Everything is made of atoms. Yes, even you.
Atoms consist of a core and some electrons. The core is made of protons and neutrons.
And depending on the number of protons, you get different elements.
Water is made of Hydrogen and Oxygen. This is some Sodium. Hm, I wonder what happens
when you mix them…oh, whoopsie. Quantum mechanics tells us that this is not
what atoms actually look like, they look more like this, but we’ll get to that later.
For now, just think of atoms as having multiple electron “shells”. The electrons in the
outermost shell are called “valence electrons”. Most of chemistry is really just the behaviour
of these electrons. Every element is listed in the periodic table.
All elements in the same column or “group” have the same number of valence electrons.
For the main groups, the number of valence electrons is just the group number from 1
to 8. Except for helium. It’s too small to have 8 electrons, it can only have 2. But
still, it acts like a noble gas, so it’s kind of just grouped in with those. Luckily,
the transition metals also follow a nice pattern! That was a lie, it’s kind of a mess. But
that’s not so important for now, so we’ll get to that later.
Elements with the same number of valence electrons tend to show similar behaviour in chemical
reactions. For example, the first group, without hydrogen, is called the “alkali metals”.
Here’s some things they have in common: They have one valence electron. They’re
shiny metals. They’re kind of soft. And they do this sometimes.
All elements in the same row or “period” have the same number of shells. This number
increases from top to bottom. Also, the mass gets bigger from left to right, as each element
gains a proton, an electron and some neutrons. Depending on the number of neutrons in the
core, you get different isotopes of the same element, most of which are pretty unstable,
and fall apart, releasing ionizing radiation. Fun fact! That stuff will kill you.
If an atom has the same amount of electrons as protons, it has no charge. If it has more,
it has a negative charge, and if it has less, it has a positive charge. Charged atoms are
called “ions”, negative ions are “anions” and positive ions are “cations”.
The periodic table is also pretty much a dictionary, as every cell tells you:
The name and symbol of an element, the number of protons in the core, which is also the
total number of electrons and the atomic mass, which is the mass of neutrons and protons
combined. The periodic table is roughly divided into
three categories: Left of this line are the metals. Right of it are the non-metals, which
are mostly gases, and the line is called the “semimetals”, which have properties that
fall somewhere inbetween. Two or more atoms bonded together form a molecule.
If you have at least two different elements, you get a “compound”. Oh yeah, this is
probably a good time to mention that compounds often behave completely differently than the
elements they’re made of. Like, put together an explosive metal and a toxic gas, and you
get, of course, an even more explo- table salt. You get tablesalt.
There’s many ways to write molecules, for example the Molecular formula, where you just
count the number of each atom in a molecule, and write them as a subscript number next
to the element symbol. But that has some problems. Look at these
two molecules: They have the same molecular formula, but obviously, they’re not the
same. They’re isomers. Showing this difference is probably kind of
important: It’s the only thing that separates graphite from diamonds, because they’re
both just fancy versions of carbon, and I don’t think anyone’s going to go “mmm,
yes, this dusty black blob is indeed very expensive”.
One way to show the structure of an atom is a Lewis-Dot-Structure, which represents the
valence electrons and bonds as dots and lines. That is also going to help us understand why
atoms bond in the first place. You see, everything in the universe wants
to get to a state of lower energy. That’s why a ball on a hill will roll down by itself,
because that decreases its potential energy. This trend also applies to atoms: The state
of lowest potential energy is having a full outer shell of electrons, which is most often
eight, or in the case of hydrogen and helium, two. If you think back to the periodic table,
you’ll see that all noble gases already have a full outer shell, which is why they
don’t really want to react with anything. If two atoms don’t have a full outer shell,
but can achieve one by sharing electrons, they will naturally do so, the same way a
ball will go downhill, as their combined energy would be lower than if they were separate.
The sharing of electrons is called a “covalent bond”. These bonds are also caused by the
positively charged nucleus of an atom tugging on electrons of another atom. The strength
of this pull is called “electronegativity”. In the periodic table, the electronegativity
increases from bottom left to the top right. Therefore, Fluorine has the strongest pull.
