Atoms, nucleus number

As any group is descended the size of the atom and number of electrons shielding the outer electrons from the nucleus increases and the ionisation energy falls (see Table 6.2.)... 

The most important types of radioactive particles are alpha particles, beta particles, gamma rays, and X-rays. An alpha particle, which is symbolized as a, is equivalent to a helium nucleus, fHe. Thus, emission of an alpha particle results in a new isotope whose atomic number and atomic mass number are, respectively, 2 and 4 less than that for the unstable parent isotope. 

A neutron is characterized by having no electrical charge but has one unit of atomic mass, the same as that of a proton (Figure 46.2). Neutrons, like protons, reside in the atomic nucleus and contribute to the mass of the atom. The chemistry of an atom, like its size, is determined by the electrons in the atom. The mass of the atom is characterized mainly by the total number of neutrons and protons in the nucleus (atomic binding energies are ignored in this discussion). For mass spectrometric purposes of measurement, it is the mass that is important in establishing m/z values. 

While the number of protons in an atomic nucleus characterizes each element, the mass of the nucleus comprises the total number of protons and neutrons. 

ATOMIC NUMBER The number of protons in an atomic nucleus. 

An alplia p uticle is an energetic helium nucleus. The alplia particle is released from a radioactive element witli a neutron to proton ratio tliat is too low. The helium nucleus consists of two protons and two neutrons. The alplia particle differs from a helimn atom in that it is emitted witliout any electrons. The resulting daughter product from tliis tj pe of transformation lias an atomic number Uiat is two less tluin its parent and an atomic mass number tliat is four less. Below is an e. aiiiple of alpha decay using polonium (Po) polonium has an atomic mass number of 210 (protons and neutrons) and atomic number of 84. 

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

Atom-kem, m. atomic nucleus, -kette,/. chain of atoms, atomic chain, -lage, /. atomic layer atomic position, -lehre, /, doctrine of atoms, atomic theory, -mechanik, /. mechanics of the atom, -modell, n, atomic model, -nummer, /, atomic number, -ord-nung, /. atomic arrangement, -refraktion, /. atomic refraction, -rest, m. atomic residue (= Atomrumpf). -ring, m. ring of atoms, -rumpf, m. atomic residue or core (remainder of an atom, as after removal of some electrons), -schale, /, atomic shell, -strabl, m. atomic ray, -tafel, /, atomic table, atomtbeoretisch, a. of or according to the atomic theory,... 

In a neutral atom, the number of protons in the nucleus is exactly equal to the number of electrons outside the nucleus. Consider, for example, the elements hydrogen (Z = 1) and uranium (Z = 92). All hydrogen atoms have one proton in the nucleus all uranium atoms have 92. In a neutral hydrogen atom there is one electron outside the nucleus in a uranium atom there are 92. 

In Chapter 2 we briefly considered the structure of the atom. You will recall that every atom has a tiny, positively charged nucleus, made up of protons and neutrons. The nucleus is surrounded by negatively charged electrons. The number of protons in the nucleus is characteristic of the atoms of a particular element and is referred to as the atomic number. In a neutral atom, the number of electrons is equal to the number of protons and hence to the atomic number. 

NAME OF ABUNDANCE IN ATOMIC MASS NUCLEUS NUMBER OF... 

You have been told that the atomic nucleus bears a positive charge and is surrounded by a number of negatively charged particles called electrons. Also, the nucleus is supposed to contain most of the mass of the atom and to be made of protons and neutrons, each of which has nearly two thousand times the mass of the electron. How do we know that atoms are built this way How do we know that there is such a particle as an electron Again, weight relations associated with chemical reactions provide key evidence. 

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 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. 

The Structural Basis of the Magic Numbers.—Elsasser10 in 1933 pointed out that certain numbers of neutrons or protons in an atomic nucleus confer increased stability on it. These numbers, called magic numbers, played an important part in the development of the shell model 4 s it was found possible to associate them with configurations involving a spin-orbit subsubshell, but not with any reasonable combination of shells and subshells alone. The shell-model level sequence in its usual form,11 however, leads to many numbers at which subsubshells are completed, and provides no explanation of the selection of a few of them (6 of 25 in the range 0-170) as magic numbers. 

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. 

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. 

Elastic scattering is the most likely interaction between fast neutrons and low atomic mass number absorbers. The interaction is sometimes referred to as the "billiard ball effect." The neutron shares its kinetic energy with the target nucleus without exciting the nucleus. 

The electrons in the outermost shell of electrons surrounding an atomic nucleus, the number of which determines the valency of the atom. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

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