Subatomic Particles and the Nuclear Atom

Examine the parts of a typical cathode ray tube. Note that the electrodes take on the charge of the battery terminal to which they are connected. The cathode ray travels from the cathode to the anode. 

The fact that the cathode ray is deflected in a magnetic field indicates that it is composed of charged particles. 

A tiny hole located in the center of the anode produces a thin beam of electrons. A phosphor coating allows the position of the beam to be determined as it strikes the end of the tube. Because altering the gas in the tube and the material used for the cathode have no effect on the cathode ray, the particles in the ray must be part of all matter. 

Scientists continued their research using cathode ray tubes, and hy the end of the 1800s they were fairly convinced of the following  

The next significant development came in 1909, when an American physicist named Robert Millikan (1868-1953) determined the charge of an electron. 

One of the most important results of Dalton s theory was that it generally persuaded scientists that atoms exist. As a result, scientists began research to discover the exact nature of the atom. 

Thomson concluded that the ray consisted of a stream of negatively charged particles that had been dislodged from atoms. These particles became known as electrons. Thomson went on to determine that the mass of an electron is much less than that of a hydrogen atom. 

in 1909, Robert Millikan accurately determined the mass of an electron to be 1/1840 the mass of a hydrogen atom. He also determined the electron s charge to be 1 -. This value was accepted as a fundamental unit of electrical charge. 

In 1932, Chadwick demonstrated the existence of the neutron, a nuclear particle having no charge but with nearly the same mass as a proton. The following table summarizes the subatomic particles that comprise the atom. 

Particle Symbol Electrical charge Relative mass Actual mass (g) 

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A nuclear reaction involves the rearrangement of subatomic particles within the nucleus of an atom and the exchange or release of energy from within the nucleus. And since any particular element is defined by the number of protons and neutrons in its nucleus, a change in the number of either of these basic particles means the transmutation of the element into a different isotope or into an entirely different element. 

We start by assuming that our sample consists of a monatomic gas, like He or Ne (or any other monatomic gas, like Hg vapor). Such a sample has only three types of energy states electronic, nuclear, and translational. Of these three, electronic and nuclear states are states within the atoms. Only translational energy states relate the position of the atom as a whole, rather than relating the relative positions of the subatomic particles of the atom. 

The observation that atoms of a single element can have different masses helped scientists refine the nuclear model still further. They realized that an atomic nucleus must contain subatomic particles other than protons and proposed that it also contains electrically neutral particles called neutrons (denoted n). Because neutrons have no electric charge, their presence does not affect the nuclear charge or the number of electrons in the atom. However, they do add substantially to the mass of the nucleus, so different numbers of neutrons in a nucleus give rise to atoms of different masses, even though the atoms belong to the same element. As we can see from Table B.l, neutrons and protons are very similar apart from their charge they are jointly known as nucleons. 

Most nuclear reactions involve the breaking apart of the nucleus into two or more different elements or subatomic particles. If we know all but one of the particles, then the unknown particle can be determined by balancing the nuclear equation. When chemical equations are balanced, we add coefficients to ensure that there are the same number of each type of atom on both the left and right of the reaction arrow. However, in order to balance nuclear equations we ensure that there is the same sum of both mass numbers and atomic numbers on the left and right of the reaction arrow. Recall that we can represent a specific isotope of an element by the following symbolization ... 

Chemists and physicists have collaborated since the middle of the twentieth century to make new elements substances never before seen on Earth. They are expanding the Periodic Table, step by painful step, into uncharted realms where it becomes increasingly hard to predict which elements might form and how they might behave. This is the field of nuclear chemistry. Instead of shuffling elements into new combinations - molecules and compounds - as most chemists do, nuclear chemists are coercing subatomic particles (protons and neutrons) to combine in new liaisons within atomic nuclei. 

In order to end up with an element that was not in the reactants, the particles in the nucleus of an atom—the protons and neutrons—would have to change. This is a different type of reaction, called a nuclear reaction. Some nuclear reactions occur naturally in elements that are described as radioactive. The nuclei of radioactive elements are unstable. Since they are unstable, they can fall apart and give off subatomic particles. Eventually, through a process called radioactive decay, these unstable elements are transformed into a stable (non-radioactive) element. When an atom of one element is changed into an atom of another element through a nuclear reaction, it is called transmutation. 

The ultimate structures or components of reality (top) are subatomic particles, when I was a high school student, only a few such particles were known and many scientists thought that electrons, protons, and neutrons were the basics whose arrangement in patterns accounted for the way the world was. Now literally hundreds of subatomic particles have been "discovered." The word is enclosed in quotation marks because, of course, no one has actually ever seen a subatomic particle. They are assumed to exist because their presence enables sensible interpretation of various kinds of instrumental readings. Thus modern physicists picture the universe as composed of hundreds of subatomic particles being influenced by three basic types of forces (1) the nuclear binding forces, which operate only at the extremely tiny distances inside atomic nuclei i i - uis pi... 

Since this time, a number of discoveries have been made regarding additional subatomic particles however, these are related to the domain of nuclear physics. The basic components of the atom that relate to its chemical behavior are the proton, neutron, and electron. It is to these that we now turn our attention. 

