Chemical elements The periodic table The periodic table is a very useful invention for chemists the scientists who study the substances around us It is a chart containing all the known chemical elements It even has spaces for elements not yet discovered To understand the periodic table how it was invented and why it is needed by chemists it is important to know about atoms and elements Inside an atom Nucleus containing protons and neutrons Neutron Proton Electron Atoms Books birds buildings and all the other things around you are made from tiny things called atoms You are made from atoms too Atoms are so small that a million of the biggest ones would make a pile less than half a millimeter high We cannot see atoms even with a microscope It can be difficult to believe that something invisible exists Just over a hundred years ago a few scientists still did not believe in atoms but the results of many experiments show that atoms are real Not only that they are made from even smaller things called subatomic particles Inside an atom There are three main types of subatomic particle Protons and neutrons are the heaviest They join together to make a 6 This is a diagram of a lithium atom Its nucleus is surrounded by three electrons and it contains three protons and four neutrons cluster in the middle of the atom called the nucleus Electrons are much lighter subatomic particles and clouds of them surround the nucleus THAT’S A FACT The nucleus of an atom is around 100,000 times smaller than the atom as a whole and most of an atom is empty space The name atom comes from the Greek word atomos meaning something that cannot be split
Elements A chemical element is a substance made from one type of atom Some elements such as gold and iron have been known for thousands of years Others have been discovered much more recently Helium often used in balloons was discovered in 1895 Americium used in smoke alarms was discovered only in 1944 Scientists continue to discover new elements today The atomic number proton number of an atom is the number of protons it contains All the atoms of an element have the same atomic number Hydrogen and Everything is made from atoms even you Oxygen atoms make up 65 percent of your body and carbon atoms make up 18 percent of it Hydrogen atoms make up most of the rest along with small percentages of many other elements carbon are two different elements Every hydrogen atom contains just one proton but every carbon atom contains six protons There are over a hundred different elements and no two elements have the same atomic number The periodic table helps chemists understand all the different elements and what they do 7
The periodic table of elements A closer look The periodic table might seem confusing at first so let’s look at its main features Each box contains information about a single element such as hydrogen or oxygen You will not find water or any other compound in the periodic table This is because compounds are made from two or more elements chemically joined together For example water is a compound made up of two hydrogen atoms and one oxygen atom Chemical symbols Each element has its own chemical symbol made from one two or sometimes three letters The first letter is always a capital letter and any others are lower case Some symbols are easy to understand such as H for hydrogen and C for carbon Others such as Cu for copper are not so obvious THAT’S A FACT The ancient Romans had copper mines on Cyprus an island in the Mediterranean Sea They called the metal aes Cyprium which became shortened to cuprum That is why the symbol for copper is Cu and not Co which is the symbol for cobalt group 1 1 1 H hydrogen 7 3 Li lithium Na sodium 23 39 11 19 K potassium 37 85 Rb rubidium 133 55 Cs cesium group 2 4 9 Be beryllium 24 12 Mg Ca Sr strontium 137 56 Ba barium 223 87 226 88 Fr francium Ra radium relative atomic mass name Example 2 4 He helium atomic number symbol group 8 26 56 Fe iron group 9 27 59 Co cobalt group 7 25 55 Mn magnesium 20 40 group 3 21 45 group 4 22 48 group 5 23 51 group 6 24 52 calcium 88 38 scandium 39 89 titanium 40 91 chromium 96 42 manganese 98 43 zirconium 178 72 niobium 73 181 molybdenum 74 184 technetium 75 186 ruthenium 76 190 rhodium 77 192 Tc 101 44 103 45 Ru Rh V vanadium 93 41 Nb hafnium tantalum tungsten 266 106 Sg Re rhenium Os osmium Bh Hs 261 104 262 105 264 107 277 108 268 109 rutherfordium dubnium seaborgium bohrium hassium meitnerium Sc Y yttrium lanthanides 57-71 actinides 89-103 139 57 140 58 141 59 144 60 145 61 150 62 152 63 157 64 La lanthanum 227 89 Ac Ce cerium 232 90 Th praseodymium 91 231 neodymium 238 92 promethium 237 93 samarium 244 94 europium 243 95 gadolinium 247 96 Sm Pu Eu Am actinium thorium protactinium uranium neptunium plutonium americium Ti Zr Hf Rf Pr Pa Cr Mo W Pm Np Ta Db Nd U Ir iridium Mt Gd Cm curium period 1 period 2 period 3 period 4 period 5 period 6 period 7 8
The numbers Atoms are not only very small they are also very light They are so light that it makes little sense to use their actual masses in grams Instead their relative atomic mass is used This is the mass of the atom compared to the mass of a carbon atom Hydrogen atoms are the lightest Their relative atomic mass is just 1 The relative atomic mass of carbon is 12 so a carbon atom weighs 12 times more than a hydrogen atom The top left number in each box of the periodic table is the relative atomic mass of the element The top right one is the atomic number the number of protons in that element’s atoms arranged in order of increasing atomic number As you go from left to right across a period the atomic number on the right increases by one each time Groups A vertical column in the periodic table is called a group The elements in a group have similar chemical properties For example the elements in group 1 are very reactive metals The elements in group 18 are very unreactive nonmetals You are going to find out more about elements like these and how understanding them helped in the invention of the periodic table Periods A horizontal row in the periodic table is called a period The elements are The periodic table of elements The number on the left in each box is the element’s relative atomic mass The number on the right is its atomic number proton number group 13 group 14 group 15 group 16 group 17 9 11 14 16 12 19 5 6 7 8 27 13 28 B boron Al aluminum 31 70 Ga gallium C carbon Si silicon 14 32 73 Ge germanium 119 50 Sn tin Pb lead Fl 82 114 nitrogen 15 31 N P phosphorus 75 33 As arsenic 122 51 Sb Bi bismuth 288 115 Uup O oxygen S sulfur 32 79 16 34 Se Te Po Lv polonium 116 293 fluorine 35 17 chlorine 35 80 F Cl Br I iodine 210 85 At astatine Uus selenium 128 52 bromine 53 127 204 81 207 antimony 83 209 tellurium 84 209 metals non-metals metalloids 47 112 48 115 49 group 10 group 11 group 12 59 30 63 5 29 65 28 Ni nickel 106 46 Pd palladium 78 195 Pt platinum Ds Tb terbium 247 97 Bk Cu copper 108 Ag silver 197 Au gold 79 111 Rg Dy Cf cadmium 80 201 Hg mercury Zn zinc Cd Cn Ho Es indium In Tl thallium Uut ununtrium Er erbium Fm group 18 4 2 He helium Ne neon Ar argon 20 40 84 10 18 36 Kr krypton 54 86 131 Xe xenon 222 Rn radon Uuo ununoctium period 1 period 2 period 3 period 4 period 5 period 6 period 7 9 271 110 272 285 112 284 113 289 294 117 294 118 darmstadtium roentgenium copernicium flerovium ununpentium livermorium ununseptium 159 65 162 66 165 67 167 68 169 69 173 70 175 71 dysprosium 98 251 holmium 99 252 257 100 258 101 Tm thulium Md Yb ytterbium 102 259 No Lu lutetium 262 103 Lr berkelium californium einsteinium fermium mendelevium nobelium lawrencium
