7.1 - Atomic Radius Flashcards
Atomic radius: Is refering to the size of the electron cloud, not the nucleus
* while the nucleus weighs almost all of the atom, the electrons are what determine the radius
Atoms get larger as thye go down a group, and they get larger as they go from R –> L
* this seems slightly counter intitive because masses get larger from L –> R. This is due to the effective nuclear charge (shown below)
Francium is the fattest element (you get fat in france)
below its showing the increase in atomic radius from R –> L
* however, al is the heavist while Na is the lightest, but this doesnt matter
The sodium atom has 11 protons in the nucleus, meaning it has a +11 charge
* so we get a more and more charged nucleus as we go from L –> R across a period
So that outermost electron shell (valence electrons) determines that anatomic radius. That distance is defined by how much those outer electrons are attracted to the nucleus (which has positively charged protons in the middle)
* So like we said, the # of protons increases from L –> R
* So because sodium has less protons in its nucleus those valence electrons are less attracted to the nucleus, making them further away from the nucleus. So less protons = less overall mass, however, it still has a bigger radius
higher effective nuclear charge (calculated in the equation below) = those electrons are more attracted to the nucleus (meaning their radius is smaller)
* note valence electrons don’t contirbute to the shielding electrons because they dont block eachother from being attracted to the nucleus (dont get in the way of eachother because they’re in the same orbit)
* These effective nuclear charges match the charge on each group (for instance carbons group has a +4 effective nuclear charge) –> makes sense because this incraeases from L –> R, meaning the electrons are more pulled into the nucleus meaning the atomic radii are shinking in this direction (but the mass is increasing)
As you go down the atomic radii increases, this is because you have more shell electrons blocking the valence electrons from being attracted to the protons in the nucleus.
**As you go from L –> R the atomic radii shrinks because there are more protons in the nucleus, and the same # of shielding electrons (because the outer valence electrons don’t count as shielding electrons and those are what are being increased from L –> R)
Bond length is defined as the length from nucleus to nucleus of 2 atoms
* so you can basically just add the 2 radii of the atoms together to find the bond length
* With covalent bonding us atomic radii, and w/ ionic bonding us ionic radii (were doing atomic for now)
So larger atoms make longer bonds, because their radii are larger (and the bond length is the sum of both atomic radii)
There are sometimes questions like this: Which one has the longest bond:
* C - F
* C - Cl
* C - Br
* C - I
* You would pick the C - I bond because iodine has the largest atomic radius (as you go down a group you get more shielding electrons, which means the valience electrons arent as closely attracted to the protons in the nucleus, meaning the v electrons devaite further from the nucleus, meaning the atomic radii is larger [doesnt correspond w/ atomic mass])
Atomic radii assumes you have neutral atoms. When talking about Ionic radii it assumes we have cations and anions (they have charges)
When you go from a neutral atom to a cation, you’re going to lose at least one electron (makes it more positive)
* if you lose electrons from you electron cloud, than your electron cloud gets smaller (so you lose your valence electrons and your atomic radii shrinks)
* so cations have a smaller ionic radii than if they were in their elemental form w/ an antomic radii (makes sense, as cations they lose electrons, making them not have as large of an atomic radii)
Anions:
* gaining electrons = bigger atomic radii (essentially adding more things that are repelled by eachother, making the atomic radii even larger)
* So S^2- = much larger than S
* so antions have a larger ionic radius than the atomic radius they would have as a neutral atom
Questions will be like this: Which of the following has an ionic radius that is larger than its corresponding atomic radius?
* You’re proably going to have a bunch of metals and a non metal
* its the non-metal, because they have larger ionic radii because when they turn into ions they gain electrons, increasing their ionic radii because they’re all repelled by eachother and shield eachother and make it harder for themselves to be attracted to the positively charged nuceli
Ways to compare ionic radii:
They can do it if they all have exactly the same charge:
* Mg^2+
* Ca^2+
* Sr^2+
* Ba^2+
* This means their ionic radii are smaller than the atomic (because were taking away electrons)
* but they’re proably all smaller by the same amount because we’ve taken away 2 electrons from all of them
* So we would basically just find which one has the largest atomic radii and use that because they’re all smaller by the same amount
* Ba is the largest in this case (its got more core electrons, meaning its shielding the protons from the valence electrons more, meaning the valence electrons are less attracted to the protons in the nucleus, meaning they’re more spread out = larger atomic radii)
comparisions in atomic radii
Isoelectric series
* series of atoms that have the same electron configuration (but the protons vary)
All of the below have the same # of electrons - these are all isoelectronic. So the difference between these atoms/ions isnt in the electron cloud because its all the same. Its all about the protons in the nucleus. The one w/ the most protons in the nucleus will have the smallest atomic radii, because the electrons are going to be more attracted to it
* S^2- (nuclear charge = +16) <— largest
* Cl- (+17)
* Ar (+18)
* K+ (+19)
* Ca^2+ (+20) <— so this will be the smallest in the series
Fr
So remember, as you go down the anatomic radius increases because more electrons are shielding the protons, making the valience electrons less attracted and therefore wider
As you go from R –> L you have decreased protons in the nucleus when compared to electrons (because you’re basically just adding valence electrons), meaning the anatomic radius will increase (they all repell eachother away)
answer = Al, because Al has the least protons in the nucleus compared w/ the # of electrons
So the longest bond is assigned to the 2 biggest atoms.
O is smaller than S so it can’t be that one
I is the biggest thing bound to S, so S-I is the answer
So remember, these all lost an electron, meaning their radius must’ve shrunk
* since they all lost one they’re all proportionally the same
* Since just pick the smallest atom
Li+ is the smallest because it has the least core electrons from shielding the valence electrons, meaning they can be more attracted to the proton nucleous
So this means that as an ion its radius is smaller than as an elemental atom.
* This is true of cations because they lose an electron, meaning all the other electrons are less repelled by eachother and can get closer to the nucleus
Al is the only possible answer (only metal)
Meaning it added electrons, which all repel eachother so the ionic radius would be larger
* so it must be a non-metal anion
P
So they all have the exact same # of electrons (they’re equivilants), however they have differing #’s of protons.
Sr^2+ has the most protons and the same number of electrons as the others, meaning that it holds its electrons closer, meaning its anatomic radii is the smallest.
Te^2-
This is because it has the least protons to the # of electrons (they all have the same # of electrons). Therefore it holds them least closely, giving it the largest anatomic radii
