structura atomului , 2010

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  • 7/27/2019 Structura atomului , 2010


    3. Atom structure

    3.1. Atom components

    At the end of the last century (1897), it has been proved that atoms,

    very small and stable particles are systems composed by two parts:

    - a central nucleus, positively charged, heavy, thus, the whole mass of the

    atom is concentrated inside it;

    - an electronic cover,composed by electrons that move around the nucleus,

    negatively charged, with a smaller mass than that of the nucleus and thenumber of negative charges are charges compensate by the positive charges

    of the nucleus.

    Every atom, according to experiments made by Rutherford, has a

    single nucleus in which the entire positive charge and almost the whole mass

    of the atom is concentrated [1, 2]

    Central nucleus is formed by nucleons that areprotons, which have apositive electrical charge and are almost 1836 times bigger than electrons

    and neutrons, which dont have an electrical charge and which are about

    1839 times bigger than the electrons. Before 1961, only electrons, protons

    and neutrons where accepted as subatomic particles. Today, it is known that

    protons and neutrons are made by of very small particles, called quarks.

    Quarks are the smallest known parts of matter, which dont existanytime alone. They are always found in combination with other quarks in

    bigger particles of matter. Protons and neutrons, particles which form atoms,

    are formed by quarks. There are six types of quarks: up, down, charm,

    strange, top, bottom. All quarks have a certain mass and electrical charge.

    Ordinary matter, that is matter made up of atoms, contains only the smallest

    two quarks, up and down. [3] Charm and strange quarks are found in cosmic

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    radius. Researchers obtained top and bottom quarks in the laboratory, the

    heaviest quarks, but they didnt find them in nature.

    Table 3.1: Symbol, charge and mass of the six quarks

    Name Symbol charge Mass (approx.,GeV/c2)





















    The majorities of particles discovered at high energies in accelerators

    of particles or in cosmic radiations are made up of combinations of quarks.

    These particles are classified in two big categories: mesons (formed by two

    quarks) and baryons (formed by three quarks, for example proton and


    The proton is formed by two quarks: up and down. The neutron is

    formed by two quarks: down and up. Thus, the proton charge is u(+2/3)

    +u(+2/3) +d(-1/3) = +1, and the neutron charge is u(+2/3) +d(-1/3) +d(-1/3)

    =0, as was experimentally measured.

    In 2001, particles formed by penta-quarks where observed, in

    experiments of high energies physics. Although the experimental situation is

    not very obvious, the theory admits the existence of this type of particles.

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    The electron has a neutral partner from electrical point of view,

    almost without mass, called neutrino. The electron and neutrino are leptons.

    In conclusion, atoms are composed only by quarks and leptons.Bosons are particles for transporting the interaction forces. Thus,

    electrons are bounded to the nucleus through photons which transport the

    electromagnetic force. Protons and neutrons are maintained together in

    nucleus through gluons which transport the nuclear force.

    The numbers of negative charges from the electronic cover is equal

    to the number of positive charges existing in the nucleus and are noted withZ, called the atomic number.

    Based on these suppositions, a lot of phenomenon known from

    antiquity, the periodical table of elements, valence and chemical bond

    nature, electrical phenomenon (emission and absorption of light) could be


    Isotopes are atoms species which belong to the same element,differing through their mass, but are through their physical and chemical

    properties, they have identical atomic numbers Z, but different mass

    numbers A. [4] Mass number A of one isotope is equal to the sum between

    the protons number Z and neutrons number N, from the nucleus:

    A = Z + N (3.1)

    Different nuclear species are noted using symbols of elements, and in

    the four corners, definitive characteristics are attached:

    Ionisation state eMass number AChemical


    of elementAtomic number Z Number of neutrons N

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    Only 21 elements from the periodical system are monoisotopic that is

    in other words, atomic nucleus of single specie. Almost all 21 have odd

    number Z (11Na, 55Cs, 4Be, 13Al, 9F). [5]The others elements from nature represent mixtures of isotopes in

    constant proportions, excepting the easy elements (H, C, O, etc.).

    Elements with even Z have more isotopes, as for example Ca: 96.97

    % Ca4020 , 0.64 % Ca4220 , 0.145 % Ca

    4320 .

