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The Nucleus BY: MEGAN & JULIA

Structure  A neutral atom has the same number of protons and

electrons, which is also equal to the atomic number.

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Structure (continued)  The sum of the number of neutrons and the number

of protons will equal the mass number.

Mass Number Atomic Number

55.845 26

Fe

Element

Isotopes  Atoms of the same element that have different

numbers of neutrons are called isotopes, they have the same number of electrons and behave the same chemically but behave differently in nuclear reactions. The nucleus of an isotope is called the nuclide.

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Electric Forces  Electric forces attract electrons to the positive

nucleus, this force should also make protons repel each other inside the nucleus.

Strong Nuclear Force  There must be some other force to prevent the

protons from repelling. That force is called the „Strong Nuclear Force‟: it overpowers the electrical repulsion to keep protons together. This is the same for: protons-protons protons-neutrons neutrons-neutrons

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Strong Nuclear Force (continued)  This force only works over short distances as the

distance becomes greater, the electric force becomes more important. Both neutrons and protons are referred to as nucleons.

Mass Defect  Chemistry is the study of matter, whereas physics is

the study of energy. Therefore in order to compare the two we must use Einstein's equation where E is the energy in joules, m is the mass in kg and c is the speed of light in m/s.

E=mc2

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 When we add energy we actually add mass. If we

look at the mass of a nucleus compared to the mass of the nucleons that make up a nucleus, the assembled nucleus‟s mass should always be less then the total mass of ALL the nucleons together in order for it to be stable. Energy must be added to take the nucleus apart.  If the masses were equal, the nucleus would fall apart.  The difference between the mass of the nucleus and the total mass of all the nucleons is called the mass defect.

 Using E=mc2 the binding energy can be found from

the experimentally determined mass defect.  The unit of mass in nuclear physics is the atomic mass unit “u” one u is 1/12 the mass of 12/6C nucleus. u =1.6605x10-27 kg (u is constant) mproton= 1.007276 u mneutron= 1.008665 u

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 The binding energy per nucleon increases as the

mass number A approaches 56 (iron), nuclei heavier than iron will have smaller binding energies.  iron56 is the most tightly bound nucleus (most negative binding energy)

Consider an atom of the commonest isotope of chlorine, Cl-35. The actual mass of this atom, determined by experiment, is 34.980175 u. The combined mass of the constituent particles may be determined as follows: Mass of 17 protons = 17 x 1.00728 = 17.12376 u Mass of 18 neutrons = 18 x 1.00867 = 18.15606 u Mass of 17 electrons = 17 x 0.00055 = 0.00935 u Combined Mass = 35.28917 u The difference in mass is called the mass defect of the atom (or nucleus), In this case, the mass defect is about 0.309 u or 5.13 x 10-28 kg

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https://www.youtube.com/watch?v=n_PM719Sz2w In a nuclear reaction, if the resultant nucleus is bound tighter than the original nucleus, energy is released. Therefore: if mtotal of the products is < mtotal of the original nuclei some of the mass turned into energy, this energy is released in the reaction.

Example  The mass of

is 6.6447x10-27 kg, calculate the binding energy per nucleon in MeV mproton= 1.007276 u mneutron= 1.008665 u 4 2

He

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