As we mentioned in the introduction to this series,
we are going to spend a considerable amount of time up front performing a survey of the
entire periodic table.
There are so many elements on this table that are never mentioned in an organic chemistry
syllabus, and honestly are rarely mentioned in general, such that even many chemistry
students do not know the elements that some of these symbols refer to.
For many such elements it is due to their extreme rarity or instability, which makes
them much less relevant than other elements, but nevertheless, it is time to work our way
through this table, leaving no element behind.
This survey will proceed as follows.
First we will cover the main group elements one group at a time.
Those are the elements in the s-block and p-block.
Then we will cover the d-block, or transition elements, followed by the f-block, or the
lanthanides and actinides.
But before we cover any of these, we are going to start out by discussing one litte misfit
of an element.
We will spend this tutorial talking about hydrogen.
Hydrogen will be covered on its own, as it somewhat defies the organization of the periodic
table.
It is technically a Group 1 element, but Group 1 is the alkali metals, and hydrogen is a
nonmetal.
And since hydrogen makes up around 75% of the visible matter in the universe, it is
ubiquitous in astronomy as well as chemistry, and deserves a few minutes all to itself.
Hydrogen is the smallest and simplest of all the elements.
It houses a single proton in its nucleus, and therefore when neutral, possesses only
one electron.
In this ground state, its electron configuration is 1s1, meaning its lone electron resides
in the 1s orbital, which is a spherical orbital.
There are two other common oxidation states.
The first would be +1, which has an electron configuration of 1s0, meaning an empty outermost
shell, or simply no electrons at all, which is why we often refer to H+ as simply a proton.
The second would be -1, resulting in H-, or a hydride ion, which has an electron configuration
of 1s2, or two electrons in the 1s orbital.
This means a full outermost shell, since the first energy level includes only this single
s orbital.
Neutral hydrogen, with an oxidation state of zero, is capable of making a single covalent
bond, which involves the overlap of the 1s orbital with some other orbital to produce
a sigma bond, which can occur in any direction.
Additionally, hydrogen has three naturally occurring isotopes.
1H is called protium, containing just the one proton and no neutrons.
2H is called deuterium, containing one neutron.
3H is called tritium, containing two neutrons.
Protium is by far the most abundant isotope at 99.98% abundance.
This is followed by deuterium at 0.0156%, which has application as a moderator in nuclear
reactors, and is also frequently utilized for isotopic labeling experiments in chemistry.
Finally we have only trace amounts of tritium, which is radioactive, decaying via beta emission
with a half-life of 12.4 years.
It is formed in the upper atmosphere due to collisions between atmospheric particles and
cosmic rays.
These isotopes can be separated by fractional distillation or electrolysis.
Given that hydrogen atoms can make one covalent bond, hydrogen is diatomic in its elemental
form.
H2 is a colorless, odorless gas and we can describe its bonding behavior with this molecular
orbital diagram.
The two 1s atomic orbitals will combine to form a sigma bonding orbital as well as a
sigma antibonding orbital, and the two electrons will reside in the bonding orbital, producing
the single covalent bond between the atoms.
So how do we make hydrogen gas?
One method is by reacting metals like zinc and iron with dilute acid.
The metal atoms are oxidized, electrons are transferred to the protons, and these combine
to form diatomic hydrogen.
Another method is by the steam reforming of methane with nickel catalyst.
Methane and water are heated to 1000 degrees Celsius, which produces carbon monoxide and
hydrogen gas.
Finally there is the water-gas shift reaction, where carbon monoxide and water react over
iron and copper catalysts to produce carbon dioxide and hydrogen gas.
We will learn more about this type of transition metal catalysis later in the series.
To look more closely at the hydrogen molecule, the single covalent bond allows diatomic hydrogen
to act approximately like a spring.
We actually discussed this in some detail when learning about the quantum harmonic oscillator
over in the modern physics series.
When discussing this topic we talked about how the Morse potential for a diatomic molecule
is approximated by the quantum harmonic oscillator at low energies.
To elaborate in the context of diatomic hydrogen, it is the case that when comprised of two
deuterium nuclides, it will have a lower zero-point energy than when comprised of protium, or
a smaller distance from this ground state to the bottom of the well.
Be sure to visit these quantum mechanics tutorials for more information about what this means
and its derivation.
Thermodynamically speaking, D2 has a greater bond enthalpy than H2, just as D2O, or “heavy
water”, has a greater bond enthalpy than H2O, and even a higher boiling point, as a
more massive molecule will require additional kinetic energy to be liberated into the gas
phase.
Moving on to reactivity, H2 is quite stable.
We can see this by looking at the activation energy for homolysis, which will be equivalent
to its bond enthalpy.
Hydrogen will frequently form binary hydrides with s-block metals.
These are nonvolatile and crystalline, taking on the form MH or MH2, depending on the oxidation
state of the metal.
The evidence for their formation is that they conduct electricity when molten, releasing
hydrogen gas at the anode, or oxidizing electrode.
Hydrogen will also form metal hydrides with d-block and f-block metals.
First there are bulk nonstoichiometric hydrides, where hydrogen atoms tuck into the metal lattice
wherever they can fit, sometimes even in non-integer ratios.
This works with all f-block metals, as well as d-block metals from Groups 3 to 5, as well
as chromium and lead.
Then there are surface hydrides, most commonly seen with Group 10 metals like platinum, palladium,
and nickel.
Here, the hydrogen atoms are adsorbed onto the surface of the metal, activating them
for further reaction with organic compounds.
This type of chemistry is the basis for much of the catalysis we will discuss later in
the series.
Then there are also molecular hydrides, which are covalent, such as diborane.
These will be mentioned as we proceed through the table and cover the other elements present
in these compounds.
Some other key reactivities for molecular hydrogen are as follows.
At high temperatures it will react with molecular oxygen to form water quite exothermically.
It will react with metal oxides to form lower oxidation state oxides and water, or elemental
metals and water.
It will react with molecular nitrogen to form ammonia in something called the Haber process.
This is a key reaction in the manufacture of fertilizers, and it is quite involved,
requiring extreme conditions and ferric oxide catalyst.
We will discuss this in more depth later in the series.
Hydrogen also has a number of other important applications.
Given that it is the lightest chemical substance, it is lighter than all other atmospheric components
and therefore buoyant in air.
For this reason it was used in early balloons, though it was quickly found to be less than
ideal due to its flammability, a lesson that was learned the hard way through tragedies
like the Hindenburg disaster.
Although in this particular case, the Germans had been using hydrogen because America would
not sell them helium.
Hydrogen is also used as fuel for nuclear fusion.
This is performed inside the sun and all other stars, where the extreme heat and gravitational
pressure force hydrogen nuclei to overcome the intense electrostatic repulsion and fuse
to form helium nuclei.
This is called the proton-proton chain reaction, which occurs over several steps.
Here on Earth we are unable to reproduce these intense conditions inside the sun, so we require
other strategies.
We will not discuss them in depth here, but they include electromagnetic confinement,
involving plasma in a magnetic toroid, such as with the famous Tokamak, and inertial laser
confinement, involving lasers and a pellet of fusion material.
These utilize several different fuel cycles, which start with collisions between different
nuclides.
These could be deuterium and tritium, deuterium and deuterium, deuterium and helium-3, or
even protium and boron-11.
These each have their own advantages and disadvantages, involving ease of confinement, amount of energy
produced, as well as rarity and radioactivity of starting materials.
And with that we have learned quite a bit about hydrogen, its properties, its formation,
and its reactivity, and we have therefore completed just a tiny portion of our survey
of the periodic table.
Let’s move forward and check out some other elements.