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AP®︎/College Chemistry
Course: AP®︎/College Chemistry > Unit 1
Lesson 6: Periodic trends- Periodic trends and Coulomb's law
- Atomic and ionic radii
- Ionization energy: group trend
- Ionization energy: period trend
- First and second ionization energy
- Worked example: Identifying an element from successive ionization energies
- Electron affinity: period trend
- Electronegativity
- Periodic trends
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Atomic and ionic radii
Atomic and ionic radii are found by measuring the distances between atoms and ions in chemical compounds. On the periodic table, atomic radius generally decreases as you move from left to right across a period (due to increasing nuclear charge) and increases as you move down a group (due to the increasing number of electron shells). Similar trends are observed for ionic radius, although cations and anions need to be considered separately. Created by Jay.
Want to join the conversation?
- I dont understand why anion of an element is bigger? As we know by a trend in modern periodic table that across a period the number of valence electrons increase by one but still the atomic size decreases,so why does this not apply for the example of anion being bigger as there also only one extra valence electron is getting added?(14 votes)
- The trend you mentioned is so because as there is an additional proton with an increase of an electron. Since there is no electron shielding for the same period element, the additional proton pulls all the electron closer. Thus the radius is shorter as you go right the periodic table. However, it is not the same for ions. An anion means the number of proton stays the same while an additional electron comes in the orbital. The positive charge does not increase, so the radius will be larger due to the stronger electron repulsion. And vice, versa, a cation will have significantly smaller radius because an electron goes away while the positive charge stays the same.(32 votes)
- OK so I understand how a lithium atom would be smaller if an electron is removed b/c that's eliminating a whole energy level, but why would a beryllium atom be smaller if it became a cation and an electron was removed, since... a remaining electron would still be present in the outer shell? Do I have a knowledge gap here?(5 votes)
- When an electron is lost, the other electrons feel a stronger attraction to the nucleus. Does that make sense?(9 votes)
- How do you even measure that small a distance? ( Atomic radius )(2 votes)
- Probably the most common way to determine these distances is using a method called X-ray crystallography. Along with being able to measure distances between atoms it allows us to determine the structure of a molecule. In effect being able take a picture of the molecule.
This method involves first creating a crystal composed of the molecule with sufficient size, high purity, and a rectangular prism shape. Which in my experience is the most time consuming. Afterwards we place the crystal in an instrument where X-rays are directed toward it and the molecules in the crystals scatter those X-rays. Those scatterings essentially give us pictures of the molecule which we can combine to yield the structure of the molecule and allows us to determine the distances between the atoms.
Hope that helps.(7 votes)
- what does pm mean?(2 votes)
- Post meridiem, or after midday. Na just kidding.
In a chemistry context pm stands for picometers. It’s a unit of length equal to 10^(-12) m, or a trillionth of a meter. We have length this small to measure the sizes of atoms.
Hope that helps.(3 votes)
- Why do we always assume that electrons revolve around the nucleus? Why do they have to revolve around the nucleus? Can we explain all these with the quantum mechanical model of atom, where electrons just move randomly around the nucleus?(1 vote)
- because we don't need quantum mechanical model to describe and calculate radii (and other stuff), revolving model works just fine and it simplifies calculus big time. it's almost impossible to calculate anything with quantum model without computers. quantum model is mainly used for describing orbitals and energy of atoms and molecules(3 votes)
- (Atam) So, I was wondering what how do you calculate a radii of any element? 8:01(2 votes)
- Are bigger elements more reactive and unstable?(2 votes)
- If you mean by bigger atoms with more protons then yes. But if you mean bigger by atomic radius then is not always the case for example a neutral atom of Chlorine is smaller than a Cl- atom and yet the bigger Cl anion is more stable than the neutral Cl. That's because its outer shell is filled and that's what atoms generally want.(1 vote)
- Shouldn't chlorine anion be smaller in size since the number of electrons increases Columb force also increases?(2 votes)
- No it is bigger. The nuclear charge stays the same but the number of electrons has increased. This nuclear charge has to be distributed for all of the electrons so it gets weaker. This "stretches" the atom and increases the ion's size.
Remember: anions are always greater than the neutral atoms of the element. It's the opposite for cations.(1 vote)
- can someone please help me explain this "shielding effect", am really confused(1 vote)
- You know how putting on a jacket makes you feel the cold less? It's a similar idea to that.
The inner electrons are like a jacket for the outer electrons. They take away some of the charge from the nucleus that the outer electrons feel.(3 votes)
- Will this help in grinding for IChO in Canada?(2 votes)
- KA would most likely help. At the very least it would serve as an introduction to many of the topics on the IChO. It would be best to seek out additional chemistry teaching materials too. Studying from a variety of sources and becoming proficient in them would certainly help your odds.
