Geos 306, Lecture 2
Introduction, Origin of the Elements, The Nature of the Atom
Origin of the Elements
For the purposes of this course we define mineralogy
as "the study of geologic materials at the atomic level".
All the properties of a mineral are determined by its chemistry
and its crystal structure.
The most important elements, by abundance, on Earth are O, Fe, Si, Mg.
Mineralogy is dominated by the study of minerals that are
composed of these four elements. Eg: Earth's core: Fe (iron); lower mantle: bridgmanite, MgSiO3,
upper mantle: olivine (Mg,Fe)2SiO4, pyroxene, (Mg,Fe)SiO3; Earth's crust: quartz SiO2
On the crust of the earth and on the crust of the moon, the number of mineral species incorporating a given element
correlates with the the abundance of the elements. (Hazen et al 2015)
Not only is the inorganic planet dominated by these elements, but so is the living part.
For instance, hemoglobin in our blood and chlorophyll in plants both display very similar molecular structures that differ mostly by one atom.
Fe is in hemoglobin and Mg is in chlorophyll (Wiki Porphyrin).
Recently it has been shown that the free oxygen in our atmosphere is related to the changing of the valence of Fe
between Fe3+ and Fe2+, with the help of primordial microbes.
An exciting area of research is the investigation of the co-evolution of the bio- and geo-sphere.
(Hazen et al 2008)
In the figure below, from Hummer et al, is a visualization of the increaing oxidation state of Mn as the Earth evolves.
So, where did the atoms on Earth come from?
The early history of earth followed the steps listed below:
formation of neutrons, protons and electrons
coalescence into clouds, then galaxies, then stars
evolution of a star
large stars evolve to supernova
Earth formed from the debris of at least one supernova
The atoms in the minerals that make up the earth originated
in a supernova, similar to the often photographed crab nebula, shown below in different wavelengths.
In some cases, such as ours, the supernova debris coallesced into a spinning protoplanetary disk, as shown in the figure below.
In time, the debris continued to coallesce and the sun, planets, moons, the entire solar system,
developed to where it is today. The evolution of our solar system, and our own planet, is not complete.
The Nature of the Atom
12C (6 protons + 6 neutrons),
Definition of the atom: "That which cannot be divided" (Democritus, 530
For our purposes we assume that the structure of the atom is composed of
2 parts, nucleus and electrons. The nucleus is made of protons
Neutrons are the heaviest of these particles.
They are unstable (1/2 life
= 12 minutes) except in the presence of protons. They decay to form a proton
and an electron.
The proton has slightly less mass than a neutron, but more importantly,
it has a positive electrical charge. Protons can exist in close proximity with other
protons only if neutrons are also present. Neutrons are the glue that holds
a nucleus together. The number of protons in an atom is called the atomic
number. This number defines an element. It also allows us to order
the elements into a list. An example of such a list is the
The number of neutrons in a given element is not fixed. The number of protons
+ the number of neutrons is called the mass number. Two atoms with
the same atomic number but having different numbers of neutrons are called
13C (6 protons + 7 neutrons),
14C (6 protons + 8 neutrons).
One of the isotopes is always the most stable. Isotopes with odd mass
numbers are the least stable. That means that 13C is less stable
than 12C or 14C.
Associated with each proton is an electron. It has 1/2000th
the mass of a proton, and an electrical charge that is equal in magnitude but opposite
in sign to the charge of a proton.
Early models of the atom include the raisin muffin model, Rutherford's
model and the Bohr model. The Bohr model led to the concept of quantized
energy levels in an atom and wave/particle duality. Currently, the
best solution to obtain the location and energies of electrons is provided
by the Schrodinger equation.
Electrons are the glue that hold atoms together. As such, the nature
of electrons and their interactions is the most important concept in chemistry.
Review the electronic structure of the atom. It can be described
in terms of quantum numbers, with each electron associated with a unique
set of quantum numbers. These are:
n = principal quantum number, (shell number or row number)
l = azimuthal quantum number (shape, s = sharp, p=
principal, d = diffuse, f = fundamental, etc.)
m = magnetic quantum number, defines the orientation in a magnetic
field, eg px, py, pzetc.
s = spin quantum number, related to the axial spinning of an
electron in the particle concept. We use the up and down arrow symbols.
n can have any positive integer value, The number 1 is the first
row of the periodic table, 2 is the second row etc.
l can have values 0,1,2,
,n-1. So l = 0 is the s orbital,
m goes from -l to l. So we can have 1s,2s,2px,2py,2pz,
s can only be -½ or ½.
Remember that this model is for the H atom, we hypothesize that the other
atoms have these orbital shapes.
O 1s2, 2s22p4
Mg 1s2, 2s22p6, 3s2
Si 1s2, 2s22p6, 3s23p2
Fe 1s2, 2s22p6, 3s23p64s23d6
Note that the energy levels do not increase simply by filling all n=1,
then n = 2, then n = 3, etc. The order in which the energy levels
fill can be found by examining the order of elements in the periodic table!
Be able to make spin diagrams for various
- Wenk and Bulakh: pages 1-11, 580-585
- Neese: pages 3-5, 39-46
- Broecker, W.S. (1985) How to Build a Habitable Planet. Eldigio Press, Palisades, New York Chapter 1&2
- Kirshner, R.P. (1994) The earth's elements. Scientific American, October, pages 59-65 pdf of paper
- Klein: pages 42-53
- Amaldi, G. The nature of matter, Chicago Press, 1966.