C1 Atomic structure and the Periodic table

C1 Atomic structure and the Periodic table

1. Atoms

   • Atoms contain protons, neutrons and electrons. Atoms have a radius of about 0.1 nanometers. There are a few different modern models of the atom- but chemists tend to like this model the best. The nucleus is in the middle of the atom, it contains protons and neutrons. It has a positive charge because of the protons, almost the whole mass of the atom is concentrated in the nucleus. The electrons move around the nucleus in electron shells. They're negatively charged and tiny, but they cover a lot of space. The volume of their orbits determines the size of the atom. Electrons have virtually no mass.
     • Number of protons equals number of electrons. Atoms are neutral- they have no charge overall. This is because they have the same number of protons and electrons. The charge on the electrons is the same size as the charge on the protons, but opposite- so the charges cancel out. In an ion, the number of protons doesn’t equal the number of electrons. This means hit has an overall charge. Atomic number and mass number describe an atom. The nuclear symbol of an atom tells you its atomic (proton) number and mass number. The atomic number tells you how many protons there are, the mass number tells you the total number of protons and neutrons in the atom. To get the number of neutrons, just subtract the atomic number from the mass number.

2. Elements

   • An element is substance made up of atoms that all have the same number of protons in their nucleus. Elements consist of atoms with the same atomic number, atoms can have different numbers of protons, neutrons and electrons. It's the number of protons in the nucleus that decides what type of atom it is. For example, an atom with with one proton in its nucleus is hydrogen and an atom with two protons is helium. If a substance only contains atoms with the same number of protons it's an element, there are about 100 different elements. So all the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons. Atoms can be represented by symbols, either one or two letters. Some are obvious; oxygen=O, some aren't, sodium=Na.
     • Isotopes are the same except for extra neutrons. Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons. So isotopes have the same atomic number but different mass numbers. A very popular example is carbon-12 and carbon-13. Because many elements can exist as a number of different isotopes, relative atomic mass (Ar) is used instead of the mass number when referring to the element as a whole.

3. Compounds

   • Elements can mix and match to make lots of new substances called compounds. Atoms join together to make compounds, when elements react, atoms combine with other atoms to form compounds. Compounds are substances formed from two or more elements, the atoms of each are in fixed proportions throughout the compound and they're held together by chemical bonds. Making bonds involves atoms giving away, taking or sharing electrons. Only the electrons are involved, it's usually difficult to separate the original elements of a compound out again, a chemical reaction is needed to do this. A compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to for positive ions and the non-metal atoms gain electrons to form negative ions. The opposite charges mean they're strongly attracted to each other, this is called ionic bonding. A compound from non-metals consists of molecules. Each atom shares an electron with another atom, this is called covalent bonding.
     • A formula shows what atoms are in a compound. Just as elements can be represented by symbols compounds can be represented by formulas. The formulas are made up of elemental symbols in the same proportions that the elements can be found in the compound. Examples- C02, H20, NH3 and HCL.

4. Chemical equations

   • Chemical changes are shown using chemical equations, one way to show a chemical reaction is to write a word equation, it's not as quick as symbol equations and you can't tell straight away what's happened to each of the atoms, but it's easy. Symbol equations show the atoms on both sides. Chemical changes can be shown in a kind of shorthand using symbol equations. Symbol equations show the symbols or formulas of the reactants and products.
     • Symbol equations need to be balanced, there must always be the same number of atoms on both sides- they can't just disappear. You balance the equation by putting numbers in front of the formulas where needed, you can't change formulas you can only put numbers in front of them.

5. Mixtures and separation techniques

   • Mixtures are easily separated- not like compounds. Unlike in a compound, there's no chemical bond between the different parts of a mixture. The parts of a mixture can be either elements or compounds, and they can be separated out by physical methods such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. Air is a mixture of gases, mainly nitrogen, oxygen, carbon dioxide and argon. The gases can all be separated out fairly easily. Crude oil is a mixture of different length hydrocarbon molecules.
     • Paper chromatography- 1) Draw a line near the bottom of filter paper 2) Add a spot of the ink to the line and place the sheet in a beaker of solvent (water) 3) The solvent used depends on what's being tested, some compounds dissolve well in water, but sometimes other solvents, like ethanol is needed 4) Make sure the ink isn't touching the solvent 5) Place a lid on top of the container to stop the solvent evaporating 6) The solvent seeps up the paper, carrying the ink with it. 7) Each different dye in the ink will move up the paper at a different rate so the dyes will separate out. Each dye will form a spot in a different place 8) if any of the dyes in the ink are insoluble in the solvent you've used, they'll stay on the baseline 9) When the solvent has nearly reached the top of the paper, take it out of the beaker and leave it to dry. 10) The end result is a pattern of spot called chromatography.
   • Filtration separates insoluble solids from liquids, filtration can be used if your product is an insoluble solid that needs to be separated from a liquid reaction mixture. It can be used in purification as well. There are two ways to separate soluble solids from solutions, Evaporation and Crystallisation.
     • Evaporation- 1) Pour the solution into an evaporating dish 2) Slowly heat the solution. The solvent will evaporate and the solution will get more concentrated. Eventually, crystals will start to form 3) Keep heating the evaporating dish until all you have left are dry crystals. Crystallisation- 1) Pour the solution into an evaporating dish and gently heat the solution. Some of the solvent will evaporate and the solution will get more concentrated. 2) Once some of the solvent has evaporated, or when you see crystals start to form, remove the dish and leave to cool. 3) The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution. 4) Filter the crystals out of the solution and leave them in a warm place to dry. You could also use a drying oven. Filtration and crystallisation can be used to separate rock salt.

6. Distillation

   • Distillation is used to separate mixtures which contains liquids. There are two types- simple and fractional. Simple distillation is used to separate out solutions and fractional distillation is used to separate a mixture of liquids.
     • Simple: 1)Simple distillation is used for separating out a liquid from a solution. 2)The solution is heated, the part of the solution that has the lowest boiling point evaporates first. 3)The vapour is then cooled, condenses and is collected. 4)The rest of the solution is left behind in the flask. 5) You can use simple distillation to get pure water from seawater, the water evaporates and is condensed and collected. Eventually you'll end up with just the salt left in the flask. 6) The problem with simple distillation is that you can only use it to separate things with very different boiling points- if the temperature goes higher than the boiling point of the substance with the higher boiling point, they'll mix again.
     • Fractional:

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