Review of Basic Chemistry PDF

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Review of Basic Chemistry...

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8/31/2017

Review of Basic Chemistry

Structure of Matter I. Life thrives on this planet because we have so much water here and because water has truly unique properties. No other planet in our solar system has been shown to have substantial amounts of liquid water on its surface. Water is a very effective absorber and transporter of heat and an extremely effective solvent because of its bonding. To understand the bonding in water we must first briefly review the basic structure of matter. II. The structure of matter is different from the STATES OF MATTER. The three so-called states of matter are solid, liquid, and gas. Water is the only substance to occur naturally on earth, in all three states. III. Matter is made up of tiny particles called atoms. A. All the atoms of a given element are alike and behave in the same way chemically. There are 115 elements, 88 of which occur naturally. The heaviest elements with atomic numbers > 109 are only recently discovered and have not had all their properties elucidated. B. Atoms of different elements have different properties. In the course of an ordinary chemical reaction, no atom of one element disappears or is changed into an atom of another element. 1. The chemical behavior of oxygen is different from that of hydrogen. When H and O combine to form water, all the H atoms and all the O atoms are present in the water and no new atoms of another element are formed during the process. 2. Compound substances are formed when atoms of more than one element combine. a. In the compound, water, hydrogen and oxygen are combined with each other. For every oxygen atom present, there are two hydrogen atoms. C. Atoms contain a tiny, positively charged, massive center called the ATOMIC NUCLEUS. 1. For our purposes we will only consider the most important particle in the nucleus which is the proton. Protons have a mass nearly equal to the hydrogen atom and carry a unit positive charge. 2. Electrons are tiny particles carrying a unit negative charge and weighing about 1/1837 the amount of the lightest atom hydrogen, which contains one proton. Electrons occur in orbits or shells arranged around the nucleus. 3. All nuclei contain a whole number of protons exactly equal to the number of electrons in the neutral atom. 4. The number of protons contained in an atom is a fundamental property of the corresponding element and is known as its ATOMIC NUMBER. For example, the simplest, lightest atom is hydrogen. It has 1 proton and 1 electron and therefore, its atomic number is 1. Element 2 is helium with 2 protons and 2 electrons. Each addition of a proton (with its matching electron) produces another element with a higher atomic number and a higher mass. IV. During the course of hundreds of thousands of experiments conducted by chemists and physicists it has been recognized that electrons determine the chemical behavior of an element. These electrons are in constant and rapid motion around the nucleus. (Chemical behavior should be distinguished from the nuclear behavior of an element, which is determined by the particles in the nucleus.) A. Scientists have discovered that certain arrangements of electrons are more stable and, therefore, favored over others. http://core.ecu.edu/geology/woods/CHEMintroA.htm

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1. The different orbits in which electrons move around the nucleus of an atom can contain different numbers of electrons. It has been determined that the most stable configuration is one in which the outer shell contains the maximum possible number of electrons. For the innermost shell this is 2 electrons, but for all the surrounding outer shells this is 8 electrons. B. The elements in the periodic table are arranged relative to the number of protons in their nucleus. Since a given atom is neutral this number of positively-charged protons is matched by an equal number of negativelycharged electrons. Therefore not all elements have filled outer shells of electrons and so the neutral atoms with unfilled outer shells tend to gain or lose electrons and to combine with other elements in order to achieve the stable outer shell configuration. 1. As an example - hydrogen has only one proton and, therefore, only one electron. Therefore it has a strong tendency to combine with other elements in such a way as to share or trade electrons so that it has two electrons in its outer shell and is "stable". As an ion in solution it exists with a single positive charge because it tends to give up its single outer-shell electron to another nucleus. 2. Helium, which is the second element in the periodic table, has 2 protons in its nucleus and has 2 electrons in its outer shell. This provides it with a filled outer shell since the first shell of orbiting electrons can only contain 2 electrons. Helium, therefore, is very chemically inert and doesn't trade or share electrons with other atoms. a. The other elements that sit in the column under helium in the periodic table (neon, krypton, xenon, and radon) also have filled outer shells, are very chemically unreactive and are therefore called the inert gasses. C. Most of the elements in the periodic table do not have filled outer shells so they seek to combine with other elements in order to achieve, if only for part of the time, a filled outer shell of electrons. D. There are two main ways in which atoms achieve filled outer shells of electrons. They combine with other atoms to form two major types of bonds - ionic and covalent. V. Larger particles and bonding A. Isolated atoms are rarely encountered in nature. Only in a very few elementary substances, the noble or inert gases, is the individual atom the structural unit of which the substance is composed. Most elementary and compound substances are made up of other types of structural units, bound together by different types of bonds. The nature of the bonds between particles is the underlying cause of many of the properties of Earth materials B. Two of the most important of the larger particles are molecules and ions. 1. MOLECULES are aggregates of atoms held together by relatively strong forces called chemical bonds. a. HCl, H2O, Cl2 and NH3 are examples of molecular substances in which the fundamental structural unit is an aggregate of atoms. Compared to the strength of chemical bonds holding together molecules the bonds between molecules are relatively weak. b. Most of the molecular substances we deal with every day are much more complex than these simple compounds. http://core.ecu.edu/geology/woods/CHEMintroA.htm

