GENERAL CHEMISTRY CHAPTER 2 10/16/2010 1 GENERAL CHEMISTRY: CHAPTER TWO BASIC ATOMIC STRUCTURE INTRODUCTION: 1.1 Preamble In order for us to appreciate the nature of chemical reactivity, we will need to learn about the basic factors that determine the structures of the reactant and product, molecules and ions. To understand the structures and dynamics of molecules and ions we must first grasp the nature of the structures of their individual component atoms. 1.1 Basic Particles From a chemist’s perspective, it is sufficient to start out by accepting the existence of just three component atomic particles: the proton, the neutron and the electron. These particles were identified and characterized between 1890 and 1930. The electron was “discovered” by J.J. Thompson (below) in 1897 by analyzing cathode rays.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 2 J.J. Thompson A primitive cathode ray tube. The proton was “discovered” by Lord Rutherford (below) in 1918, also by analyzing, cathode rays.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 3Rutherford observed that when a beam of alpha particles was shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. The neutron was discovered by Sir James Chadwick. When beryllium is bombarded with alpha particles it gives rise to an electrically neutral penetrating radiation. Chadwick bombarded the hydrogen atoms in paraffin with beryllium emissions, and also used helium, nitrogen, and other elements as targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a massGENERAL CHEMISTRY CHAPTER 2 10/16/2010 4approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of Nature. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work. A proton has a mass that is very close to 1 atomic mass unit (amu) and a positive charge with a value of +e, where e is the atomic charge unit, also known as the ‘charge on the electron’. The mass of a neutron is close to that of the proton and also to 1 amu, but not exactly the same as either. The name is intended to signify that it is an electrically neutral particle (i.e. has a zero electrostatic charge). The atomic mass unit was first defined to be equal to the mass of a single hydrogen atom. The precise definition has been refined over the years, without making any substantial change to its magnitude. The Unified atomic mass unit is equal to 1.660538782 × 10−24 grams. An electron has a mass of 1/1838 amu and a negative charge with a value of -e. In practical units that charge is equal to - 1.602 * 10-19 coulombs.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 5 1.2 Atomic Numbers: For an atom to be electrically neutral, it must have equal numbers of protons and electrons. That number is commonly represented by the symbol Z and is called the atomic number. The protons and neutrons, of an atom, are tightly packed together to form a nucleus. The size of the nucleus, relative to that of the atom as a whole, has been compared to that of a golf ball inside the Astrodome. Neutrons and protons are collectively known as ‘nucleons’. The electrons spread out to fill the remainder of the atomic volume. A critical part of descriptive theoretical chemistry involves furnishing descriptions of the spatial arrangement of the atomic electrons. 1.3 Isotopes All atoms, of the same element, have the same atomic number, Z. That means that the nuclei, of that element, all contain the same number of protons. More significantly perhaps, they also have the same numbers of electrons. Atoms of the same element do not necessarily, however, all have the same number of neutrons.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 6 It is instructive to consider atoms of the element chlorine. There are two common stable varieties (isotopes). One variety has 18 neutrons in its nucleus the other has twenty. Since both protons and neutrons have masses that are close to 1 atomic mass unit and the electron masses are almost trivially small, the mass of an atom, expressed in atomic mass units, is numerically very close to the sum of the numbers of neutrons and protons. That sum is referred to as the mass number of the isotope. The chlorine atoms with 17 protons and 18 neutrons have mass numbers of 35. Atoms of that isotope are represented symbolically by 35Cl and verbally by chlorine-35. Atoms of the isotope with 20 neutrons have a mass number of 37 and are represented by 37Cl or chlorine-37.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 71-4 Isotope Stability We have made no mention of a chlorine-36 isotope. The reason for that is that a combination of 17 protons and 19 neutrons is not stable and even if formed it would very quickly disintegrate and undergo a transmutation to an atom of a different element. We note, in passing, that stable isotopes with odd numbers of both protons and of neutrons, are relatively rare. The frequencies of different combinations of odd and even numbers, in stable isotopes, are shown in the next figure. Frequencies of stable Even - Odd combinations of protons and neutrons.GENERAL CHEMISTRY CHAPTER 2 10/16/2010 8Seeking a rationale for the odd-even distribution is an intriguing topic outside the scope of these notes. We can usefully distinguish between three types of combinations of neutrons and protons. There are those that are classified as being stable. That means that the nuclei of those particular isotopes will remain unchanged indefinitely. If one creates a graph showing the possible numbers of neutrons that can combine, in a stable fashion with specific numbers of protons, one generates a band that is referred to as the band of stability. For the elements with the lowest atomic numbers, the stable isotopes have roughly equal numbers of protons and neutrons. As the atomic number increases so the neutrons become more and more dominant until, for the heaviest atoms, there are roughly three neutrons for each pair of protons.GENERAL CHEMISTRY CHAPTER 2 10/16/2010