C.1. Atoms, Bonds and Moles

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  • Created by: sana.aaa
  • Created on: 19-01-18 18:20

Atoms :

  • all substances are made of tiny particles called atoms
  • an atom is the smallest part of an element that can exist
  • elements can have different properties, such as silver, chromium, and gold are shiny solid metals.
  • other metals such as oxygen, nitrogen, argon, and chlorine are non-metals and are gases at room temp.
  • group 1- the alkali metals
  • group 2- the alkaline earth metals
  • group 7- the halogens
  • group 0- the noble gases
  • molecule- a grouping of 2 or more atoms bonded together.
  • different types of atoms bonded together are called compounds
  • chemical bonds hold the atoms tightly together in compounds
  • some compounds are made up from just 2 types of atoms, most compounds consist of more than 2 different types of atoms.
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Chemical Equations :

  • chemical equations show the reactant (the substances you start with) and the products (the new substance made) in a reaction.
  • the chemical equation can be represented by a word equation (for example hydrogen  +oxygen---> water)
  • you can investigate what happens to the mass of reactants compared with the mass of products in a reaction
  • using a symbol equation helps you to see how much of each substance involved in this equation
  • when an equation is balanced- there is the same number of each type of atom on both sides of the equation
  • this is important because atoms cannot be CREATED or DESTROYED
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Law of Conservation of Mass :

  • THE TOTAL MASS OF THE PRODUCTS FORMED IN A REACTION IS EQUAL TO THE TOTAL MASS OF THE REACTANTS---  this is called the Law of conservation of mass
  • this law can appear to be broken when the reactions are carried out in open containers (such as test tubes and conical flasks). For example, if you weigh a  sample of calcium carbonate before you heat it, then weigh it again after it has been heated you will see that the mass has decreased. This is because carbon dioxide is also formed in this reaction, therefore meaning that the gas has escaped into the air. 
  • similarly, a piece of copper increases in mass when heated in air, the apparent extra mass comes from the oxygen gas that the copper reacts with to make COPPER OXIDE.
  • you can check if an equation is balanced by counting the number of each atom on either side of the equation. if the numbers are equal then it is a BALANCED EQUATION
  • state symbols can also be added such as (g), (l) and (aq)
  • "s" is for solids
  • "g" is for gases
  • and "aq" is for substances dissolved in water, called aqueous solutions
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Separating Mixtures :

  • a mixture is made up of two or more substances (elements or compounds) that are not chemically combined together
  • mixtures have no fixed composition
  • the different elements or compounds in a mixture can be separated again more easily
  • there are no chemical bonds between atoms of the different substances in a mixture
  • compounds, however, have a fixed composition
  • chemical reactions must be used to separate the elements in a compound
  • the chemical bonds between atoms of the different elements in the compound
  • before the substances in a mixture can be identified, they are separated from each other
  • the techniques available include:
  • filtration
  • crystallisation
  • chromatography
  • distillation
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Filtration :

  • this technique is used to separate substances that are insoluble in a particular solvent from those that are soluble in the solvent.
  • for example:
  • when you try to separate sand, salt (sodium chloride, and water), the sand is collected on the filter paper. 
  • then it can be washed with distilled water to remove any salt solution left on it.
  • the wet sand is finally dried in a warm over to evaporate any water off, this leaves the sand as pure and dry.
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Crystallisation :

  • to obtain a sample of pure salt(sodium chloride, NaCl) from the salt solution following filtration, you would need to separate the sodium chloride in the solution (called the filtrate) from the water.
  • this done by evaporating the water from the sodium chloride solution, the best way this can be done is by heating it in an evaporating dish on a water bath
  • using a water bath is a gentler way of heating that heating the evaporating dish directly on a tripod and gauze mat.
  • heating should be stopped when the solution is at the point of crystallisation- this is when the small crystals first appear around the edge of the solution or when crystals appear in a drop of solution extracted from the dish with a glass rod.
  • the rest of the water is then left to evaporate off the saturated solution at room temp to get a good sample of sodium chloride crystals
  • a flat-bottomed crystallisation dish or petri dish can be used for this final step, to give a larger surface area for the water to evaporate from.
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Distillation :