It’s just, unbelievably desperate for an electron.
If the difference in electronegativity is bigger than about 1.7, you get an ionic bond.
A good example is Sodium Chloride. Chlorine would do anything for an electron, while Sodium
has one too many and just kind of wants to get rid of it anyway. “Perfect!” they
both say, forming an ionic bond, where sodium loses an electron and turns into a cation,
and chlorine gains an electron and turns into an anion. That seems pretty important…you
might wanna remember it. The most common place you see Ionic bonds
is in salt, yes, also table salt, but more generally, when metals and nonmetals bond,
you get “a” salt, which is just a grid of ions.
Speaking of metals, a pure metal forms “metallic bonds”. You can imagine this as a huge grid
of the positively charged nuclei, which are surrounded by freely moving electrons. You
see, in a metal grid, the valence electrons are kind of promiscuous, or as nerds call
it, “delocalized”. They can move freely in a giant playground of nuclei, instead of
being loyal to just one. This kind of bond is responsible for the properties
of metals, like conducting electricity and heat, and also, being malleable, as in being
kind of bendy. Like, you can hammer on this stuff until it’s the most deformed, unelegant
and ugly looking piece of material ever known, and it will just limp along as if nothing
ever happened. If the difference in electronegativity is
lower than about 0.5, the electrons are shared pretty equally and you get a nonpolar covalent
bond. If it’s bigger than 0.5 but smaller than around 1.7, one of the elements is pulling
on the electrons pretty hard. Not quite hard enough to completely steal an electron, but
definitely hard enough to skew the electrons a bit, making it a polar covalent bond.
An example is water. Oxygen has a very high electronegativity compared to hydrogen. As
a result, it pulls the electrons of hydrogen so hard, that they kind of belong to oxygen,
giving it a partial negative charge, and leaving hydrogen with a partial positive charge. The
presence of two poles with opposite charge is called a “electric dipole”.
All permanent dipole molecules can interact with each other, and really, with anything
that has a charge. As a result, the molecules will tug on each other and arrange themselves
in a way that oppositely charged ends are next to each other. The forces acting between
them are called “intermolecular forces” or IMFs.
A specific example is hydrogen bonds, where hydrogen bonds to something very electronegative,
like Fluorine, Oxygen or Nitrogen, creating strong dipoles that tug on each other.
The polarity of water also explains why it’s one of the most versatile solvents to exist.
It can pull apart molecules by tugging on charges, and it keeps them apart by surrounding
a particle with its oppositely charged end. This way, it can dissolve almost anything
with an uneven distribution of charges. Water cannot dissolve nonpolar molecules though.
It’s the reason why water and oil don’t mix, since fat molecules are nonpolar, while
water is polar. Just remember the ancient saying: “Similia
Similibus Solventur”, or in a language that’s actually spoken: similar things will dissolve
similar things. But even if molecules are not polar at all,
there can be electrostatic forces acting between them. How? Electrons move around inside atoms,
and by pure chance they can end up on one side of the atom, creating a momentary dipole,
which influences other particles next to it to become a dipole as well. At least for a
very short time, as the electrons keep moving and the dipole disappears. This is called
“Van der Waals forces”. Fun fact! Soap works because the molecules
that it’s made from, which are called “surfactants” have a polar “head”, and a nonpolar “tail”.
This way, when in water, they can surround for example nonpolar fat molecules and form
“micelles”, which, along with the water, transport the dirt particles away.
These are the most important bonds and forces ranked by strength: [Ionic Bonds, Covalent
Bonds, Metallic Bonds, Hydrogen Bonds, Van der Waals Forces] There are three main states of matter: Solid,
liquid and gas. Solids are tightly packed in a fixed structures,
where the particles can only wiggle. Unless, you know, you smash them. In liquids, the
particles can move freely but are still confined to a fixed volume, as the forces between them
are still strong enough to keep them together, and the particles in gases have enough energy
to just do whatever they want and fill up all the volume you give them.