    Hydrogen (H) (Standard atomic mass: 1.00794(7) u) has three naturally

    occurring isotopes, denoted1


    H, and3

    H. Other, highly unstable nuclei (4

    Hto 7H) have been synthesized in the laboratory but not observed in nature.

    Hydrogen is the only element that has different names for its isotopes in

    common use today [6, 7]:

    1H (protium) (99.98%). 2H (deuterium)(0.0026 0.0184%) is not radioactive, and does not

    represent a significant toxicity hazard. Water enriched in moleculesthat include deuterium: heavy water.

    3H (tritium) is radioactive, decaying into helium-3 through decaywith a half-life of 12.32 years.

    4His a highly unstable isotope of hydrogen. It has been synthesizedin the laboratory by bombarding tritium with fast-moving deuterium

    nuclei. It decays through neutron emission and has a half-life of (1.39 0.10) 1022 seconds.

    5His a highly unstable isotope of hydrogen. It has been synthesizedin the laboratory by bombarding tritium with fast-moving tritium

    nuclei. It decays through double neutron emission and has a half-life

    of at least 9.1 1022 s.

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    6H decays through triple neutron emission and has a half-life of31022 s.

    7Hwas first synthesized in 2003 by a group of Russian, Japanese andFrench scientists at RIKEN's RI Beam Science Laboratory by

    bombarding hydrogen with helium-8 atoms.[8]

    Isotopes are frequently used in chemistry, in the study of chemical

    reactions kinetics, as tracer elements (in agricultural application), at

    synthesis of labeled compound, in the study of chemical compounds, of

    structures and chemical reactions as well as in biochemistry. The14

    C isotopeis used in archaeological dating. The isotopes are used in medicine

    (treatment and diagnosis) and nuclear medicine (diagnostic tests of this type

    are done by injecting a radionuclide or radioisotope into the bloodstream

    intravenously). On the other hand, the radioisotope Technetium-99 can be

    attached to certain pharmaceuticals to be transported to the bones. Any

    increased physiological function, such as due to a fracture in the bone, willusually mean increased concentration of the tracer.

    Nuclides are nuclear species, characterized by a mass number A,

    atomic number Z and energy state, with the condition of that, the life of this

    state to be is high enough (t > 10-8s). [9,10]

    If nuclides depart from stability line, a radioactivity in emission of

    protons or neutrons can exist.

    For stable nucleus:



    = (3.2)

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    Fig. 3.1. Function of maximum nuclear stability A = f(Z)

    If Z < 20, Z A/2. In figure 3.1 it is graphically representedZ = f(A), according to relation (3.2).

    Some elements, as uranium or radium, have the property to issue

    invisible radiations, which can cross different metallic foils, can impress

    photographic covered plates or cause fluorescence of some substances.

    More frequent, the representation N = f(Z) is used (figure 3.2). The

    return to stability line is made by radiation emission .

    Fig. 3.2. Function of maximum nuclear stability N = f(Z)

    3.2. Electric elementary charge

    Notion of electricity descends from elektron (amber, in Greece)

    and it was introduced by Gilbert (sec. XVI) for defining the attraction force

    of a stick of electrified amber through friction, on easy objects. The

    attraction between the stick of glass and a piece of electrified amber, led to





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    the definition of two electrified forces, a negative one and a positive one. In

    the case of attraction, we can observe the presence of an electric current

    which circulates from the negative charged glass to the positive chargedamber.

    Attraction force (F) between two opposite charges, q1 and q2 situated

    at r distance, is expressed by the Coulomb law:





    = (3.3)

    Elementary electric charges have the following characteristics:

    - are additive;

    - are indestructible (they cant be detached of electrons);

    - are identical;

    - positive elementary charge (p+ proton), situated in nucleus or in free

    state, is equal in absolute value with negatively elementary charge (e- -

    electron), but it has a mass of approximate 1840 times higher;

    - charges the negative or positive bodies (excess or deficit of electrons);

    - can be conductors of electricity, the charges are freely, in large spaces,

    independent of temperature. All these take place in the case of metals, alloys

    and electrolyte solutions;

    - can be isolators or dielectrics, charges are joined, in the case of inert gases,

    covalent molecules and ionic substances in solid state;

    - can be semiconductors, the number of free charges depends on the


    Theelectrochemicalequivalentof an element is the quantity from the

    respective element, removed from (or towards) one electrode, by a quantity

    of electricity equal to 1 coulomb = 1 ampere second.