  • crystallisation separates a soluble solid from a solvent but sometimes you need to collect the solvent itself instead of just letting it evaporate off into the air
  • distillation allows some countries with a lack of fresh water to use seawater, and purify it to obtain usable water
  • in simple distillation, a solution is heated and boiled to evaporate the solvent, the vapour given off then enters a condenser. this is an outer glass tube with water flowing through it that acts as a cooling "jacket" around the inner glass tube from the flask. here the hot vapour is cooled and condensed back into a liquid for collection in a receiving vessel.
  • any dissolved solids will remain in the heated flask.
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Fractional Distillation :

  • distillation can also be used to separate the mixture of miscible liquids, such as ethanol and water.  the word miscible describes liquids that dissolve in each other, mixing completely.
  • they do not form the separate layers seen in mixtures of immiscible liquids that have allowed to settle, such as the oil and water layers in a salad dressing.
  • the miscible liquids will have different boiling points, so you can use this to distill off and collect the liquid with the lowest boiling point first.
  • it is difficult to get pure liquids from the mixtures of liquids with similar boiling points by simple distillation, as vapours are given off from each liquid before they actually reach their boiling
  • so to aid separation you can add a fractionating column to the apparatus for distillation- this is usually a tall glass column filled with glass beads, fitted vertically on top of the flask being heated.
  • the vapours must pass over and between the glass beads in the fractionating column before they reach the condenser. the temp in the fractionating column is highest at the bottom of the column, getting lower as the vapours rise up. the substance with the highest boiling point will condense more readily on the cooler glass beads nearer the bottom of the column and drip back down into the flask beneath. the lower substance with the lower boiling point will continue rising and pass over into the condenser, where it is cool enough to turn back into liquid state and be collected.
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Fractional Distillation Example :

  • the boiling temp of ethanol is 78*C and the bp of water is 100*C, so if the temp is kept around 80*C, the liquid collected will be mainly ethanol. 
  • you can test the difference between the starting mixture of ethanol and water and the distillate collected by applying a lighted splint to a small volume of each in an evaporating dish.
  • ethanol is a flammable liquid, but it is not flammable when mixed with an excess of water.
  • the distillate will ignite when a flame is applied as the ethanol is collected should only have a small amount of water present.
  • it burns with a clear blue flame.
  • fractional distillation is used to separate ethanol from a fermented mixture in the alcoholic spirits industry and in the use of ethanol as a biofuel.
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Paper Chromatography :

  • one technique that is used to separate substances from mixtures in solution is PAPER CHROMATOGRAPHY, it works because some compounds in a mixture will dissolve better than others in the solvent chosen.
  • a capillary tube is used to dab a spot of the solution on a pencil line near the bottom of a sheet of absorbent chromatography paper
  • the paper is then placed standing in a solvent at the bottom of a beaker or tank
  • the solvent is allowed to soak up the paper, running through the spot of the mixture
  • the RELATIVE SOLUBILITY determines how far they travel up the paper
  • the MORE SOLUBLE a substance is in the solvent, the further up the paper it is carried
  • different solvents can be used to maximise separation
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Early ideas about atoms :

  • the ancient Greeks were the first to have ideas about particles and atoms
  • it wasn't until early 1800's that these ideas became linked to strong experimental evidence when JOHN DALTON put forward his ideas about atoms
  • from his experiments he suggested that substances were made up of atoms that were like tiny hard spheres. he also suggested that each chemical element had its own atoms that differed from other in their mass
  • Dalton believed that these atoms could not be divided or split- they were fundamental building blocks of nature
  • he suggested that the atoms re-arranged themselves and combined with other atoms in new ways
  • his ideas help visualise elements, compounds, and molecules, as well as particles in solids, liquids, and gases.
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Evidence for Electrons in atoms :

