Atom, atomic mass orbitals

Increasing atomic mass accounts for both these trends. The volume occupied by an individual atom in the metallic lattice varies slowly within the d block, so the more massive the nucleus, the greater the density of the metal. Toward the end of each row, density decreases for the same reason that melting point decreases. The added electrons occupy antibonding orbitals, and this leads to a looser array of atoms, larger atomic volume, and decreased density. 

The substances we call elements are composed of atoms. Atoms in turn are made up of neutrons, protons and electrons neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus. Nuclide is the general term referring to any nucleus along with its orbital electrons. The nuclide is characterized by the composition of its nucleus and hence by the number of protons and neutrons in the nucleus. All atoms of an element have the same number of protons (this is given by the atomic number) but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic weight of the element). Atoms with different atomic mass but the same atomic numbers are referred to as isotopes of an element. 

Figure 1. The relation between central density and the mass of various degenerate star models. Chandrasekhar s curve is for white dwarfs with a mean molecular weight 2 of atomic mass units. Rudkjobing s curve is the same except for inclusion of the relativistic spin-orbit effects Rudkjobing (1952). The curve labeled Oppenheimer and Volkoff is for a set of neutron star models. The solid line marked Wheeler is a set of models computed with a generalized equation of state, from Cameron (1959).

The atomic nucleus is made up of protons and neutrons. The number of protons determines the atomic number and hence the identity of an element and is equal to the number of orbital electrons, a feature necessary to ensure the electrical neutrality of the atom. The atomic mass of the nucleus is made up by the additional neutrons that are present. Hence ... 

Elements are defined by the number of protons in the nucleus of each atom. The number of nuclear protons is equal to the number of electrons orbiting the nucleus. The nucleus of carbon contains six protons. This value is known as the atomic number for carbon. In nature, carbon occurs largely in a form in which the nucleus also contains six neutrons. The atomic mass of carbon is defined as the sum of the number of protons pins neutrons. Consequently, this form of carbon is called carbon-12, or About 98.9% of carbon in nature is Most of the rest is carbon-13, and contains seven neutrons in the nncleus. Smaller amounts of carbon occur that contain five or eight neutrons. These are known, respectively, as carbon-11, and carbon-14, These variations on the theme of carbon are called isotopes. Carbon-11 and carbon-14 are radioactive and decay spontaneously carbon-12 and carbon-13 are stable. 

ASPARTATE CARBAMOYLTRANSFERASE ATMOSPHERE ATOM PERCENT EXCESS TRACER/TRACEE RATIO COMPARTMENTAL ANALYSIS ISOTOPE EXCHANGE KINETICS ATOMIC MASS UNIT ATOMIC ORBITAL ATOMIZATION ATP... 

Isotopes of an element all have the same number of protons in their nuclei (and of electrons orbiting them), but differ in their number of neutrons. Neon-20 has ten protons (an atomic number of 10) and ten neutrons neon-22 has ten protons and twelve neutrons. The atomic mass of an isotope is the total number of protons and neutrons in its nucleus here 20 and 22 respectively. Chemists denote a particular isotope of an element by writing its atomic mass as a superscript before the elemental symbol °Ne, Ne. 

Another mode of decay is possible for man-made proton-rich nuclei. It has been shown that certain of such nuclei can capture one of their orbital electrons, lowering the atomic number by one unit and leaving the mass number unchanged. The net change is the same as that in positron decay, but orbital-electron capture or K capture occurs when the mass of the parent atom exceeds that of the daughter atom, but by less than 0.00110 atomic mass unit. A typical example is the conversion of Be7 (mass 7.01916) to Li7 (mass 7.01822) ... 

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