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    The elementary electric charge e is a universal constant,

    characteristic to some particles with repose mass, quantified at the values of

    electric charge q and has the value obtained from the electrolysis laws(Faraday). The number of charges from a faraday is equal to the Avogadro


    e =23


    F 96500N 6.023 10


    = 1.602.10-19coulombs/electron (3.4)

    3.3. Dimensions of electronJ. J. Thomson (1897) discovered the electron and concomitantly he

    calculated the rated and e/me ratio from deviation of cathode radius in

    electric and magnetic fields:

    gram/coulombs10759,1m/e 8e = (3.5)

    From the constant value found for e/me, independent of the gas

    nature present in the tube of discharge or of material used for obtaining thecathode, it was he concluded that electrons have negative charge and they

    are the fundamental constituents of matter.

    Electron mass in repose is of 1837 times smaller than hydrogen atom

    mass and is calculated; knowing that the electron charge is 1, from the

    value of ratio e/me and of elementary electric charge:






    3.4. Electromagnetic radiation. The photon

    The energy of electromagnetic field is determines the absorption and

    emission of electromagnetic radiation and due to this reason it is used in

    spectroscopy and photochemistry.

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    An electromagnetic field is a perturbation which is propagated in

    vacuum with rate of light in vacuum c 3108 m s-1.

    3.4.1. Undulationtheory of light

    Undulatory theory of light conceives the light as a succession of

    electromagnetic waves (Huygens, 1672), in contrast with corpuscular theory

    which assimilates it with a flux of particles (Newton).

    An electromagnetic field can be seen as being formed by two

    components, an electric field (which acts on charged particles or on

    polarized bodies in repose or motion) and a magnetic field (which acts only

    on charges in motion); every field produces a force which can accelerate the

    particle. An electromagnetic field is generated by charges in motion. For

    example, the electrons are moving forward and backwards in an aerial and

    thus generate an electromagnetic perturbation [11] which is propagated in


    An electromagnetic field can induce movement in charged particles,

    as is taking place in aerial of a radio apparatus at reception. The

    electromagnetic field is propagating as a sinusoidal wave and is

    characterized by wave length (which is the distance between the

    neighboring maximum of the wave), wave frequency, total amplitude AT,

    which is the maximum value of perturbation and of intensity, which is direct

    proportional with the square of amplitude (figure 3.3).

    Fig. 3.3: Propagation of electromagnetic field



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    Wave frequency () represents the number of waves which pass in a

    second through a certain point.

    S.I. = time


    = Hz (Hertz);= = length. time-1 (are dimensions of a rate)

    Number of wave (), the inverse of wave length, represents the

    number of wave lengths from a centimeter:

    = 1/=/c (3.7)

    where: = cm-1.

    In table 3.2 is presented the classification of electromagnetic radiusfunction of frequency and of wave length, together with the types of motions

    which absorb or emit energy of a certain wave length.

    Table 3.2: Classification of electromagnetic radiation [12]

    Motion Radiation type > 1m

    radio1 m

    microwave 1 mmmolecular rotation

    10-3 minfraredfar off 10-5 m

    molecular vibrationnear infrared





    10-6m=1 m700 nm700-620 nm560-510 nm450-40010-7 m

    electronic excitation

    10-8mvacuum ultraviolet 10-9m=1nm

    10-10m=1excitement of electronicheart 10-11mX radius

    10-12m=1pm radius 10-12-10-13mnuclear excitation [13]

    cosmic radius

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    The spectrum of electromagnetic radiations is divided after criterion

    of wave length in some domains, from low to high frequencies (figure 3.4):

    - radio radiations (waves);- microwaves;

    - hertzien radiations;

    - infrared radiations;

    - luminous radiations;

    - ultraviolet radiations;