Subatomic particles (protons, neutrons, and electrons) can all be considered as spinning on their axes. In many atoms (such as 12C) these spins are paired against one another, such that the nucleus of the atom has no overall spin. However, in other nuclei (such as II and 13C) the nucleus does possess an overall spin. The rules for determining the nuclear spin are as follows ... 

Matters are made up of small particles such as molecules and atoms. Thermodynamic laws have been postulated and inferred without looking into the micro-properties or microstates within the systems. A branch of thermodynamics has evolved, which tries to interpret thermodynamic properties based on the properties of micro constituent of the system. This branch is called the Statistical Thermodynamics. An offshoot is the Nuclear Thermodynamics , where matter is treated as another form of energy and role of atomic and subatomic particle forms are studied in determining thermodynamic properties. 

The atom is composed of many types of subatomic particles, but only three types will be important in this course. Protons and neutrons exist in the atom s nucleus, and electrons exist outside the nucleus. The nucleus (plural, nuclei) is incredibly small, with a radius about one ten-thousandth of the radius of the atom itself. (If the atom were the size of a car, the nucleus would be about the size of the period at the end of this sentence.) The nucleus does not change during any ordinary chemical reaction. (Nuclear reactions are described in Chapter 21.) The protons, neutrons, and electrons have the properties listed in Table 3.1. These properties are independent of the atom of which the subatomic particles are a part. Thus, the atom is the smallest unit that has the characteristic composition of an element, and in that sense, it is the smallest particle of an element. 

Every atom has an extremely dense nucleus that contains most of the atom s mass. The nucleus contains positively charged protons and neutral neutrons, both of which are referred to as nucleons. You may have wondered how protons remain in the densely packed nucleus despite the strong electrostatic repulsion forces produced by the positively charged particles. The answer is that the strong nuclear force, a force that acts only on subatomic particles that are extremely close together, overcomes the electrostatic repulsion between protons. 

The concept of the elements depended on two different but ultimately complementary ideas about matter. The first idea was ancient that the elements were the fundamental building blocks of nature. Whether there were 1, 2, 3, 4, or 92 elements was in a sense less important than the power of the concept to explain nature and direct research. The second idea came with the discovery of the structure of the atom and the physics that made that discovery possible that an element represented a specific combination of subatomic particles determined by physical laws. The creation of controlled nuclear fission and the invention of accelerators and cyclotrons made a kind of modem alchemy possible, allowing the creation of new elements that were not found in nature but that still met the new conditions to be considered elements. 

Fermi, Enrico. (1901-1954). An Italian physicist who later became a U.S. citizen. He developed a statistical approach to fundamental problems of physical chemistry based on Pauli s exclusion principle. He discovered induced or artificial radioactivity resulting from neutron impingement, as well as slow or thermal neutrons. He was professor of physics at Columbia (1939) and awarded the Nobel Prize in physics in 1938. He was the first to achieve a controlled nuclear chain reaction, directed the construction of the first nuclear reactor at the University of Chicago (1942), and worked on the atomic bomb at Los Alamos. He also carried on fundamental research on subatomic particles using sophisticated statistical techniques. Element 100 (fermium) is named after him. 

Medieval alchemists spent years trying to convert other metals into gold without success. Years of failure and the acceptance of Dalton s atomic theory early in the nineteenth century convinced scientists that one element could not be converted into another. Then, in 1896 Henri Becquerel discovered radioactive rays (natural radioactivity) coming from a uranium compound. Ernest Rutherford s study of these rays showed that atoms of one element may indeed be converted into atoms of other elements by spontaneous nuclear disintegrations. Many years later it was shown that nuclear reactions initiated by bombardment of nuclei with accelerated subatomic particles or other nuclei can also transform one element into another—accompanied by the release of radiation (induced radioactivity). 

From an electrostatic point of view, it is amazing that positively charged protons can be packed so closely together. Yet many nuclei do not spontaneously decompose, so they must be stable. In the early twentieth century when Rutherford postulated the nuclear model of the atom, scientists were puzzled by such a situation. Physicists have since detected many very short-lived subatomic particles (in addition to protons, neutrons, and electrons) as products of nuclear reactions. Well over 100 have been identified. A discussion of these many particles is beyond the scope of a chemistry text. Furthermore their functions are not entirely understood, but it is now thought that they help to overcome the proton-proton repulsions and to bind nuclear particles (nucleons) together. The attractive forces among nucleons appear to be important over only extremely small distances, about 10 cm. 

Continued contraction, now driven by gravitational collapse causes temperatures to rise. Molecules break down to atoms, ions, and subatomic particles. The highest temperatures are in the center of the mass, and it is here that nuclear burning begins giving birth to a protostar (Fig. 2.4). 

Apart from use of experimental values of atomic - rather than nuclear - and electronic masses and of electric charges, the basis of this calculation has an empirical component. The calculation is certainly not made genuinely from first principles or ab initio, firstly because the composition of the basis set is predetermined, by those who have published this basis set [12] and by the authors of Dalton software [11] who have incorporated it, according to its success in reproducing experimentally observable quantities and other calculated properties. Secondly, the solution of Schrodinger s equation is based on a separation of electronic and nuclear motions, essentially with atomic nuclei fixed at relative positions, which is a further empirical imposition on the calculation efforts elsewhere to avoid such an arbitrarily distinct treatment of subatomic particles, even on much simpler molecular systems, have... 

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