C4- Chemical Patterns

A brief summary of the C4 module taken from the OCR 21st Century Science Chemistry textbook.

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The Periodic Table

  • The relative atomic mass (RAM) scale shows the mass of an atom of a element compared with the mass of hydrogen (RAM 1).
  • The periodic table of elements gets its name from the repeating patterns you see when chemists line up the elements in order.
  • Each row on a periodic table is a period. Each column is a group of similar elements with similar properties.
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The Alkali Metals

  • The metals in group 1 (lithium, sodium and potassium) are very reactive and have to be kept under oil to stop them reacting with moisture or oxygen in the air.
  • Alkali metals are soft (like cheese) and they are shiny but tarnish quickly in air. The shiny surface of the metal becomes dull when cut.
  • Lithium fizzes gently and floats on the surface of water. It produces alkali Lithium hydroxide (LiOH) and hydrogen.
  • The reaction with sodium is more energetic (yellow flame and skates on water's surface); potassium is even more violent (catches fire, metal spits).
  • Hot sodium and chlorine react to produce a yellow flame and clouds of white NaCl crystals- table salt.
  • The term salt is used to cover all compounds of metals and non-metals.
  • There are clear trends in the properties of alkali metals; both physical properties(density and melting point) and chemical properties (reactivity with water and chlorine).
  • The elements are dangerously reactive but the compounds are useful.
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Chemical Equations

  • In a chemical change, there is no change in mass as the number of each type of atom is the same.
  • Chemical equation- A summary of a chemical reaction showing the reactants and the products with their physical states.
  • You cannot write an equation if you don't know-
  • 1. All the starting chemicals (reactants).
  • 2. Everything that is formed during the change (products).
  • Balanced equation- An equation showing the formulae of the reactants and products. The equation is balanced when there is the same number of each atom on both sides.
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The Halogens

  • Fluorine, chlorine, iodine and bromine are all very reactive non-metals.
  • Non-metals typically have low melting and boiling points.
  • Chlorine is a green gas at room temp, bromine is a red liquid and iodine is a grey solid.
  • The halogens are molecular (like most non-metals)- they each consist of molecules with atoms joined in pairs.
  • The forces between molecules are weak, so it is easy to separate them and turn them into gases.
  • All the halogens can kill bacteria and chlorine especially bleaches (others less).
  • Hot iron glows brightly in chlorine (bromine and iodine less).
  • The halogens are too hazardous for everyday use but their compounds are useful (chlorine for salt, pool treatment and plastics; bromine for drugs and pesticides; iodine for medicines, dyes and photographic chemicals).
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The Discovery of Helium

  • Different chemicals produce different flame colours when held in a flame.
  • Kirchhoff built a spectroscope by putting a glass prism into a wooden box and inserting 2 telescopes at an angle.
  • Each element has its own characteristic spectrum (consisting of a set of lines) when its light passes through a prism. The spectroscope was used to record the line spectra of many elements.
  • A solar eclipse in 1868 helped scientists to discover helium. During an eclipse, it is possible to study the spectra of the light from the hot gases around the edge of the sun.
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Atomic Structure

  • A model of an atom can be used to understand how atoms join together to form compounds, and how they re-group during chemical reactions.
  • The nucleus of an atom consists of protons and neutrons, with orbiting electrons. Protons are +ive and neutrons are neutral.
  • The electrons are -ive and the mass of an electron is so small, it can be ignored.
  • In an atom, the number of +ive protons = number of -ive electrons. So the atom has no charge.
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Electrons in Atoms

  • Heating atoms gives them energy; this forces the electrons to move to higher-energy orbits further from the nucleus.
  • These electrons drop back from outer to inner orbits, giving out light energy.
  • Each energy jump corresponds to a particular colour in the spectrum. The bigger the energy jump, the closer to the blue end of the spectrum.
  • There are regions around the nucleus where electrons are more likely to be found- clouds of negative charge. Each electron clouds is a shell around the nucleus.
  • Each shell exists when there are electrons in them and all electrons in a shell have the same energy.
  • The innermost shell with least energy fills first. When full, next shell fills.
  • Electron configuration is the number and arrangement of electrons in an atoms of an element.
  • 1st shell can hold 2 electrons. 2nd shell- 8 electrons. 3rd shell- 18 electrons.
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Electronic Structures and the Periodic Table

  • A modern periodic table shows the elements in order of proton number.
  • Across the periods, the proton number increases by 1 and the number of electrons increases by 1 also.
  • When atoms react, the electrons in the outer shell get involved as chemical bonds break and new chemicals form.
  • Elements have similar properties if they have the same number and arrangement of electrons in outer shell.
  • Elements with 1 or 2 electrons in the outer shell are metals. Elements with more are generally non-metals.
  • At the end of each period is a noble gas (helium, neon, argon, krypton, xenon, radon) - a group of very unreactive elements.
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  • Salts are compounds of non-metals and metals.
  • Salts (such as sodium chloride) are crystalline- the crystals of NaCl are shaped like cubes.
  • Salts have higher melting and boiling points than compounds that are made up of small molecules (such as chlorine or bromine).
  • There are many soluble salts, including most of the compounds of alkali metals with halogens. Some are insoluble.
  • Molten compounds are more likely to conduct electricity than solids. Soluble salts also conduct electricity.
  • Electrode- a conductor made of a metal or graphite through which a current enters or leaves a chemical during electrolysis. Electrons flow into the -ve electrode (cathode) and out of the +ive electrode (anode).
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Ionic Theory

  • Electrolysis- Splitting up a chemical into its elements by passing an electric current through it.
  • +ively charged particles are attracted to the cathode, -ively charged particles are attracted to the anode.
  • When the charged particles reach the electrodes they move to, they turn back into atoms.The charges particles are called ions.
  • Sodium Chloride is made up of +ive sodium ions and -ive chloride ions- they attract each other.
  • Crystals of NaCl are closely packed in a solid. The ions cannot move so they can't conduct electricity. When it is molten or dissolved in water, they can.
  • Metals form +ive ions and non-metals generally form -ive ions.
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Ionic Theory and Atomic Structure

  • Atoms form ions by gaining or losing electrons. Metals lose electrons, leaving more protons- the ions are positively charged.
  • When Na and Cl atoms turn into ions, they end up with the same electron configuration as the nearest noble gas (generally true for simple ions).
  • The anode is positive as electron flow out of it into the battery. The cathode is negative as the flow of electrons from the battery goes through it.
  • Not all ions have single positive or negative charges. All compounds are overall electrically neutral. PbBr2- Br is -ive, so Pb must be 2+ive.
  • The charges of simple ions show a periodic pattern.
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Chemical Species

  • Chemical species- The same element can take different chemical forms with distinct properties.
  • Chlorine has 3 species- Atom, Molecule and Ion.
  • Chlorine atoms do not normally exist in a free state; very reactive and quickly destroy the ozone layer. They rapidly pair up to form chlorine molecules.
  • Chlorine is a gas at room temp and consists of chlorine molecules. They are reactive enough to damage human tissues.
  • Chloride ions are chemically active but are not harmful. In salts, they are essential to life and occur in living tissues.
  • Sodium has 2 species- Atom and Ion.
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