Protons and atomic number

Consider, for example, the mass of one chlorine atom, containing 17 protons (atomic number) and 18 neutrons ... 

A quick estimate as to the nuclear spin being integer, half integer, or zero can be made from the number of protons (atomic number) and neutrons (atomic mass - atomic number) contained in that nucleus. A summary is given in Table 1.1. 

A particular isotope is represented by the symbol X, in which Z represents the number of protons (atomic number) and A represents the total number of protons and neutrons (mass number) in the nucleus. 

Rutherford proposed the nuclear model of the atom to account for the results of experiments in which alpha particles were scattered from metal foils. According to this model, the atom consists of a central core, or nucleus, around which the electrons exist. The nucleus has most of the mass of the atom and consists of protons (with a positive charge) and neutrons (with no charge). Each chemically distinct atom has a nucleus with a specific number of protons atomic number), and around the nucleus in the neutral atom are an equal number of electrons. The number of protons plus neutrons in a nucleus equals the mass number. Atoms whose nuclei have the same number of protons but different number of neutrons are called isotopes. 

The periodic table of the elements has had a profound influence on the development of modern chemistry and physics, including quantum mechanics. Physicists are often interested in nuclear structure and nuclear reactions. The periodic table can be used to establish the numbers of protons (atomic number) and neutrons (via the relative isotopic mass) in a specific isotope of an element. 

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. 

All the atoms of a particular element have the same number of protons in the nudeus. This number is a basic property of an element, called its atomic number and given the symbol Z ... 

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 discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

Every element has a unique nuclear charge and a specific and unchanging number of protons. The number of protons in the nucleus is called the atomic number and is symbolized Z. All atoms with the same value of Z belong to the... 

When determining symbols for nuclides, the key is to remember that the atomic number and number of protons are the same and that the mass number is the sum of the number of protons plus the number of neutrons. 

The number of protons plus neutrons in an atom is termed the mass number. The number of protons (which also equals the number of electrons) is the atomic number. When elements are arranged in order of their atomic numbers and then arranged in rows, with a new row starting after each noble gas, the scheme is termed the periodic table. A simplified version is shown in Table 3.2. 

Fig. 7 Chemical structure of berberine with atom numbering and proton labels. NMR spectra of berberine titrated with increasing concentrations of calf thymus DNA at 298 K (a) and at 350 K (b). Reprinted from [165] with permission from the publisher...

One seemingly obvious relationship in the periodic table is the one between atomic number and atomic size. Clearly, as the number of protons and electrons in an atom increases so should the atomic radii. Unfortunately, it s not that simple. A glance at Figure 5.2... 

Figure 4.18. Minimum detectable concentrations as a function of atomic number and proton energy for thin organic specimens in a typical PIXE arrangement. (Reproduced by permission of Johansson...

The representation of the periodic system in this book shows yet another perspective. Each element has not only its own history but also its own identity. This is determined by the number of protons in the nucleus (the atomic number) and the corresponding number of electrons in the atomic shell. These electrons, in turn, give each element their properties, their "personalities", so to speak. There are relationships, but each element is unique in the sum of its properties. The text describes the particularities of each element, and the chosen picture indicates a scene from everyday life where we would encounter... 

The atomic number, Z, is the number of protons in the nucleus. Both the proton and neutron have masses that are approximately 1 atomic mass unit, amu. The electron has a mass of only about 1/1837 of the proton or neutron, so almost all of the mass of the atoms is made up by the protons and neutrons. Therefore, adding the number of protons to the number of neutrons gives the approximate mass of the nuclide in amu. That number is called the mass number and is given the symbol A. The number of neutrons is found by subtracting the atomic number, Z, from the mass number, A. Frequently, the number of neutrons is designated as N and (A - Z) = N. In describing a nuclide, the atomic number and mass number are included with the symbol for the atom. This is shown for an isotope of X as AZX. 

Mass number The total number of neutrons and protons in the nucleus of the element, and equal to the sum of the atomic number and the number of neutrons. 

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 ... 

Isotrope, Having the same atomic number (and position in the Periodic Table of Elements) but different masses. The difference is due to extra neutrons in the nucleus. For example hydrogen, one of three isotopes, has an atomic number of 1 and a mass of 1 the naturally occurring deuterium has a mass of 2 because it has an extra neutron in its nucleus the artificially produced tritium has another neutron for a mass of three. All three have one proton and electron and, hence, an atomic number of 1. 

