Neutrons and mass number

All hydrogen atoms have 1 proton in their nucleus (otherwise they wouldn t be hydrogen), but most (99.985%) have no neutrons. These hydrogen atoms, called protium, have mass number 1. In addition, 0.015% of hydrogen atoms, called deuterium, have 1 neutron and mass number 2. Still other hydrogen atoms, called tritium, have 2 neutrons and mass number 3. An unstable, radioactive isotope, tritium occurs only in trace amounts on Earth but is made artificially in nuclear reactors. As other examples, there are 13 known isotopes of carbon, only 2 of which occur commonly, and 25 known isotopes of uranium, only 3 of which occur commonly. 

The number of neutrons in an atom is equal to the difference between the mass number and the atomic number, or (A - 7). For example, if the mass number of a particular boron atom is 12 and the atomic number is 5 (indicating 5 protons in the nucleus), then the number of neutrons is 12 - 5 = 7. Note that all three quantities (atomic number, number of neutrons, and mass number) must be positive integers, or whole numbers. 

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. 

Rutherford found that a second type of radiation was attracted to the positively charged electrode. He proposed that this type of radiation consists of a stream of negatively charged particles. By measuring the charge and mass of these particles, he showed that they are electrons. The rapidly moving electrons emitted by nuclei are called (3 particles and denoted (3". Because a (3 particle has no protons or neutrons, its mass number is 0 and it can be written Je. 

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. 

STRATEGY Write the nuclear equation for each reaction, representing the daughter nuclide as E, with atomic number Z and mass number A. Then find Z and A from the requirement that both mass number and atomic number are conserved in a nuclear reaction, (a) In a decay, two protons and two neutrons are lost. As a result, the mass number decreases by 4 and the atomic number decreases by 2 (see Fig. 17.7). (b) The loss of one negative charge when an electron is ejected from the nucleus (Fig. 17.8) can be interpreted as the conversion of a neutron into a proton within the nucleus ... 

Distinguish clearly between (a) neutron and nucleus, (b) mass number and atomic weight, (c) atomic number and mass number, (d) atomic number and atomic weight, (e) atomic weight and atomic mass unit, and (/) atomic mass and atomic mass unit. 

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. 

You should carefully examine the changes in both atomic number (protons) and mass number (protons plus neutrons) ... 

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. 

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. 

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. 

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... 

An outstanding feature of inorganic mass spectrometry is its determination of precise and accurate isotopic abundances and isotope ratios. Isotopes of the same element (of the same number of protons or atomic number of element, Z) are, by definition, nuclides with different mass m and mass number A (A = Z + N) due to the different number of neutrons (N) in the nucleus. Isotope analyses are of special interest for characterizing the composition of samples with respect to stable and unstable isotopes in quite different concentration ranges - from the analysis of matrix elements down to the trace and ultratrace concentration level.1-9 Of 1700 isotopes, nearly 16 % (264 isotopes) are stable. The chemical elements Tc, Pm, Th, U and the transuranic elements do not possess stable isotopes. 

Neutron excess is the difference between the number of neutrons and the number of protons in an atomic nucleus. This is found by subtracting the atomic number of that nuclide from the neutron number or by subtracting twice the atomic number from the mass number. 

Figure 7.10 Contours of the Q value for the emission of a 12C nucleus as a function of neutron and proton numbers calculated with the liquid drop model mass formula. The contour lines me separated by 10 MeV. The dotted curve indicates the line of B stability [Eq. (2.9)].

The mass number (A) is the sum of the protons and neutrons present in the atom. The number of neutrons can be detennined by (A - Z). The symbol for denoting the atomic number and mass number for an element X is as follows ... 

The proton, p, has a mass of 1.007277 u and a unit charge of +1. This charge is equal to 1.6022 x 10 19 coulombs a coulomb is the amount of electrical charge involved in a flow of electrical current of 1 ampere for 1 sec. The neutron, n, has no electrical charge and a mass of 1.008665 u. The proton and neutron each have a mass of essentially 1 u and are said to have a mass number of 1. (Mass number is a useful concept expressing the total number of protons and neutrons, as well as the approximate mass, of a nucleus or subatomic particle.) The electron, e, has an electrical charge of -1. It is very light, however, with a mass of only 0.000549 u, about 1/1840 that of the proton or neutron. Its mass number is 0. The properties of protons, neutrons, and electrons are summarized in Table 1.1. 

Figure 4.2 Distribution of stable nuclides as a function of proton neutron ratio and mass number...

Each element is distinguished by the number of protons in the nucleus (the atomic number). The number of neutrons is usually similar to the number of protons, although the number of neutrons may vary. Atoms with the same number of protons but different numbers of neutrons are called isotopes. For example, the most common kind of carbon atom has six protons and six neutrons in its nucleus. Its mass number (the sum of the protons and neutrons) is 12, and we write its symbol as 12C. About 1% of carbon atoms have seven neutrons the mass number is 13, written 13C. A very small fraction of carbon atoms have eight neutrons and a mass number of 14. The 14C isotope is radioactive, with a half-life (the time it takes for half of the nuclei to decay) of 5730 years. The predictable decay of 14C is used to determine the age of organic materials up to about 50,000 years old. 

There seems to be some kind of relationship between the number of protons and neutrons in a nucleus. For chlorine, 17 protons to 18 neutrons seems to be good - it is a stable relationship. In addition, 17 protons to 20 neutrons also seems to be good also for chlorine - it is also a stable relationship. In other words, all the chlorine atoms you might find will have an atomic number of 17, and mass numbers of either 35 or 37. The mixtures of these naturally occurring chlorine atoms in the gas are in such a ratio that the average atomic mass of normal chlorine is 35.5 (Figure 12.1). 

What will be its atomic number and mass number (mass of protons and neutrons in the nucleus) ... 

I Atomic mass and mass number are not the same. Atomic mass refers to the naturally occurring mixture of isotopes mass number refers to an individual isotope. Atomic mass is an average and is never an exact integer mass number is a sum (of the number of protons plus the number of neutrons) and is always an integer. Except for the artificial elements, mass numbers are not given in the periodic table. 

Complete a table like that of Problem 3.25 for uncombined atoms having (a) symbol (b) atomic number 12, 14 neutrons (c) mass number 3, 1 electron (d) 27 protons, 33 neutrons (e) mass number 83, 47 neutrons and (/) 6 electrons, 8 neutrons. 

The number of neutrons in the nucleus of a particular element can vary. Isotopes are two atoms of the same element having a different number of neutrons. The mass number of an atom is the total number of protons and neutrons in the nucleus. Isotopes have different mass numbers. 

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