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Mass number

The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=Aâˆ’Z.

The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: . This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

## Mass number changes in radioactive decay

Different types of radioactive decay are characterized by their changes in mass number as well as atomic number, according to the radioactive displacement law of Fajans and Soddy. For example, uranium-238 usually decays by alpha decay, where the nucleus loses two neutrons and two protons in the form of an alpha particle. Thus both the atomic number and the number of neutrons decrease by 2 (Z: 92â†’90, n: 146â†’144), which decreases the mass number by 4 (A = 238â†’234); the result is an atom of thorium-234 and an alpha particle ():

On the other hand, carbon-14 naturally decays by radioactive beta decay, whereby one neutron is transmuted into a proton with the emission of an electron and an anti-neutrino. Thus the atomic number increases by 1 (Z: 6â†’7) and the mass number remains the same (A = 14), while the number of neutrons decreases by 1 (n: 8â†’7). The resulting atom is nitrogen-14, with seven protons and seven neutrons:

Another type of radioactive decay without change in mass number is emission of a gamma ray from a nuclear isomer or metastable excited state of an atomic nucleus. Since all the protons and neutrons remain in the nucleus unchanged in this process, the mass number is also unchanged.

## Mass number and isotopic mass

The mass number gives an estimate of the isotopic mass measured in atomic mass units (u). For 12C the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of 12C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35Cl has a mass number of 35 and an isotopic mass of 34.96885.

There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:

1. The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
2. The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein's mass-energy equivalence relation E = mc2. For 35Cl the isotopic mass is less than 35 so this must be the dominant factor.

## Atomic mass of an element

The mass number should also not be confused with the relative atomic mass (also called atomic weight) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the unified atomic mass unit. This weighted average can be quite different from the near-integer values for individual isotopic masses.

For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subject to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).

Atomic number

In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, the atomic number is also equal to the number of electrons.

The atomic number, Z, should not be confused with the mass number, A, which is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), the atomic mass of an atom is roughly equal to A.

Atoms having the same atomic number Z but different neutron number N, and hence different atomic mass, are known as isotopes. Most naturally occurring elements exist as a mixture of isotopes, and the average atomic mass of this mixture determines the element's atomic weight.

## History

Loosely speaking, the existence of a periodic table creates an ordering for the elements. Such an ordering is not necessarily a numbering, but can be used to construct a numbering by fiat. Dmitri Mendeleev claimed he arranged his tables in order of atomic weight ("Atomgewicht") However, in deference to the observed chemical properties, he violated his own rule and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering the elements by proton number, Z, but this number was not known or suspected at the time.

A simple numbering based on periodic table position was never entirely satisfactory. Besides iodine and tellurium, several other pairs of elements (such as cobalt and nickel) were known to have nearly identical or reversed atomic weights, leaving their placement in the periodic table by chemical properties to be in violation of known physical properties. Another problem was that the gradual identification of more and more chemically similar and indistinguishable lanthanides, which were of an uncertain number, led to inconsistency and uncertainty in the numbering of all elements at least from lutetium (element 71) onwards (hafnium was not known at this time).

In 1911, Ernest Rutherford gave a model of the atom in which a central core held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms. This central charge would thus be approximately half the atomic weight (though it was almost 25% off the figure for the atomic number in gold (Z&nbsp;=&nbsp;79, A&nbsp;=&nbsp;197), the single element from which Rutherford made his guess). Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also known as element number, atomic number, and symbolized Z). This proved eventually to be the case.

The experimental situation improved dramatically after research by Henry Moseley in 1913. Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided to test Van den Broek and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fit the Bohr theory's demand that the frequency of the spectral lines be proportional to a measure of the square of Z.

To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminum (Z&nbsp;=&nbsp;13) to gold (Z&nbsp;=&nbsp;79) used as a series of movable anodic targets inside an x-ray tube. The square root of the frequency of these photons (x-rays) increased from one target to the next in a linear fashion. This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the proton number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 membersâ€”no fewer and no moreâ€”which was far from obvious from the chemistry at that time.

The conventional symbol Z presumably comes from the German word (atomic number).

## Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z. The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element's ele

From Encyclopedia

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Question:What is the mass number of an element with atomic number 18 and 17 neutrons?

Answers:atomic mass is protons plus neutrons (all in the nucleus) atomic number is the number of protons (or electrons) in an atom so mass = 18+17 = 35

Question:My face is truely red right now. I have no clue what the element is for this problem. I am helping my younger brother with his 'atomic structure review' worksheet. And I honestly have no clue what these are all about. So, what 'element' has a mass number of 55? now, I feel really stupid. :P I cannot believe I didn't think of the periodic table. Man, its been too long since I was a seventh grader.

Answers:OMG! I should have know this without finding a periodic chart. I used to be a biology major at BYU. I had four semesters of chemistry under my belt. Oh, well, I don't have to use this info any more, I guess. Mn

Question:AS Chemistry homework, really need help, thanks.

Answers:The atomic no. of Ca is 20. Atomic no. is also the mass or Proton no. and is equal to nucleon no. all of them are 20. See the periodic tables of elements.

Question:18. if the atomic number of an atom is 11, how many electrons does that atom have, explain. 19.if an atom has an atomic number of 6 and a mass number of 14, how many protons, electrons, and neutrons are in the atom? 20.what part of Daltons theory was modified after the discovery of isotopes? 22. what is the main difference between bohrs model of the atom and the atomic theory that is currently accepted ?

Answers:An element's atomic number is the number of protons in its nucleus. Number of protons specifies atom type. The atomic number also specifies order on the periodic table. For an electrically neutral atom, atomic number also corresponds to count of electrons. Mass number is the number of nucleons (both protons and neutrons) in an atom's nucleus. This is roughly equal to the mass of the atom, because protons and neutrons are the most significant contributes to the atom's mass. Different neutron counts can exist among one particular atom, and atoms differing in neutron counts of the same element are called isotopes. The atomic mass is an overall average of the effective mass of billions of atoms distributed evenly based on isotopic abundance. 18. Key word: "atom". Atoms are defined as neutral. Ions are non-neutral systems resembling the atom. Thus, because the atom is neutral, 11 protons also means 11 electrons. 19. 6 protons, 8 neutrons, 6 electrons 20. "All atoms of a given element are identical." Isotopes indicate that not all atoms of the same type are identical, because the neutron counts differ. 22. Let's just say, the Bohr model is oversimplified. The Bohr model has us visualize the atom as a miniature solar system, which is an incomplete view of reality. Electrons aren't easily located, nor are they definitely in one location at a time.