Knowing this we can define two important words: Temperature is the average kinetic energy
of particles in a system, or how much and how fast they move and entropy is the amount
of disorder. Substances tend to be solid at low temperature
and/or high pressure, which is a state of low entropy, as they’re neatly organized
and don’t move that much, and gas at high temperature and/or low pressure, where they
move around like crazy, so it’s a state of high entropy.
Strong bonds, like ionic bonds, lead to high melting points, as they take a lot of energy
and therefore a high temperature to break apart. That’s why most salts are solid at
room temperature, whereas water, which is only being held together by hydrogen bonds,
is a liquid. Well, actually (!), there’s another state
called “plasma” which is ionized gas and can exist at very high temperatures, such
as in stars, or very high electric potential. The latter is used for neon lights. Gas is
ionized in a tube with a very high voltage. Collosions of the ions with other particles
makes their electrons move to a higher energy state. Once they falls back down, the difference
in energy is released as light. The colour of the light depends on the element
that’s used in the tube, as each element has different, but fixed energy levels, and
the difference between those determines the energy and therefore the frequency of the
released light, which is what changes the colour. All possible frequencies, that an
element can emit, are called the “emission spectrum”.
All matter can be divided into two categories: Pure substances, which can consist of one
element or one compound, and mixtures. Mixtures consist of at least two pure substances
and can be homogeneous or heterogeneous. Homogeneous means the substances will mix evenly and the
mixture looks the same everywhere, like salt in water, which is a “solution”.
Heterogeneous mixtures look different depending on where you look. They have distinct regions
made of separate substances. One example is sand in water, which is called a “suspension”.
Okay, well what about milk? That looks the same everywhere, so it must be homogeneous!
Uhhh, no. Milk is something we call a “colloid”, or more precisely an “emulsion”. The difference
between salt water and milk is that the solute doesn’t fully dissolve in the solvent, meaning
there are bigger particles than in a solution, but smaller particles than in a suspension.
This allows the particles to stay evenly distributed, but not fully dissolved, placing them somewhere
between solutions and suspensions. Hey remember sodium and water? What’s going
on here? Explosions are really just chemical reactions that release a lot of energy in
a very short amount of time. Also, they expand, like, a lot.
There’s a couple types of chemical reactions: synthesis, decomposition, single replacement,
and double replacement. Here’s an example for each one.
They all happen mainly for one reason: To decrease energy and get to a more stable state.
Chemical reactions happen in certain ratios, for example, to produce water molecules, you
need twice the amount of hydrogen compared to oxygen. This is called “Stoichiometry”.
These ratios are based on the conservation of mass, which says that mass cannot be created
or destroyed, only converted. Practically, when dealing with reaction equations, you
have to make sure that there’s the same amount of atoms on each side of the equation,
and if not, balance it out element by element. As a rule of thumb, you should balance out
the metals first, then the nonmetals, and hydrogen and oxygen at the end. But, it’s
really just trial and error until everything is balanced.
Okay, but if we wanted to make this reaction happen in a lab, how would we know that we
have exactly twice the amount of hydrogen compared to oxygen? You can’t just take
20 grams of this and mix it with 10 grams of that, because the atoms don’t weigh the
same, so 10 grams of both contain a different amount of particles.
What to do? Just look up the atomic mass of the reactants
and take that amount in grams. You’ll get exactly this amount of particles. That is
1 mole, which is just an amount of something, kind of like “a dozen”. In other words,
we can interpret the reaction as 2 moles of this react with 1 mole of that, which we can
easily measure. It’s important to differentiate between
physical and chemical changes, as reactions only take place in the latter. Physical change
happens when the appearance changes but the substance does not, for example hammering
metal. A chemical change happens when the substances themselves change and this is often
accompanied by bubbles, a funky smell, or, you guessed it, explosions.
