Nucleus atomic, properties

A convenient orbital method for describing eleetron motion in moleeules is the method of molecular orbitals. Molecular orbitals are defined and calculated in the same way as atomic orbitals and they display similar wave-like properties. The main difference between molecular and atomic orbitals is that molecular orbitals are not confined to a single atom. The crests and troughs in an atomic orbital are confined to a region close to the atomic nucleus (typieally within 1-2 A). The electrons in a molecule, on the other hand, do not stick to a single atom, and are free to move all around the molecule. Consequendy, the crests and troughs in a molecular orbital are usually spread over several atoms. 

The number of protons in an element s atomic nucleus is called the atomic number, Z, of that element. For example, hydrogen has Z = 1 and so we know that the nucleus of a hydrogen atom has one proton helium has Z = 2, and so its nucleus contains two protons. Henry Moseley, a young British scientist, was the first to determine atomic numbers unambiguously, shortly before he was killed in action in World War I. Moseley knew that when elements are bombarded with rapidly moving electrons they emit x-rays. He found that the properties of the x-rays emitted by an element depend on its atomic number and, by studying the x-rays of many elements, he was able to determine the values of Z for them. Scientists have since determined the atomic numbers of all the known elements (see the list of elements inside the back cover). 

The smallest unit having the chemical properties of the element are the atoms. All atoms are made up from a number of elementary particles known as the protons, neutrons, and electrons. The protons and neutrons make up an atomic nucleus at the center of the atom, while the electrons, distributed in electron shells, surround the atomic nucleus. The atoms of each element are identical to each other but differ from those of other elements in atomic number (the number of protons in the atomic nucleus) and atomic weight (their weighted average mass) as listed in the table below. 

Chemists were interested in atoms, to be sure, but they primarily concerned themselves with molecules and with the "affinities" by which atoms combine into molecules. The molecules became larger and larger, as biologically active molecules consumed more and more of chemists interest at the very time that physicists were beginning to focus on smaller and smaller particles inside the atomic nucleus or the derivation of macroscopic properties from atomic mod-... 

Today, physical chemistry has accomplished its great task of elucidating the microcosmos. The existence, properties and combinatory rules for atoms have been firmly established. The problem now is to work out where they came from. Their source clearly lies outside the Earth, for spontaneous (cold) fusion does not occur on our planet, whereas radioactive transmutation (breakup or decay), e.g. the decay of uranium to lead, is well known to nuclear geologists. The task of nuclear astrophysics is to determine where and how each species of atomic nucleus (or isotope) is produced beyond the confines of the Earth. 

The order in Problem 7.7(c) may occur because the valence electrons of a larger atom could be more available for bonding with the C, being further away from the nucleus and less firmly held. Alternatively, the greater ease of distortion of the valence shell (induced polarity) makes easier the approach of the larger atom to the C atom. This property is called polarizability. The larger, more polarizable species (e.g. I, Br, S, and P) exhibit enhanced nucleophilicity they are called soft bases. The smaller, more weakly polarizable bases (e.g. N, O, and F) have diminished nucleophilicity they are called hard bases. 

Before beginning a discussion of nuclei and their properties, we need to understand the environment in which most nuclei exist, that is, in the center of atoms. In elementary chemistry, we learn that the atom is the smallest unit a chemical element can be divided into that retains its chemical properties. As we know from our study of chemistry, the radii of atoms are approximately 1-5 x 10-10 m, or 1 -5 A. At the center of each atom we find the nucleus, a small object (r 1-10 x 10-15 m) that contains almost all the mass of the atom (Fig. 1.1). The atomic nucleus contains Z protons, where Z is the atomic number of the element under study, Z being number of protons and is thus the number of positive charges in the nucleus. The chemistry of the element is controlled by Z in that all nuclei with the same Z will have similar chemical behavior. The nucleus also contains N neutrons, where N is the neutron number. Neutrons are uncharged particles with masses approximately equal to the mass of a proton ( 1 u). Each proton has a positive charge equal to that of an electron. The overall charge of a nucleus is +Z electronic charge units. 

Erwin Schrodinger (1887-1961) and others considered the wave properties of electrons and proposed that electrons were not orbiting around the nucleus in an atom but were in electron-cloud probability areas outside the atomic nucleus. These probability areas were designated as energy levels. 

NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum. Slight variations in this resonant frequency give us detailed information about the molecular structure in which the atom resides. 

Description Helium nucleus (not helium atom). Same properties as an electron but was ejected from the nucleus. Not a particle at all. Gamma rays are high-energy radiation. 

Early on, Niels Bohr had speculated that electrons were particles circling an atoms nucleus in quantum shells with fixed energies. Helium, he knew, has two electrons. Because it is a very stable atom, one that refuses to gain or lose electrons under most conditions, Bohr concluded that two electrons filled the lowest energy shell, which he called n= 1. Bohr offered no reason why two electrons would completely fill that energy shell he simply based his conclusion on the known properties of helium. 

Mass spectrometry is based on the physical properties of the atomic nucleus. The atomic nucleus of any chemical element consists of protons and neutrons. In an electrically neutral atom the number of positively charged protons in the nucleus equals the number of negatively charged electrons in the shells. The number of protons (Z = atomic number) determines the chemical properties and the place of the element in the periodic table of the elements. The atomic number Z of a chemical element is given as a subscript preceding the elemental symbol (e.g., jH, gC, 17CI, 2eF or 92 )-Besides the protons, uncharged neutrons with nearly the same mass in comparison to the protons (m = 1.67493 x 10 kg versus nip = 1.67262 x 10 kg) stabilize the positive atomic nucleus. In contrast to the mass of the protons and neutrons in the nucleus, the mass of the electrons is relatively small at = 9.10939 x 10 kg. 

Isotopes of a chemical element are nuclides with the same number of protons (Z) but a different number of neutrons N) in the atomic nucleus. Isotopes of a chemical element (e.g., H and of hydrogen Cl and Cl of chlorine or Fe, Fe, Fe and Fe of iron, respectively) have the same number of protons (Z) and possess the same chemical properties, but differ in the number of neutrons (N) and thus in the mass number (A). With increasing Z, the number of neutrons in a stable atomic nucleus is higher than the number of protons. For mono-isotopic elements. 

---