Environmental chemistry

1) The Electronic Configuration of Oxygen is 1s2 2s2 2p4. Explain what is meant by the term “electronic configuration”, with reference to the relative energies and shapes of atomic orbitals. Explain how the electrons are arranged in the case of an oxygen atom.

The term electronic configuration can be defined the distribution of electrons in different orbitals. The electronic configuration is what characterizes each electron in an atom. It is expressed by indicating the quantum number and its respective orbital along with the number of electrons present in it, for example 1s2 2s2 2p4 (Oxygen). The relative energies of various orbitals of single electrons depend on the value of the principal quantum number 'n' and is independent of the value of 'l'. This can be shown by an arrangement known as energy level diagram.

To arrive at the electron configurations of atoms, you must know the order in which the different sublevels are filled. Electrons enter available sublevels in order of their increasing energy. A sublevel is filled or half-filled before the next sublevel is entered. For example, the s sublevel can only hold two electrons, so the 1s is filled at helium 1s2. The p sublevel can hold six electrons, the d sublevel can hold 10 electrons, and the f sublevel can hold 14 electrons.

Oxygen atoms have 8 protons and therefore 8 electrons in their neutral state. The electronic configuration of oxygen is 1s2 2s2 2p4. This shows that Oxygen has two shells of electrons. The first is holds only two electrons and is completely filled. The second contains six electrons, with space for another two electrons to make the full complement of 8 which is necessary for stability. For oxygen to obtain a full outer shell it has to gain two electrons from other elements or share them via covalent bonding.

2) Describe and compare the chemical bonding in the following: a crystal of common (rock) salt, a sample of methane gas, a copper wire.

In NaCl (rock salt), the chemical bonding that holds the Na+and Cl-ions together is the attraction between the two opposite charges. This bonding mechanism is referred to as ionic, or electrovalent. This ion pair is held together by strong electrostatic attractions. Ionically bonded crystals typically display moderate hardness and specific gravity, rather high melting points, and poor thermal and electrical conductivity. The electrostatic charge of an ion is evenly distributed over its surface, and so a cation tends to become surrounded with the maximum number of anions that can be arranged around it.

Methane bonding is completely different it is bonding with four hydrogen atoms each with one 1s orbital, there are four covalent bonds between carbon (C) and hydrogen (H). Methane is structured that way, the four bonds would not have a symmetrical position with respect to each other. Bonds consist of electrons, and because like electrical charges repel each other, these bonds try to be as far away as possible from each other. The four bonds (in Methane) will diverge from each other, and this means that the complete spatial structure of Methane should be a regular tetrahedron with a carbon atom in the centre and the four hydrogen atoms at the vertices of the tetrahedron.

The covalent bond which occurs in the example of methane is formed when two atoms are able to share electrons whereas the ionic bond shown in the rock salt example is formed when the sharing is so unequal that an electron from an atom is completely lost to another atom, resulting in a pair of ions.

Each atom consists of protons, neutrons and electrons. At the centre of the atom, neutrons and protons stay together. But electrons revolve in orbit around the centre. Each of these molecular orbits can have a certain number of electrons to form a stable atom. But apart from Inert gas, this configuration is not present with most of the atoms. So to stabilize the atom, each atom shares half of its electrons. Covalent bonds have a definite and predictable shape and have low melting and boiling points. They can be easily broken into its primary structure as the atoms are close by to share the electrons. These are mostly gaseous and even a slight negative or positive charge at opposite ends of a covalent bond gives them molecular polarity.

In the example of copper wire, as copper is a metal it has metallic bonds joining the atoms together. A piece of copper wire will have a certain arrangement of copper atoms. The valence electrons of these atoms are free to move about the metal and are attracted to the positive cores of copper, thus holding the atoms together.

3) Describe the non-bonding interactions that occur between (a) water molecules and (b) bromine molecules in a sample of each (pure) liquid.

In a pure (nonpolar) covalent bond, both atoms have possession of the electron pair exactly the same amount of time. In a polar covalent bond, there is unequal sharing that results from an inequity in the distribution of the electrons due to the effective nuclear charge on the atoms. This polarization of the O-H interaction is critical to explaining all of the properties of water. It results in water having a dipole with the hydrogens having a slight positive charge and the oxygen having a slight negative charge.

If oxygen's position in the periodic table it taken into account, it starts with six valence electrons, and since it has two bonds with hydrogen, two of its electrons are involved in bonding pairs. This means that the oxygen has four electrons remaining. These electrons are organized into two non-bonding pairs. That is, the oxygen of water has four pairs of electrons around it two that are interacting in polar covalent bonds with hydrogen and two that are not interacting when water is in the gaseous state. Four electron pairs means that the atoms adopt a tetrahedral arrangement with the two hydrogens occupying two corners and the electron pairs occupying the other two.

4) With reference to the lecture notes and literature sources, briefly describe how 13C and 14C analyses may be used to provide environmental information.

Carbon-14 dating is a way of determining the age of certain archaeological artifacts of a biological origin up to about 50,000 years old. It is used in dating things such as bone, cloth, wood and plant fibers that were created in the relatively recent past by human activities.

­As soon as a living organism dies, it stops taking in new carbon. The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced. The carbon-14 decays with its half-life of 5,700 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely.

Metabolic flux analysis using 13C labelling is a technology to quantitatively track metabolic pathways and determine overall enzyme functions in cells. Measuring metabolic fluxes allows you to observe the functional output of the combined transcriptome, proteome and metabolome changes and bridges contemporary functional analyses to the cellular phenotype. The technique necessary for 13C based metabolic flux analysis. Is the precise measurement of the labelling pattern of targeted metabolites interpretation of large data sets given by mass spectrometry measurements with a computer model to calculate the metabolic fluxes catalyzed by thousands of cellular enzymes.

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