Modifier D is used to show the mass number of the atom being considered, this being the total number of neutrons and protons considered to be present in the nucleus. The number of protons defines the element, but the number of neutrons in atoms of a given element may vary. Any atomic species defined by specific values of atomic number and mass number is termed a nuclide. Atoms of the same element but with difierent atomic masses are termed isotopes, and the mass number can be used to designate specific isotopes. 

The fact that both the mole and the mass of an element are based on carbon-12 enables us to relate mole and mass. A molar mass is defined as the mass in grams of one mole of a substance, and it can be obtained directly from an element s atomic mass. We can use the elements hydrogen and nitrogen to illustrate this concept. Periodic table entries for both elements are shown below. The whole number above the element is the atomic number and gives the number of protons in the nucleus. The number below the element s symbol is the molar mass (as well as the atomic mass) ... 

Peeking inside the atom Protons, eiectrons, and neutrons Deciphering atomic numbers and mass numbers Understanding isotopes and caicuiating atomic masses... 

The answers to questions like these, favorites of chemistry teachers, are best organized in a table. First, look up the symbols Cl, Os, and K in the periodic table in Chapter 4 and find the names of these elements. Enter what you find in the first column. To fill in the second and third columns (Atomic Number and Mass Number), read the atomic number and mass number from the lower left and upper left of the chemical symbols given in the question. The atomic number equals the number of protons the number of electrons is the same as the number of protons, because elements have zero overall charge. So fill in the proton and electron columns with the same numbers you entered in column two. Last, subtract the atomic number from the mass number to get the number of neutrons, and enter that value in column six. Voila The entire private life of each of these atoms is now laid before you. Your answer should look like the following table. 

To solve chromium s line, notice that chromium has a mass number of 52 and an atomic number of 24. This tells you that the number of protons is 24, because the atomic number and the number of protons are the same. To determine the number of neutrons, you simply subtract the atomic number from the mass number 52-24=28 neutrons. Finally, the number of protons is equal to the number of electrons. Continue this process to fill in the info for iridium and molybdenum. 

When you re given the atomic number, the number of protons, or the number of electrons, you automatically know the other two numbers because they re all equal. Each element in the periodic table is listed with its atomic number, so by locating the element, you can simply read off the atomic number and therefore know the number of protons and electrons. To calculate the atomic mass or the number of neutrons, you must be given one or the other. Calculate atomic mass by adding the number of protons to the number of neutrons. Alternatively, calculate the number of neutrons by subtracting the number of protons from the atomic mass. 

Np-l-jHe. Type alpha. This reaction is alpha decay due to the emission of an alpha particle, He. You simply need to adjust the atomic number and mass number to correspond to the loss of two neutrons and two protons. Thus, the mass number is reduced by 4, and the atomic number is reduced by 2. You then change the chemical symbol to reflect the element that s now present due to the change in atomic number. 

A substance can be classified chemically in many ways. One of the simplest ways to classify a substance is as an element or a compound. An element is a pure substance that cannot be changed into a simpler substance by chemical means. Elements are the building blocks of nature all matter is composed of elements. The periodic table is a concise map that organizes chemical elements into columns (groups) and rows (periods) based on their chemical properties. Currently, there are 118 known chemical elements, with whole numbers 1 to 118. These numbers are referred to as the element s atomic number and give the number of protons in the nucleus of an atom of the element. For example, carbon s atomic number is 6 and each carbon atom has 6 protons in its nucleus. The first 92 elements occur naturally, and those above atomic number 92 are synthesized through nuclear reactions using particle accelerators. Element 118 was just confirmed in the fall of 2006, and by now, more elements may have been produced. 

Today, it is recognized that an atomic nucleus consists of a number of protons (particles of charge number 1+ and mass number approximately 1) and neutrons (chargeless particles of mass number approximately 1) bound together by a short-range force known as the strong force. The total charge number is then the atomic number, and the total mass number (which is less than the sums of the mass numbers of the free constituent particles by a... 

The number of nucleons is equal to the sum of the number of protons (Z = atomic number) and number of neutrons (N) in the nucleus and is defined as the mass number (A = nucleon number)... 

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