    - X (Roentgen) radiations;- "" radiations

    Radio waves are used for the transmission of television signals, for

    communication through satellite and mobile telephony.Microwaves are used

    in communication and in microwave oven, which is based on relatively

    strong absorption of radiations of this frequency in water and in vegetal and

    animal matter. Millimeter waves are used in astronomy, for example.Terahertziene waves recently started to be researched and used in practical

    applications. Infrared radiation (light) is very useful in physical-chemical

    analysis through spectroscopy. Also, it is used for transmission of data

    without wire but at small distances, as is the case of almost all remote

    controls for television and other home apparatus. Visible light is the most

    near at hand example of electromagnetic waves. Ultravioletradiation (light)

    is responsible for skin bronzing. X (or Roentgen)radius are used for a long

    time in medicine for visualizing of intern organs. Gamma radius is often

    produced in nuclear reactions.

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    Fig. 3.4: The electromagnetic spectrum [14]

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    3.4.2. The quantum theory

    Max Planck, in 1900, showed that a hot body cant emit or absorb

    light of a certain wave length in minimal quantities, only a certain minimalenergy quantity which he called energy quantum or photon ().

    Photon is the smallest energy quantity of an electromagnetic

    radiation which can exist. It hasnt a constant value, it depends on the

    radiation frequency () emitted or absorbed by a body.

    = h (3.8)

    where: h = 6.6256






    s = Plancks constant, being an universal constantof action, because it has the dimensions [energy] x [time], that is dimensions

    of an action.

    Expression (3.8) is the fundamental equation of quantum theory. A

    body can emit or absorb only a round quantum number.

    3.5. Radioactivity. Radiations , ,

    Radioactivity is defined as being the nucleus property (some

    nuclides) to spontaneously emit , particles or to suffer an electronic

    capture and disintegration .15 Radioactive radiations arent homogeneous,

    thus if the radiations emitted by a radioactive source are passed through a

    magnetic field, they are deviated differently (figure 3.5).

    The natural radioactivity is especially met at the elements situated at

    the end of the periodical system of elements. There are only some elements

    with medium atomic mass which emit radiations (40K, 82Rb , 152Sm, 176Lu,187Re). All these elements emit radiations, excepting samarium which

    emits radiation.

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    The radioactive substances emanate three types of radiations; ,

    and (figure 3.5). Radiations are deviated in electric and magnetic field in

    a smaller measure towards radiations. Radiations arent deviated inelectric or magnetic field, which is proof that they havent electric charges.

    Fig. 3.5: Representation of, and radiations

    3.5.1 Heavy charged particles (, p+

    radiations, deuterons)

    radiations are nuclides of helium, positively charged, moving with

    very high rates, which pass the thin walls of a metallic pot, accumulatinginside this as helium form.

    Heavy charged particlescan suffer threetypes of interactions [16]:

    percussions with atomic electrons (the most important); in thesepercussions radiation loses its energy in proportion of over 98% and

    the effects of percussion are excitation (detectors used in the study of

    radiations: ZnS), ionization (complete) removal of electrons in atoms ormolecules, which produces positive and negative ions) and dissociation;

    braking in the electric field of the nucleus (retransmission of and Xradiations);

    nuclear reactions which take place with a very small probability, of about10-3%.





    Pb clothing


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    3.5.2 and radiation

    radiations are negatively charged, being formed by electrons which

    are moving with very high rates (of 20 times higher than the rate ofradiations , respectively up to 99% from the light rate c). Thats why

    radiations have a higher power of penetration than radiation.

    radiations are of electromagnetic nature, as the light, but with a

    wave length smaller than X radius obtained in tubes of X radius, at very high

    voltages. They arent influenced by an electromagnetic field and they

    havent an electric charge, the ionization power being reduced. [17, 18]

    In conclusion, substances are formed by atoms. Hypothetical, there

    could be antimatter composed by antiatoms. In different experiments in very

    intense electromagnetic fields, they could isolate for short time, nucleus of

    antimatter, and positrons were detected in cosmic radiations.

    From the study of radiations send out by radioactive elements and of

    phenomenon produced by these, it was proved that the atom has a lacunastructure, being composed by a small, heavy and positively charged nucleus,

    which concentrates almost the whole mass of the atom and a small, easy and

    negatively charged electrons, which are rotating around the nucleus at very

    large relative distances compared with the nucleus dimensions.

