The Atom PDF

The Atom The smallest particle in an element is referred to as an Atom. It is made up of a nucleus which is surrounded by electrons, each having a charge of -1. The nucleus contains two types of particles:  A neutron, having a charge of 0 and,  A proton, having a charge of +1 Neutr...

The Atom The smallest particle in an element is referred to as an Atom. It is made up of a nucleus which is surrounded by electrons, each having a charge of -1. The nucleus contains two types of particles:  A neutron, having a charge of 0 and,  A proton, having a charge of +1 Neutrons and protons are capable of independent existence. An Atomic species for which the number of protons and neutrons are known is called a nuclide. For a nuclide:  the number of protons is equal to the number of electrons, such a number being called the atomic number (Symbol Z).   There is no simple relation between the number of protons and neutrons in a nuclide, however their sum is referred to as the nucleon number (Symbol A). The mass of a neutron and the mass of proton are respectively almost equal to 1 ‘unified atomic mass unit’ (Symbol: u) (also known as Dalton, Symbol: Da), where 1 1 dalton is equal to the mass of 12 of a 12C atom. Therefore, the nucleon number gives the mass of that nuclide in atomic mass units and in fact, the nucleon number is also referred to as the mass number. In order to identify different nuclides, the following notation is used: Isotopes Isotopes are nuclides of the same element having the same proton number (remember that an element is defined by the number of protons it have), but different neutron numbers. For example, hydrogen has three isotopes, which are known as protium ( 11 H ), deuterium ( 12 H ) and tritium ( 13 H ). On the other hand, chlorine has two isotopes, 35 37 which are 17 Cl and 17 Cl. Each isotope has a different mass, expressed in atomic mass units, known as the isotopic mass. For example, the isotopic mass of 23Na is 23u. The relative isotopic mass is the mass of an atom of an isotope related to one atomic mass unit. For example, the relative isotropic mass of 23Na is 23. The atomic mass of an element is the mean of the isotropic masses of its isotopes in a normal isotropic mixture. For example the atomic mass of chlorine is 35.5 u. The relative atomic mass of an element is the mean of the isotropic masses of its isotopes in a normal isotropic mixture related to one atomic mass unit. For example the atomic mass of chlorine is 35.5. Finding the atomic mass of an element; Potassium (K) has two isotopes, 39K which accounts for 93% of the normal isotropic mixture and 41K which accounts for 7%. Find the atomic mass of potassium.  93   7  Atomic mass of K =   39     41  100   100  Atomic mass of K = 39.1 u Finding the percentage of an isotope The relative atomic mass of Ne which consists of 22 Ne and 20 Ne is 20.2. Find the percentage of 20Ne in the isotropic mixture. Assuming that the percentage of 20Ne is x%. This means that (100 – x)% of 22Ne is present, since % of 20Ne and % of 22Ne must be equal to 100. Therefore,   100  x   20.2    20     x   22   100    100   20x 2200  22x 20.2   100 100 20x  2200  22x 20.2  100 2020  20x  22x  2200 2020  2200  20x  22x  180  2 x 180 x 2 x  90 Therefore 90% of a normal isotropic mixture of neon consists of 20Ne. Other examples to work: 1. Gallium (Ga) has two isotopes, 69Ga which accounts for 60% of the normal isotropic mixture and 71Ga which accounts for 40%. Find the atomic mass of potassium. 2. Lithium (Li) has two isotopes, 6Li which accounts for 7.4% of the normal isotropic mixture and 7Li which accounts for 92.6%. Find the atomic mass of potassium. Nuclear Reactions: There are two types of nuclear reactions:  Fusion nuclear reactions: When two atomic nuclei collide at very high speeds and fuse together to form a new nucleus ;and  Fission nuclear reactions: The splitting of a heavy nucleus into lighter nuclei. In server nuclear reactions, three types of radiation, alpha particles (-particles), beta particles ( -particles) and gamma radiation (-rays) are emitted. The table below presents some information about these types of radiation. Relative Range in Name of particle charge Mass (u) Velocity penetration air -particles +2 4 5% C 1 Few cm 1 -particles -1 3 – 99% C 100 Few m 1840 -rays radiation 0 0 C 10,000 Few Km C is the speed of light As one may note form the above table, -particles and -particles are charged. This means that deflections occur when these types of radiation enter an electric field. In fact, alpha particles will be attracted towards negatively charged surfaces whilst beta particles will be attracted towards positively charged surfaces. Gamma radiation is unaffected by an electric field. Balancing nuclear equations When balancing a nuclear equation, it is important that the total Mass number and the total atomic number on the left hand side of the equation is equal to the total Mass number and the total atomic number on the right hand side of the equation. In nuclear reactions we make use of the following particles: -particles() -particles ( Proton (H) Neutron (n) 4 0 1 1 2 He 1 e 1 H 0 n 238 Example of balancing a nuclear reaction: Write an equation for the decay of 92 U to 234 90 Th. Left hand side Right hand side Difference 4 extra on the left Mass Number 238 234 hand side 2 extra on the left Atomic Number 92 90 hand side Therefore, an alpha particle ( 42 He ) must be present on the right hand side. The balanced equation therefore is: 238 92 U 90 234 Th  42 He Other examples to work: 40 40 1. Write an equation for the decay of 19 K to 20 Ca. 234 2. 90 Th decays by alpha emission to produce X, which decays to Y by beta emission. Y also decays to Z by beta emission. Give the atomic and mass number of X, Y and Z. Half-Life and Activity of a radioactive Substance: The activity of a radioactive substance is equal to the number of integrations of the radioactive material per unit time. The half life of a radioactive substance is the time it takes to lose half of its activity. The following graph shows the variation of the activity of a radio-active substance with time.