenever our Florine gains electrons it becomes stable and it loses a lot of energy on the contrary whenever this guy gains electrons the reaction for this guy is endothermic because this guy the lithium in the neutal state is more stable than the anion than lithium minus one and that's exactly what we mean by electron affinity now likewise as we go from top to bottom on our group in any group on the period our electron affinity decreases and that's because our atomic radius increases as we go from top to bottom so as we go from top to bottom we go from exothermic reactions to endothermic reactions