All chemical reactions need activation energy to take place. Wood won’t just spontaneously
react with oxygen and start burning, or else, you know, the planet would be on fire, but
if you give it enough energy, it will. Catalysts reduce the activation energy needed for a
reaction, which makes it happen easier and faster. And as a neat bonus, they don’t
even get used up during the reaction, so you can just reuse them!
Because chemical reactions are changes in energy, it’s quite useful to keep track
of it. “Enthalpy” is, simply put, the internal
energy or heat content of a system. If the total enthalpy of a reaction is lower at the
end than at the beginning, heat was given off to the surroundings, which is an “exothermic”
reaction. If it’s the other way around a reaction is “endothermic”.
It’s easy to see how exothermic reactions can be spontaneous. It’s kind of like a
ball on a hill. It will only start rolling if you push it a little bit, but then it will
keep rolling on its own, just like wood keeps burning on its own. But in endothermic reactions,
you have to keep putting in energy, like pushing a ball uphill. That doesn’t just spontaneously
happen, right? Well, yes, actually, it does. To get the whole picture, we have to look
at Gibbs Free Energy, which looks at the change of enthalpy but also entropy of a system which
is dependent on temperature. If this whole thing is less than zero, the
reaction is “exergonic”, or spontaneous, because free energy was released. If it’s
bigger than zero, it’s “endergonic”, or not spontaneous, because free energy was
needed and absorbed. Here’s where temperature and entropy come
into play: Even if delta H is positive, so the reaction is endothermic, if the change
in entropy is big enough, it can offset this and make the total free energy negative, which
means a reaction is spontaneous. But this is strongly dependent on the temperature.
For example, melting an ice cube is endothermic, because it absorbs heat, but also, it increases
the entropy a lot, as the neatly organized ice turns to water, which is just kind of
a mess. This can happen spontaneously, but only if the temperature is above 0. If it’s
below 0, the water will spontaneously freeze, which is exothermic.
If it’s exactly 0, then no reaction will take place spontaneously. In other words,
if delta G is 0, we’re at equilibrium. Chemical equilibriums exist when reversible
reactions take place at the same speed in both directions, which means that even if
reactions are taking place, the concentrations of both sides stay the same, and to someone
watching from the outside, nothing seems to be happening.
We often find chemical equilibriums in phase changes, but also acid base chemistry.
According to Brondsted-Lowry, an acid is a molecule that donates protons, while bases
accept protons. A proton in this case is just a hydrogen ion.
So, with this definition, a molecule with at least one hydrogen that it can throw away
can be an acid, and anything that can pick it up can be a base. This also means that
once they react, they turn into the conjugate opposite, as an acid that gave away a proton
can now accept one back, which is what bases do.
A molecule that can act as both an acid and a base is called "amphoteric". A strong acid will dissociate almost completely
into its ionic form, giving off a lot of protons to the water and therefore creating lots of
hydronium ions. A weak acid just won’t dissociate nearly as much, giving us a lower concentration
of hydronium ions. So, to measure the strength of an acid we
can measure the concentration of Hydronium ions. This is called the “pH”.
Mathematically, it’s defined as the negative log of the hydronium concentration, which
means one step on the scale is a 10x change, and also, since it’s a negative log, the
higher the concentration, the lower the pH. For example. Pure water is in a chemical equilibrium.
There’s exactly one hydronium ion for every 10 million water molecules. In other words,
the concentration of hydronium is 1 over 10 million, or 1 times 10^-7. Taking the negative
log of this gives us a pH of 7, which is considered neutral.
Anything lower than 7 is acidic, and anything above is “basic”, unlike you.
You can do the same thing with hydroxide ions and you will get the pOH, which keep track
of basicity. Fun Fact! The pH and pOH always add up to 14, because they counteract each
other, so by knowing one, you know both! Now, if you have a strong base and a strong acid
and you pour them together, no, they will not explode, they will neutralize by forming
water along with a salt, which is neutral. For example, Hydrochloride and Sodium Hydroxide
will form water and table salt. Oh yeah, speaking of table salt, remember
how it consists of ionic bonds, because sodium transfers an electron to chlorine? Well that
is called a Reduction-Oxidation reaction or “redox”.
