Grade 8/ Notes Atoms are the basic building blocks of matter The nucleus (centre of the atom) contains two sub- atomic particles, Protons and Neutrons Electrons orbit around the nucleus at different shells. The properties of the particles are summarized in the table below Particle Relative mass Relative charge Found in Proton Neutron electron 1 1 0.0005 (negligible) +1 0 -1 Nucleus Nucleus Shells (orbitals) An atom is electrically neutral, as the number of electrons (-ve) is equal to the number of protons (+ve). Each type of atom has a unique identity number called atomic number. Atomic number or proton number is the number of protons in the nucleus Atomic mass or nucleon number is the total number of protons and neutrons. Number of electrons does not share much to the atomic mass as it is too light compared to protons and neutrons. Symbol of an element gives information about its structure. Atomic symbol Atomic number Mass number Name of element Isotopes are atoms of same element having different mass (nucleon) number or different neutron number. Isotopes of same element have similar chemical properties but different physical properties. In Nature, many elements have two or more isotopes Some isotopes are radioactive (emits radiation due to unstable nucleus) e.g. hydrogen-2 and hydrogen-3 Some isotopes are useful, such as cobalt-60, which is used for treating cancer. The table below shows some isotopes; Element Name of isotopes Hydrogen Carbon Chlorine No. of protons Nucleon number No. of neutrons No. of electrons Protium 1 1 0 1 Deuterium 1 2 1 1 Tritium 1 3 2 1 Carbon – 12 6 12 6 6 Carbon – 13 6 13 7 6 Carbon - 14 6 14 8 6 Chlorine - 35 17 35 18 17 Chlorine - 37 17 37 20 17 Atoms are too small to weigh using regular methods and to give any unit. Therefore an atom is weighed compared to the weight of an atom of carbon-12. Relative Atomic Mass (Ar) is the mass of an atom compared to the mass of carbon-12. The mass number of elements in the periodic table is the average mass of the isotopes of an element What is the difference between the isotopes Carbondifferent no. of neutrons 12 and Carbon-14? ……………………… 75 per cent of chlorine atoms are 35Cl and 25 per cent of chlorine atoms are 37Cl. This means that in 100 chlorine atoms, 75 will be 35Cl and 25 will be 37Cl. The total Ar for these chlorine atoms will be (75 × 35) + (25 × 37) = 2625 + 925 = 3550. So the average Ar for chlorine is 3550 ÷ 100 = 35.5. The relative atomic mass of an element is the average mass of all the isotopes present compared to 1/12th of the mass of a carbon–12 atom (12C = 12.00000 amu i.e. the standard). In an atom, electrons are arranged around the nucleus in shells or orbitals The way electrons are arranged in various shells is called electron configuration. Each shell accommodates a maximum number of electrons. 1st 2nd 3rd 4th 2 8 8 8 Each energy level or shell, must be filled with electrons before electrons go to the next level.