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2. IONS are formed when enough energy is available to remove electrons from the neutral atom of an element. Alternatively, an ion can be formed by adding an electron to the neutral atom of an element. Charged particles are called CATIONS if they have a positive charge, that is if they are formed by the removal of an electron, and ANIONS if they have a negative charge, that is if they are formed by the addition of an electron to a neutral atom. a. Examples of cations are Ca2+ and Na+. Two common anions are the chloride ion Cl- and the oxide ion O2-. Na+ and Cl are the ions formed when common table salt, NaCl dissolves in water. The electrical forces associated with the water molecule have the ability to break the bonds that hold the Na and Cl atoms together in the solid salt. The result is ions of Na+ and Cl- drifting about in the water. b. Since a macroscopic sample of matter must be electrically neutral, ionic compounds always contain both cations and anions. In the ionic compound calcium chloride there are unequal numbers of cations and anions. In order to maintain electroneutrality there must be two chloride ions (each of which has a single negative charge), to balance each calcium ion which has a double positive charge. c. Ions do not have to contain only a single element. Some of the most common ions and some of the most important ions to oceanographers contain 2 or more elements. The ones we will encounter the most often in this course are: HCO31CO32SO42NO31PO43C. Bonding 1. Ionic bonds in some ways are the simplest. They involve the transfer of an outer shell electron from one element to another. Ionic compounds are formed by different neutral atoms gaining or losing electrons during bond formation. a. For example, neutral atoms of sodium metal readily give up an outer shell electron to achieve their most stable configuration. Gaseous neutral chlorine, on the other hand readily accepts an electron to achieve stability. The electron is transferred out of the orbit or “sphere of influence” of the sodium atom and moves into the sphere of influence of the chlorine atom. In other words, the chlorine nucleus gains control of that electron. b. A molecule of NaCl is the smallest unit of salt, that displays all the properties of that compound. NaCl is one of the simplest of the ionically bonded minerals and its structure was the first to be worked out. 2. Covalent bonds - Electron transfer cannot explain bonding in the formation of a molecule of an elementary substance such as diamond (C), O2 or Cl2. The smallest unit of a diamond is the C atom, and since the components are identical an electron cannot be transferred from one to the other. Instead, each atom must behave in exactly the same way as every other atom of the same element. This lead to the concept of the covalent bond. Covalent compounds achieve a stable electronic configuration by sharing electrons rather than transferring them. a. When atoms that have the same tendency in terms of whether they want to give up or accept electrons form a molecule they http://core.ecu.edu/geology/woods/CHEMintroA.htm

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usually form a bond called covalent by sharing electrons rather than actually giving them up or accepting them. In this situation the electrons in question spend approximately equal amounts of time orbiting around the two atomic nuclei that are sharing them, rather than spending most of their time around one or the other of the nuclei. b. Oxygen has 8 electrons and, therefore, needs 2 more for a filled outer shell. Hydrogen, on the other hand, has a single electron in its 1 shell and is, therefore, a likely candidate to combine with oxygen in the ratio of two hydrogens to one oxygen to provide a full complement of outer electrons for all three members of the molecule. c. In the case of the water molecule and many other molecules, as well, these electrons are not always shared equally between the two elements. That is, the shared electrons do not spend equal amounts of time orbiting around each member of the bond. D. In the case of the water molecule oxygen tends to control the shared electron for a larger proportion of the time than does the hydrogen atom. The result is that the region around the oxygen atom is generally negatively charged and the region around the hydrogen atoms is positively charged. Furthermore, both of the hydrogen atoms are located on the same side of the water molecule giving this side of the water molecule a distinctly positively-charged character. The water molecule is therefore called a dipolar molecule because of this separation of charges around it. The dipolar nature of the water molecule and the resultant hydrogen bonds that form between water molecules are largely responsible for its uniqueness.

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