  • at the end of the 1800's, a scientist called J.J.Thompson discovered the ELECTRON.
  • this is a tiny, negatively charged particle that was found to have a mass about 200 times smaller than the lightest atom. Thompson was experimenting by applying high voltages to gases at low pressure.
  • Thompson did experiments on the beams of particles, they were attracted to a positive charge, showing they must be negatively charged themselves.
  • he called the tiny, negatively charged particles- ELECTRONS.
  • Thompson proposed a different model of the atom.
  • he said that tiny negatively charged electrons must be embedded in a cloud of positive charge.
  • he knew that atoms themselves carry no overall charge, so any charges in an atom must balance out
  • he imagined the electrons as the bits of plum in a plum pudding
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Evidence for the Nucleus :

  • the next breakthrough in understanding the atom came about 10 YEARS LATER.
  • GEIGER AND MARSDEN were doing an experiment with radioactive particles. they were firing dense, positive charged particles (ALPHA PARTICLES) at the thinnest piece of gold foil they could make.
  • they expected the particles to pass straight through the gold atoms with their diffuse cloud of positive charge (as in Thompson's plum pudding model)
  • the results shocked them- their results allowed them to create a new model for the atom
  • RUTHERFORD suggested that THOMPSON'S atomic model was not possible because the positive charge must be concentrated at a tiny spot in the centre of the atom. otherwise, the large, positive particles fired at the foil could never be repelled back towards their source. 
  • it was proposed that the electrons must be orbiting around this NUCLEUS (centre of the atom), which contains very dense positively charged protons.
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Evidence for Electrons in Shells :

  • the next important development cam in 1914, when NIELS BOHR revised the atomic model again
  • he noticed that the light give out when atoms were heated only had a specific amount of energy
  • he suggested that the electrons must be orbiting around the nucleus at set distances, in certain fixed energy levels (or shells)
  • the energy must be given out when excited electrons fall from a high to the low energy level
  • bohr matched his model to the energy values observed
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Evidence for Neutrons in the nucleus :

  • scientists at the time saw that there were two types of sub-atomic particles inside the nucleus
  • they had evidence of protons but a second sub-atomic particle in the nucleus was also proposed to explain the missing mass that had been noticed in atoms
  • these NEUTRONS must have no charge and have the same mass as a PROTON
  • because neutrons have no charge, it was difficult to detect them, in experiments
  • it wasn't until 1932 that JAMES CHADWICK did an experiment that could only be explained by the existence
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Structure of the Atom:

  • atoms are made of protons, neutrons, and electrons
  • protons have a relative charge of +1, and a relative mass of 1
  • electrons have a relative charge of -1, and a relative mass of 0
  • neutrons have a relative mass of 1
  • atoms are neutral
  • atomic number = the number of PROTONS
  • number of PROTONS= number of ELECTRONS
  • mass number= number of protons + neutrons
  • atoms of the same element have the same number of protons (and hence electrons)in their atoms
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Ions, Atoms and Isotopes :

  • atoms that gain electrons form negative ions
  • if atoms lose electrons they form positive ions
  • isotopes are atoms of the same element with different numbers of neutrons
  • isotopes have identical chemical properties, but their physical properties, such as density, can differ- and they may or may not be radioactive
  • the chemical properties are the same because their reactions depend on their electronic structures- as their atoms have the same number of protons and electrons , the electronic structure will be the same for all isotopes of an element
  • sometimes extra neutrons can make the nucleus unstable, so it is radioactive
  • not all isotopes are radioactive- they are simply atoms of the same element that have different masses
  • an ion is a charged atom
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Electronic Structure :

  • the electrons in an atom are arranged in energy levels or shells
  • the lowest energy level (1st shell) can hold up to 2 electrons
  • the second shell can carry 8 electrons- so can the 3rd
  • the outermost shell of an element's atoms determines the way in which that element reacts
  • the elements in group 0 of the periodic table are called NOBLE GASES
  • they are very unreactive elements because their atoms all have a very stable arrangement of electrons with eight electrons in the outer shell, except for helium with 2 electrons in the outer shell (the first shell).
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