If sodium chloride forms out of it’s pure elements, the sodium gets oxidized as it loses
an electron, and the chlorine gets reduced, as it gains an electron. Logically, Sodium
is the oxidant, and chlorine is the reductant. Of course not, that would make sense, it’s
other way around. More accurately, redox reactions are reactions
that change the oxidation numbers of elements, which are kind of like imaginary charges.
There’s just a few rules you have to know to figure those out:
Hydrogen is mostly +1, Oxygen is mostly -2, halogens are mostly -1, single elements are
always 0, and the numbers of all atoms in a molecule always have to add up to the molecule’s
charge. So this would total 0, while single ions just have their charge as the oxidation
number. For example, in sulfuric acid, we have 4 oxygens, which totals -8, we have two
hydrogens, which brings the total to -6, and since the whole molecule is neutral, sulfur
must have an oxidation number of +6. Just by looking at the oxidation numbers of
reactants and products you can deduce the flow of electrons, which gives you these equations.
If redox reactions happen in acidic or basic solutions, you can balance out the charges
with the ions, and fix the stoichiometry with water.
Okay, now to this weird looking thing. I spared
you from it because for describing electrons, this is very simple, and this not. But, this
is actually like, pretty wrong, electrons don’t orbit in circles. Here’s how it
actually works: All electrons inside an atom are described
by four quantum numbers. N, l, ml, and ms. N corresponds to the shells, so all electrons
with the same n are in the same shell. Within the shells we have subshells, with
multiple orbitals, which are three dimensional regions in space where electrons could be.
We know these exist thanks to schrödinger’s equation, which gives a probabilistic wave
function. You can imagine it as cloud, and the denser it is, the more likely an electron
is to be there if we were to look for it. L describes the shape and ml the orientation
of orbitals in a subshell. There are four subshells called s, p, d and
f. If electrons have the same l, they’re in the same subshell. If electrons have the
same n, l, and ml, they are in the same orbital. Also, the number of orbitals increases by
two for every bigger subshell, starting at just one for s.
The last quantum number describes an intrinsic property of electrons called “spin”, which
can have two values. Some guy named Pauli said two electrons can
never have the exact same quantum numbers inside one atom. Since ms can only have two
values, every orbital defined by n l and ml, can hold a maximum of 2 electrons with opposite
spin. Therefore the s subshell can hold 2 electrons,
the p subshell can hold 6, d can hold 10, and f can hold 14.
Now, the quantum numbers restrain each other like this, which means that the first shell
can only have an s subshell, the second can have an s and a p subshell, and so on.
This means that the first shell can hold a total of 2 electrons, the second can hold
8, the third can hold 18, and generally, the number of electrons a shell can hold follows
the rule 2n2, with n being the principal quantum number. The principal quantum number, and
therefore total number of shells increases from top to bottom in the periodic table,
from 1 to 7. Every element has a different number of electrons
that fill up these orbitals, and the different subshells and orbitals are filled in a specific
order, called the “Aufbauprinciple”: just write down the subshells like this and draw
diagonal lines from top right to bottom left. To get an electron configuration, just look
up the number of electrons of the element in the periodic table, and fill up the subshells
in this order, until there are no electrons left. This would be the electron configuration
of Sodium. You can also look up the next smallest noble
gas and shorten it by just referring to its electron configuration as the base, because
those shells are full, and don’t change for any bigger elements. This is also how
you can figure out the valence electrons for transition metals. Just look up their electron
configuration, ignore the full shells of the next smallest noble gas, and the remaining
electrons are the valence electrons! Easy peasy.
Anyways! All this knowledge going to cost you one subscribe and a thumbs up, thank you
very much, your comment is my delight, and I shall now guide you, fine person, to the
exit, where the next lesson is excitedly waiting for you.
