Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1.1.1 The Changing Atom You will be able to… • Describe how the model of the atom has changed over the years, and how it continues to do so. • Understand that scientific knowledge is always evolving. • Describe how new theories are accepted by scientists. Atoms and their fundamental particles • Chemistry is the study of matter and the changes it undergoes • Matter is defined as being composed of atoms (first proposed by Democritus in 400BC) • Dalton’s Atomic Theory (1808) – All matter consists of atoms which cannot be created, destroyed or split Elements and atoms • Elements are substances composed of only one type of atom • The composition of atoms was established by a series of experiments • The ‘plum pudding’ model had suggested that atoms were made up of positive ‘sponge’ material with negative electrons distributed throughout like currants The composition of atoms • However, this was all changed in 1909, when Rutherford’s students, Geiger and Marsden, performed an experiment where they fired alpha particles* at a sheet of gold foil and found that some immediately bounced back! • This led to Rutherford proposing that atoms are made up of a positive nucleus with the electrons at some distance away The Geiger-Marsden Experiment The radioactive source produces alpha particles which are collimated into a beam and directed at a gold foil. The alpha particles scatter off the foil and are detected by a flash of light when they hit the deflecting screen. The Rutherford model of the atom • Atoms are made up of a positive nucleus • Electrons adopt ‘planetary orbits’ around the nucleus • Atoms are mainly space! 1.1.2 Atomic Structure You will be able to… • Describe protons, neutrons and electrons • Describe the distribution of mass and charge in an atom • Describe the contribution of protons and neutrons to the nucleus in terms of atomic number and mass number • Explain the term isotopes • Deduce the atomic structure in atoms and ions What is atomic number? What are the particles in this helium atom? neutron electron helium nucleus proton Atomic number = the number of protons in an atom What is the atomic number of helium? Protons, Neutrons, Electrons particle relative mass charge proton 1.0 1+ neutron 1.0 0 electron 1/2000 1- Atoms of elements contain equal numbers of protons and electrons. How does this affect the charge of such atoms? Atoms of elements have no charge, they are neutral. What is the charge on a fluorine atom? 19 F 9 9 protons charge = +9 9 electrons charge = - 9 10 neutrons charge = 0 total charge = 0 How are the number of electrons and atomic number related in a neutral atom? They are the same. Isotopes Ions Ions are atoms that have either extra electrons added or electrons removed. e.g. Atoms Protons Electrons Lose 1 electron +1 ion Gain 1 electron Protons Electrons So in ions the number of electrons no longer equals the number of protons -1 ion Protons Electrons 1.1.3 Atomic Masses You will be able to… • Explain why 12C is used as the standard measurement of relative mass. • Define the terms relative isotopic mass and relative atomic mass. • Calculate relative atomic masses. • Work out relative molecular masses and relative formula masses. What is an element? • A substance that consists of atoms having the same chemical properties • A substance that cannot be broken down into simpler components using chemical techniques e.g Hydrogen Helium Oxygen Copper Gold H He O Cu Au What is an atom? • The smallest particle of an element that has the chemical properties of that element • Atoms are the building blocks of elements What is a compound? • A substance consisting of atoms of two or more elements in a definite ratio i.e a specific combination of atoms • A specific combination of elements that can be broken down by chemical techniques • Examples – – – – Sodium Chloride Water Ethanol Viagra NaCl H 2O C2H6O C22H30N6O4 S What is a Molecule? • A substance containing two or more atoms • Examples – – – – Hydrogen gas Nitrogen gas Ethanol Viagra H2 N2 C2H6O C22H30N6O4 S Diatomic What is an ion? • When an atom loses or gains an electron it is called an ion e.g Sodium ready loses an electron to make a cation Na Na+ + eChlorine readily accepts an electron to make an anion Cl + e- Cl- What is an ion? • Ionic compounds are compounds resulting from a reaction between ions e.g Sodium Chloride (Salt) Na+ + Cl- NaCl As the compound contains equal numbers of anions and cations then the compounds themselves have no overall charge Relative Atomic Mass • Atoms are amazingly small. In order to get a gram of hydrogen you would need to count out around 602,204,500,000,000,000,000,000 atoms • So instead, their masses are compared with the mass of an atom of the carbon-12 isotope • On this scale one atom of carbon weighs exactly 12 units Why carbon-12? • Before 1961 we used oxygen as the standard for atomic masses • But chemists were using naturally occurring oxygen, which has a mixture of the isotopes oxygen16, oxygen-17 and oxygen-18. • Physicists had chosen the single isotope oxygen-16. • So we had two different sets of atomic masses • In 1961 chemists and physicists agreed to compromise on carbon-12 • So if one atom of the carbon-12 isotope weighs exactly 12 units • An atom of the commonest isotope of magnesium weighs twice as much as that and so is said to have a relative isotopic mass of 24 • An atom of the commonest isotope of hydrogen weighs only one twelfth that of the carbon-12 isotope, so have a relative isotopic mass of 1 • The basic unit on this scale is therefore 1/12 of the mass of a 12C atom. Relative Molecular Mass Mr • This is the average mass of a single molecule and is sometimes termed RMM (Mr) • To calculate Mr add up the relative atomic masses of the different atoms in the molecule H2 H2O HCl C6H12O6 Mr = 1 x 2 = 2 Mr = (1 x 2) + 16 = 18 Mr = 1 + 35.5 = 36.5 Mr = (12 x 16) + (1 x 12) + (16 x 6) = 180 Key definitions: • Relative isotopic mass: The mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12 • Relative atomic mass, Ar: The weighted mean mass of an atom compared with onetwelfth of the mass of an atom of carbon-12 Isotopes and relative atomic mass JFe worked example A sample of bromine contains 53% bromine-79 and 47% of bromine-81. Determine the relative atomic mass of bromine. Calculate Ar (Br) Questions!