I have to warn you though that the IChO exam is formidable for secondary school students. The difficulty of the material they test on is what you would expect at a college or university chemistry course. Here on KA they can give you a good start in general chemistry with their AP chemistry material, but the IChO also tests organic chemistry (also found on KA), physical chemistry, and spectroscopic techniques like proton NMR, C-13 NMR, and IR.
Hope that helps.(1 vote)
Video transcript
In this video, we're going to
look at atomic and ionic radii. And first, we'll start
with the atomic radius. So if you think about
an atom as a sphere, the idea of atomic
radius is simple. You would just take
this as a sphere here, and then a sphere of course
would have fixed and defined radius. And so that would be one
way of thinking about it. The problem is that
an atom doesn't really have a fixed, defined radius
like this sphere example, because there's a
nucleus and then there's this electron
cloud, or this probability of finding your electron. So there's no real, clear
defined boundary there, and so it's difficult to have
a fixed and defined radius. So what chemists do is they
take two identical atoms. So let's say these are
two atoms bonded together, the same element. And if you find their
nuclei-- so let's say that that's
their nuclei here-- and you measure the distance
between those two nuclei, so this would be our distance
d between our two nuclei. If you take half
of that distance, that would be a
good approximation of the atomic radius
of one of those atoms. And so that's the idea
behind the definition of atomic radius. Let's look at the trends
for atomic radius, and first we'll start
with group trends. And so here we have
two elements found in group one, so
hydrogen and lithium. And let's go ahead and
sketch out the atoms first. And so we start
with hydrogen, which has atomic number
of 1, which means that it has one
proton in the nucleus. So here's our nucleus for
hydrogen, so one proton. In a neutral atom,
the number of protons equals the number of electrons,
and so therefore there must be one electron. So go ahead and sketch
in our electron here. And we'll make
things really simple and just show this simple
version of the atom, even though we know it doesn't
really exactly look like this. And when we do lithium,
atomic number of 3, so that means three protons
in the nucleus of lithium. So this is representative
of lithium's nucleus with three protons
and three electrons. Two of those electrons
are in the inner shell. So let me go ahead and show
two of lithium's electrons in the inner shell, so that
would be in the first energy level. And then we would need
to account for one more, so lithium's third electron
is in the second energy level or at the outer shell
in this example. And so here we
have our two atoms. And you can see as
you go down a group, you're going to get an
increase in the atomic radius. And that's because as
you go down a group, you're adding electrons
in higher energy levels that are farther away. So in this case, we
added this electron to a higher energy level
which is farther away from the nucleus, which means
that the atoms of course would get larger and larger. So you're adding
more stuff to it, so it's kind of a simple idea. Let's look at
period trends next. As you're going across
a period this way, so as you're going
this way, you're actually going to get a
decrease in the atomic radius. And let's see if we
can figure out why by once again drawing some
simple pictures of our atoms. And so lithium with
atomic number of 3, so we've already
talked about that. So there are three protons
in the nucleus of lithium. So I'm going to go ahead
and write that in here. So 3 positive charge for
the nucleus of lithium. And we have to account
for the three electrons. So once again two
of those electrons were in an inner shell,
so there we go, and then we had one electron
in an outer shell, so the picture is
something like this. Now, let's think
about what's going to happen to that outer electron
as a result of where it is. So this outer electron, this
one right here in magenta, would be pulled
closer to the nucleus. The nucleus is
positively charged, that electron is
negatively charged, and so the positively
charged nucleus is going to pull that
electron in closer to it. At the same time,
those negatively charged inner shell electrons
are going to repel it. So let me go ahead and
highlight these guys right here. These are our inner
shell electrons. Like charges repel. And so you could think about
this electron right here wanting to push this
outer electron that way, and this electron wanting to
push this electron that way. And so the nucleus
attracts a negative charge, and the inner shell electrons
repel the outer electron. And then we call this shielding,
because the inner shell electrons are shielding
that magenta electron from the pole of the nucleus. So this is called
electronic shielding or electron screening. Now, it's going to be
important concepts. So now let's go ahead
and draw the atom for beryllium, so
atomic number 4. And so here's our
nucleus for beryllium. With an atomic number
of 4, that means there are four protons in the
nucleus, so a charge of four plus in our nucleus. And we have four electrons
to worry about this time, so I'll go ahead and
put in the two electrons in my inner orbital in
our first energy level. And then we have two electrons
in our outer orbital, or our second energy level. And so again, this is
just a rough approximation for an idea of what
beryllium might look like. And so when we think
about what's happening, we're moving from a charge
of 3 plus with lithium to a charge of 4
