# C2

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• Created by: Katie
• Created on: 20-05-13 16:00
• C2
• The mass of atoms
• The top number when you look at an atom on the periodic table is the mass number
• The bottom number is the atomic number.
• The atomic number of an atom is its number of protons, which equals its number of electrons which causes it to have no charge as they cancel each other out.
• The number of protons + number of neutrons = the mass number.
• The relative mass of protons and neutrons is 1.
• Number of neutrons = mass number - atomic number
• The mass of the electrons on the shells are so tiny it is not worth measuring therefore most of the mass is contained in the nucleus.
• Isotopes
• These are atoms of the same element which always have the same amount of protons and electrons.
• Although, they don't have the same amount of neutrons.
• They have the same atomic number but different mass numbers.
• Sometimes the extra neutrons make the isotope unstable.
• Relative atomic mass
• Ar
• The average mass pf the atoms of an element compared with Carbon-12 (which is given a mass of exactly 12). The average mass must take into account of the naturally occuring isotopes of the element.
• Relative formula mass
• Mr
• The total of the relative atomic masses, added up in  the ratio shown in the chemical formula of a substance.
• Or simple terms! - we work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it.
• Moles
• Instead of saying 'the relative atomic mass in grams' or 'the relative formula mass in grams' you would say a mole.
• CaCo3 = 40+12+(16 x 3) = 100. If you had 300g of this, you would have 3 moles.
• Number of moles = mass (g) / Mr
• Yield of chemical reaction
• Amount of product produced = percentage yield x maximuum amount of product possible.
• The yield of a chemical reaction describes how much product is made.
• The percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that could be made (100%)
• Factors affecting the yield of a chemical reaction include any product being left behind in the apparatus and difficulty seperating the products from the reaction mixure.
• It is important to maximise yield and minimise energy wasted to conserve the Earths limited resources and reduce pollution.
• Reversible Reactions
• A+B = C+D
• This is supposed to have the reversible reaction symbol which shows a forward reaction and a reverse reactions.
• In a reversible reaction the products of te reaction can react to make the original reactant.