    3.5.3 Mass spectra

    Through bombarding of a sample substance with radiations and ,

    electrons from the superior layers of the atom electronic cover can be

    detaching. In accordance with the energy flux applied to molecules can be

    ionized, they can rive in ionized fragments or elementary ions [19,20].

    Another modality of ionization is realized using radiations or photons.

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    By applying an electric potential, a flux of positive ions can be

    produced through a negatively charged electrode and a flux of electrons

    through a positively charged electrode (figure. 3.6) [21, 22, 23].

    Fig. 3.6. The principle of mass spectrometry

    In sequel, the flux of positive ions is passed through a region with

    magnetic filed, they suffer deviations from rectilinear trajectory, when they

    are framing on a trajectory as arc shape in the presence of magnetic field B.

    In mass spectrometry [24, 25, 26] there can be used a photosensitiveplate or a photosensitive screen for recording the deviation of every bearer of

    electric charges.

    On the photosensitive screen C the bearer of positively electric

    charges are separated in accordance with the ratio m/q, where m is the mass

    of the bearer and q is its charge. The method is used for quantitative

    determination of sample composition.The method of deviation in magnetic field, combined with the

    acceleration in electric field [27] is also used for cyclotrons construction

    (figure 3.7). [28]










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    Fig. 3.7: The constructive principle of cyclotron

    The cyclotron (figure 3.7) is an accelerator of particles, obtained by

    advanced positive ionization of atoms. Frequently, the ionization leads to the

    extraction of 8-10 electrons from the atoms. [29] The cyclotron role [30] is

    to accelerate these particles at rates which assure them a kinetic energy,

    enough to bombard other atoms nucleus, usually heavier and from the

    category of transuranium elements, for producing nuclear reactions.[31, 32]

    3.6. Tests your knowledge

    3.6.1. The nucleus of an atom contains:

    a) protons only;

    b) neutrons only;

    c) electrons only;

    d) protons and neutrons.

    3.6.2. How many protons and neutrons are in an atom of iron that has a mass

    number of 55?a) 26 protons and 55 neutrons;

    b) 26 protons and 29 neutrons;

    c) 29 protons and 26 neutrons;

    d) 26 protons and 26 neutrons.





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    3.6.3. What is the mass number of an atom of tin that has 70 neutrons?

    a) 118.7;

    b) 70;c) 119;

    d) 120.

    3.6.4. How many electrons are in the outer electron level of the halogen?

    a) 2;

    b) 5;

    c) 1;

    d) 7.

    3.6.5. The beta particle consists of:

    a) high-energy rays;

    b) 1 neutron;

    c) 2 neutrons and 2 protons;d) 1 electron.

    3.6.6. What is the product of the alpha decay of Rn-220?

    a) Po-216;

    b) Rn-220;

    c) Rn-216;

    d) Ra-224.

    3.6.7. The radioisotopes used for diagnosis in nuclear medicine:

    a) they have short half lives.

    b) they travel rapidly through tissue.

    c) are usually gamma emitters.

    d) all of the above.

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    3.6.8. Most atoms are neutral. This means:

    a) The nucleus is only made up of neutrons.

    b) There are equal numbers of electrons and positrons in the electron shellsc) The electrons normally have zero charge.

    d) The number of electrons balances out the number of protons.

    3.6.9. Which one of these statements is true about an isotope of an element?

    a) The number of protons remains the same, but the number of neutrons is

    different.b) The number of neutrons remains the same, but the number of protons is


    c) The number of protons and neutrons remain the same, but the number of

    electrons is different.

    d) The number of protons remains the same, but electrons are added to the


    3.6.10. An isotope of cadmium has an atomic number of 48 and a mass

    number of 112. This means that the cadmium atom has:

    a) ? 48 protons

    64 neutrons and 48 electrons.

    b) ? 64 protons

    48 neutrons and 64 electrons.

    c) ? 48 protons

    112 neutrons and 48 electrons.

    d) ? 112 protons

    48 neutrons and 112 electrons.

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