Geology 111G
Lecture 3. Atomic structure; Atomic Bonding
Atomic structure
Isotopes
Ions
Atomic Bonds
Ionic Bond
Covalent Bond
Metallic Bond
Van der Waals Bond
Elemental Abundances
I. Atomic structure.
A. Matter makes up the physical universe.
1. Solid, liquid, gas.
B. Matter is composed of atoms.
1. Combinations of protons and neutrons, which form the nucleus.
2. Electrons in orbitals or shells around nucleus.
3. An atom is the smallest unit of an element that exhibits all the chemical properties of that element.
a. There are 106 elements, of which 92 are naturally occurring and generally stable.
b. Atoms do not usually occur by themselves, but are found in combination with other atoms of the same element or with atoms of different elements.
C. Components of atom.
1. Nucleus.
a. Protons have positive charge.
b. Neutrons have no charge.
c. Atomic mass equals # of protons + # of neutrons.
2. Orbitals or energy-level shells.
a. Electrons have negative charge.
b. In electrically neutral atoms, # electrons equals # protons.
D. Elements: Collections of identical atoms; most fundamental substances into which matter can be separated.
1. Combination of protons, neutrons, and electrons.
2. Atomic # corresponds to number of protons in nucleus.
a. Not to be confused with atomic mass.
b. Element with single proton (no neutron) is hydrogen (H).
1) Atomic number = 1.
2) Atomic mass = 1.
c. Element with 2 protons and 2 neutrons is helium (He).
1) Atomic number = 2.
2) Atomic mass = 4.
E. Isotopes: states of certain elements having different atomic masses.
1. This results from their having different numbers of neutrons in nucleus.
2. Note that number of protons must remain the same; otherwise, the atom is a different element by definition (atomic number different).
3. Isotopes of hydrogen.
a. Normally, hydrogen lacks a neutron.
b. If one neutron is present, atomic mass = 2; isotope is termed deuterium.
c. If two neutrons are present, atomic mass = 3; isotope is termed tritium.
4. Stable isotopes are those isotopes that are unchanging in their properties with time.
5. Radioactive isotopes are those that change their atomic mass or number with the passage of time.
a. For example: C14 is a heavy isotope of carbon (6 protons, 8 neutrons) that decays with time to nitrogen (N).
b. Normal mass of carbon is 12: 6 protons, 6 neutrons.
6. Geologic significance of isotopes is twofold.
a. Different geologic processes favor the presence of different isotopes. Thus, isotopic ratios measured in the lab may fingerprint the origin of igneous, metamorphic, or sedimentary rocks.
1) For example, oxygen isotopes (O atomic # = 8; isotopic masses = 16, 17, 18) allow discrimination of freshwater vs marine water. Freshwater limestones have relatively light oxygen isotope ratios; marine limestones have relatively heavy isotopic ratios.
b. Radiometric dating (to be covered in a later lecture) through decay of radioactive isotopes.
F. Ions: These are atoms with excess positive or negative charge.
1. Normally, atoms are electrically neutral, but they may gain or lose an electron, which gives the atom a negative (excess electron) or positive (electron deficiency) charge.
a. The electron transfer occurs to effect structural stability of electron orbitals.
1) Orbitals enclose the nucleus and define the size of a particular atom, whose dimensions are measured in angstroms (A = 1 x 10-8 cm).
b. Cation: positively charged ion.
c. Anion: negatively charged ion.
II. Atomic bonds: Atoms of different elements bond together to form compounds. This bonding is an example of a chemical reaction. Type of bonding between atoms is dependent upon the types of elements being joined. To understand our next topic, minerals, we need an understanding of bonding.
A. Ionic bond: bond type resulting when one atom gives up or gains an electron and thus becomes electrically charged; electrostatic forces then draw oppositely charged ions together.
1. Thus represents the bonding of a cation with an anion.
2. An example is table salt (NaCl), also known as the mineral halite (Na atomic # = 11; Cl atomic # = 17): sodium ion has only one electron in its outer orbital, while Cl is deficient one. Na gives up its outer electron, thus sacrificing electrical neutrality, and bonds with the Cl. The resulting compound, NaCl, is electrically neutral.
3. Molecule: Smallest unit of a compound that displays all of the compound's properties.
B. Covalent bond: Bond type resulting when adjacent atomic nuclei share an electron within their outer orbitals or electron shells to fill the vacancies in those orbitals.
1. Neither atom has its full complement of electrons, but yet each outer orbital is full. One electron is thus serving double duty.
a. Strongest of the bond types discussed.
b. Carbon atoms do this to form diamond.
c. Water molecules are bonded covalently, but the resulting molecule is polar even though it has electrical neutrality. This is due to the asymmetrical shape of a water molecule. Molecule is thus a dipole (has charge separation), which gives water its strong ability to dissolve certain substances.
C. Metallic bond: Bond type occurring in metallic elements, which outermost electrons are free to move from atom to atom. This usually results when there are so many orbitals that the electrons in the outer one are not held tightly by the nucleus.
1. High electron mobility results in the highly conductive characteristics of metals.
D. Van der Waals bond: Relatively weak attractive forces acting between neutral atoms and compounds.
1. These bonds are transient (temporary) as a result of a temporarily uneven distribution of electrons within the orbitals, giving the atom temporary polarity.
a. When the bulk of the electrons are displaced to a particular side of an atom, it becomes briefly negatively charged on that side, and is attracted to any atoms with positive polarity.
b. Weakest of the bond types discussed.
III. Elemental Abundances: Because of their different masses, elements are not evenly distributed in the Earth. Heavy elements are concentrated in the deep Earth, light elements in the crust.
A. 90 wt % of Earth's mass consists of 4 elements.
1. Fe (35-40%): most of this is in the core.
2. O (25-30%)
3. Si (13-15%)
4. Mg
(approx. 10%)
B. Crustal abundances are quite different from whole-earth abundances.
1. The above four rank differently in the crustal top ten list:
a. O (47%)
b. Si (28%)
c. Al (10%)
d. Fe (5%)
e. Other elements on the crustal top-ten list are
Mg, Ca, Na, K, Ti, H.
C. In contrast, 99% of the universe is H and He.