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Objective(s)/Standard(s) Addressed: 2b. Predict the properties and interactions of given elements using the periodic table of elements. (DOK 2) *Metals and nonmetals *Acids and bases *Chemical changes in matter (e.g., rusting [slow oxidation], combustion [fast oxidation], food spoilage) ATOMIC STRUCTURE No-one really knows what atoms are like. Scientists have built up pictures to help them explain what they observe and measure in experiments. These pictures are called "models". The model we use to explain all we need to know about atoms from a chemical point of view is the Rutherford Model, shown opposite. cloud of electrons (negatively charged) very small nucleus (positively charged) THE THREE SUB-ATOMIC PARTICLES There are three important particles which together make up an atom. Complete the table. PARTICLE WHERE FOUND proton in the nucleus neutron in the nucleus electron around the nucleus RELATIVE MASS RELATIVE CHARGE The electrons have such a small mass that we usually ignore them and think of the neutrons and protons as providing the overall mass of an atom. The more neutrons and protons in the nucleus, the heavier the atom is. Neutrons and protons are sometimes called NUCLEONS, because they are found in the nucleus of the atom. TWO IMPORTANT NUMBERS Any individual atom can be identified by two numbers. These are the ATOMIC NUMBER and the MASS (or NUCLEON) NUMBER. Both these numbers are whole numbers. ATOMIC NUMBER This is given the symbol Z and it tells us the number of PROTONS. It is also the number of electrons in the neutral atom. It identifies which chemical element the atom is. MASS NUMBER This is given the symbol A and it tells us the TOTAL number of PROTONS NEUTRONS in the nucleus. This gives us the relative mass of the individual atom. When we want to describe a particular atom we usually write these numbers with the element symbol in the format shown here for a magnesium atom. We can see that there are 12 protons. There MASS NUMBER (total no. of p + n) 24 ATOMIC NUMBER (no. of p = no. of e) 12 Mg element symbol Page 1 must be 12 electrons too. This atom has 12 neutrons (since 24-12 =12). It has a relative mass of 24. Each of the boxes below shows an atom of a different element. Write down the name of each element and the number of protons, electrons and neutrons the atoms contain. 14 N 7 name = protons = electrons = neutrons = 28 Si 14 Ag 47 207 Pb 82 name = protons = electrons = neutrons = name = protons = electrons = neutrons = 26 Fe name = protons = electrons = neutrons = name = protons = electrons = neutrons = 108 56 238 92 U name = protons = electrons = neutrons = ISOTOPES It is possible for atoms of the same chemical element to have different numbers of neutrons. The number of protons and electrons is the same, so the atoms behave in exactly the same way chemically. Different numbers of neutrons means there is a different relative mass, e.g.:1 H 1 name = hydrogen protons = 1 electrons = 1 neutrons = 0 12 C 6 name = carbon protons = 6 electrons = 6 neutrons = 6 2 H 1 name = hydrogen protons = 1 electrons = 1 neutrons = 1 13 C 6 name = carbon protons = 6 electrons = 6 neutrons = 7 3 H 1 name = hydrogen protons = 1 electrons = 1 neutrons = 2 14 C 6 name = carbon protons = 6 electrons = 6 neutrons = 8 Learn this definition of isotopes:Isotopes are atoms of the same element, with the same number of protons, but with a different number of neutrons and a different mass. Page 2 Fill in the details of protons, electrons and neutrons for each of the pairs of isotopes below 16 name = protons = electrons = neutrons = O 8 0 Sn Ca 20 name = protons = electrons = neutrons = 50 name = protons = electrons = neutrons = 44 18 120 Ca 20 O 8 name = protons = electrons = neutrons = 118 40 name = protons = electrons = neutrons = Sn 50 name = protons = electrons = neutrons = WHAT ABOUT THE ELECTRONS? There is evidence to suggest that the electrons are arranged around the nucleus in "shells", a little bit like the skins on an onion. Each shell can hold only so many electrons. When one shell is full, the remaining electrons have to go into the next shell. We represent the shells by drawing circles of increasing radius around the nucleus. 1st shell (holds 2 electrons) 2nd shell (holds 8 electrons) 3rd shell (holds 8 electrons) The electrons can be represented by dots or crosses on these circles, e.g. H has only 1 electron in the 1st shell He has 2 electrons. 1st shell is now full Mg has 12 electrons O has 8 electrons 2 in the 1st shell 6 in the 2nd shell Ne has 10 electrons 2 in the 1st shell 8 in the 2nd shell (2 shells now full) Page 3 2 in the 1st shell 8 in the 2nd shell 2 in the 3rd shell Draw similar diagrams below for the atoms indicated:B (Atomic Number 5) F (Atomic Number 9) Si (Atomic Number 14) Electron configuration: This means the arrangement of electrons in shells and can be written in shorthand as follows:He = 2 O = 2,6 Ne = 2,8 Mg = 2,8,2 Page 4 ELECTRON CONFIGURATIONS AND THE PERIODIC TABLE For GCSE Chemistry you need to be able to give the FULL electron configurations for the elements up to and including Calcium (Atomic Number 20). The Periodic Table below shows the first four Periods (rows). The Atomic Number is given for each element. Remember that this tells you the number of electrons in the atom. Fill in the electron configurations for every element up to Calcium (Ca). The first three have been done for you. Group I Period 1 Period 2 Period 3 Period 4 Group II Group III Group IV Group V Group VI Group VII Group 0 H 1 He 2 1 2 Li 3 Be 4 B 5 C 6 N 7 O 8 F 9 Ne 10 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 Ar 18 Ga Ge As Se Br Kr 2,1 Na 11 K 19 Ca 20 TRANSITION METALS Sc Ti V Cr Mn Fe Co Ni Cu Zn REMEMBER THAT Each PERIOD (row) corresponds to an electron shell. The Period Number tells you the number of shells occupied by electrons. The GROUP number tells you the number of electrons in the OUTSIDE shell (except for the Noble Gases – Group 0). Page 5 IONS Atoms sometimes lose or gain electrons when they react with other atoms. This results in imbalance between the number of protons (positive charges) and the number of electrons (negative charges). The atom is no longer electrically neutral and so we call it an ION. POSITIVE IONS These have LOST electrons. They have more protons than electrons. IMPORTANT: NEGATIVE IONS These have GAINED electrons. They have more electrons than protons. The number of protons and neutrons in the nucleus does NOT change. It is only the outer electrons that are involved in chemical reactions. Examples of each type of ion are shown below. Notice that the charge is written on the top right hand side of the element symbol. 23 Mg 12 16 magnesium ion protons = 12 electrons = 10 neutrons = 12 oxide ion protons = 8 electrons = 10 neutrons = 8 24 + Na 11 sodium ion protons = 11 electrons = 10 neutrons = 12 2+ O 8 2- 19 F 9 - fluoride ion protons = 9 electrons = 10 neutrons = 10 Fill in the table for the following atoms and ions, giving symbols as in the examples above. ELEMENT PROTONS ELECTRONS NEUTRONS Potassium 19 18 20 Calcium 20 18 20 Bromine 35 35 44 Tin 50 50 69 Chlorine 17 18 20 Aluminium 13 10 14 Copper 29 29 34 Nitrogen 7 10 7 53 54 74 Iodine Page 6 ATOM OR ION? FULL SYMBOL