plus with beryllium. And the more positive your
charges, the more it's going to attract
those outer electrons. And when you think about the
idea of electron screening, so once again we have
these electrons in green here shielding our
outer shell electrons from the effect of that
positively charged nucleus. Now, you might think
that outer shell electrons could shield, too. So you might think that oh, this
electron right here in magenta could shield the other
electron in magenta. But the problem is
they're both at pretty much the same distance
from the nucleus, so outer shell electrons don't
really shield each other. It's more of these
inner shell electrons. And because you have the same
number of inner shell electrons shielding as in the lithium
example-- so let me go ahead and highlight those again. So we have two inner
shell electrons shielding a beryllium. We also have two inner shell
electrons shielding in lithium. Because you have the
same number of shielding but you have a higher
positive charge, those outer electrons are
going to feel more of a pull from the nucleus. And they're going to be pulled
in even tighter than you might imagine, or at least tighter
than our previous example. So these electrons are
pulled in even more. And because of
that, you're going to get the beryllium
atom as being smaller than the lithium
atom, hence the trend. Hence as you go
across the period, you're always going to increase
in the number of protons and that increased
whole is going to pull those outer
electrons in closer, therefore decreasing
the size of the atom. All right. Let's look at ionic radius now. And ionic radius can
be kind of complicated depending on what chemistry
you are involved in. So this is going to be
just a real simple version. If I took a neutral
lithium atom again, so lithium-- so we've
drawn this several times. Let me go ahead and
draw it once more. So we have our lithium nucleus,
which we have three electrons. So once again I'll go ahead and
sketch in our three electrons real fast. Two electrons in the inner
shell, and one electron in the outer shell like that. And let's say you were
going to form a cation, so we are going to take away an
electron from our neutral atom. So we have-- let me
go ahead and draw this in here-- we had a three
protons in the nucleus and three electrons those
cancel each other out to be a neutral atom. And if we were to take away
one of those electrons, so let's go ahead and show
lithium losing an electron. So if lithium loses
an electron, it's going to lose that
outer electron. So the nucleus still
has a plus 3 charge, because it has
three protons in it. And we still have our two inner
shell electrons like that, but we took away that
outer shell electron. So we took away this electron
in magenta, so let me go ahead and label this. So we lost an electron, so
that's this electron right here, and so you could just
show it over here like that. And by doing so, now we
have three positive charges in our nucleus and
only two electrons. And so therefore our lithium
gets a plus 1 charge. So it's Li plus, it's a cation. And so we formed
a cation, which is smaller than the
neutral atom itself. And that just makes
intuitive sense. If you take away
this outer electron, now you have three positive
charges in the nucleus and only two electrons here. So it's pulling
those electrons in, you lost that outer electron,
it's getting smaller. And so the cation is smaller
than the neutralize atom. And so we've seen that
neutral atoms will shrink when you convert
them to cations, so it kind of makes sense that
if you take a neutral atom and add an electron,
it's going to get larger. And so that's our
next concept here. So if we took something
like chlorine, so a neutral chlorine atom,
and we added an electron to chlorine, that would
give it a negative charge. So we would get chlorine
with a negative charge, or the chloride
anion, I should say. And so in terms of
sizes, let's go ahead and draw a
representative atom here. So if this is our
neutral chlorine atom and we add an electron to it,
it actually gets a lot bigger. So the anion is bigger
than the neutral atom. And let's see if we can
think about why here. So if we were to draw an
electron configuration, or to write a noble gas
electron configuration for the neutral chlorine--
so you should already know how to do
this-- you would just write your noble
gas in brackets. So neon and then 3s2,
3p5, so seven electrons in the outer shell for
the neutral chlorine atom. For the chloride anion, you
would start off the same way. You would say neon
in brackets, 3s2. And you'd be adding
an electron to it. So it wouldn't be 3p5, it
would be 3p6 like that. And so now we would have
so this would give us eight electrons in
our outer shell, and this would give us
only seven electrons in our outer shell. Now, the explanation for the
larger size of the chloride anion in most
textbooks is, you'll see people say that the
addition of this extra electron here, so that means
that those electrons are going to repel each other more. You have eight of
them instead of seven, and so because they
repel each other more, it gets a little bit bigger. And that makes sense, but
you'll see some people disagree with that explanation,
and I haven't really seen a great
alternative offered. And so however you
want to think about it, generally the anion is
larger than the neutral atom. But in terms of the
explanation for that, you could think
about it as electrons are repelling each
other if you wanted to, despite the fact that
people disagree with that. You could think
about just more stuff as a really simple way
of thinking about it. But again, in general
for exams, think about the anion being larger.