Atomic Structure

• In an element, all the atoms are the same 
• Compounds contain two or more elements chemically combined 
• Compounds usually have different properties than the combined elements
• Mixtures contain a mixture of different elements or compounds but they are not chemically combined
• Molecules has any element chemically combined- even the same element
• To separate a mixture- you can use filtration, distillation, chromatography or crystallisation
• The periodic table is an organised array of chemical elements in order of increasing atomic number
• Elements are arranged into columns called groups
• Elements are arranged into rows called periods
• Atoms contain a nucleus with protons and neutrons surrounded by electron shells with electrons
• Electrons contain a negative charge (-1 coulombs)
• Neutrons contain a neutral charge (0 coulombs)
• Protons contain a positive charge (+1 coulombs)
• Atoms have no charge as the number of protons=number of electrons
• Atomic number=proton/electron number
• Atomic mass= number of protons/electrons + number of neutrons
• Isotopes are the same element with different number of neutrons
• Ions are atoms with an overall charge
• The periodic table contains 8 groups of elements (not including the transition metals)
• Periodic means to occur at regular intervals
• Elements in a group react in a similar way
• Elements in a group all contain the same number of electrons in their outer shell
• Johann Döbereiner noticed chemicals with similar chemical properties often occur in threes; this is known as triads

  E.g. Lithium, potassium and sodium are all metals which react quickly with water
• Dmitri Mendeleev developed the first modern periodic table
• Mendeleev arranged the elements in order of increasing atomic weight
• Mendeleev would swap elements around which were not patterns
• Mendeleev noticed some elements were not discovered and simply left gaps in the periodic table
• Mendeleev predicted the properties of elements based on the previously found elements
• Ordering elements by atomic number creates the correct order of the elements
• Relative atomic mass (RAM) is used to determine the atomic mass of an element based on its isotopes
• Relative atomic mass = (mass number of isotope 1 x percentage abundance of isotope 1) + (mass number of isotope 2 x percentage abundance of isotope 2) divided by 100
• E.g. Boron= (10 x 20) + (11 x 80) = 1080/ 100= 10.8
• Magnesium= (24 x 79) + (25 x 10) + (26 x 12) = 2458/100= 24.58
• Metals are found on the left and centre of the periodic table
• Group 1 and 2= Highly reactive metals
• The centre= Transition metals (less reactive)
• This gives metals the same electronic structure as a group 0/noble gas
• When metals react, they lose electrons to achieve a full outer energy level
• Metals always form positive ions
• Group 0 are the noble gases