atomic structure and the periodic table Flashcards
define an atom
the smallest part of an element that can exist
define an element
a substance that only contains one type of atom
define a compound
they contain two or more elements chemically combined in fixed proportions
what are chemical reactions usually
they often involve the formation of one or more new substances, and often involve a detectable energy change
how can compounds be separated
they can be separated back into their elements by chemical reactions
define mixtures and their chemical properties
they consist of two or more elements or compounds that aren’t chemically bonded together; the chemical properties of each substance in the mixture are unchanged
how can mixtures be separated
by using physical separation techniques rather than chemical reactions, e.g., filtration, crystallisation etc. these physical processes do not involve chemical reactions and no new substances are made.
why are compounds not the same as molecules
compounds must be two or more different elements joined together, molecules are just two or more of any elements joined together, they could be the same element
what is filtration used for
it’s used to separate an insoluble solid from a liquid it is suspended (not dissolved) in
method of filtration
1) use a filter funnel and filter paper folded in the shape of a cone
2) pour the mixture into the filter paper and funnel
3) the solid is too large to fit through the pores of the filter paper
4) therefore only the water passes through and the solid remains on the filter paper - so they are separated
what is crystallisation used for
it is used to obtain and separate a soluble solid from a liquid that it is dissolved in
method of crystallisation
1) leave the solution to evaporate slowly at room temperature
OR
1) heat the solution on a bunsen burner, tripod and glaze
2) this causes the water to evaporate, leaving crystals of the solid behind
3) ensure that the heat does not affect the solid itself; some solids will undergo thermal decomposition when heated
why does crystallisation work
this works because water will have a lower boiling point than the solid dissolved in it, therefore leaving the solid crystals
what is simple distillation used for
like crystallisation, it separates a soluble solid from a liquid. however, this is used when we actually want to keep the liquid and not simply evaporate it off
method of simple distillation
1) evaporate the liquid by heating it to a high enough temperature using a Bunsen burner
2) condense the vapour by cooling it in another container
what is fractional distillation used for
it is used to separate a mixture of 2 different liquids; these liquids must have different boiling points for this physical separation technique to work
method of fractional distillation
1) connect a flask to a fractionating column full of glass beads and again to a condenser and another beaker
2) heat the flask using a Bunsen burner until the temperature of the thermometer at the top of the column reaches the boiling point of the first liquid
3) at this point, both liquids will be evaporating but only the liquid with the higher boiling point will condense and drop back down once it reaches the top of the column
4) the lower boiling point liquid will not condense and will pass into the condenser, cool and form a pure liquid
5) the remaining liquid will be the other higher-boiling point liquid
what is chromatography used for
it is used to separate substances in a solution based on their different solubilities
method of chromatography
1) draw a pencil line on the bottom of chromatography paper
2) draw a dot of first colour on pencil line and dot of second colour on the pencil line
3) place the bottom of the paper in a solvent like water and ethanol
4) the solvent makes its way up the paper and the ink is dissolved into it
why is a pencil line used in chromatography
pencil does not dissolve in the solvent
knowledge of the atom before the discovery of the electron
atoms were thought to be tiny spheres that could not be divided
what did the discovery of the electron lead to
JJ Thomson’s plum pudding model; this suggested that the atom is a ball of positive charge with negative electrons embedded in it
what disproved the plum pudding model and how
Rutherford’s alpha particle scattering experiment; if the plum pudding model was true, the particles should have passed straight through or only be slightly deflected at most because the positive charge was thought to be very spread out throughout the atom
how did the alpha particle scattering experiment work (include results)
positively-charged alpha particles were fired at an extremely thin sheet of gold foil
RESULTS:
- most alpha particles went straight through, showing that the atom is mostly empty space
- few alpha particles deflected, showing that there is something small and dense in the atom (the nucleus)
- few alpha particles repelled, showing that there is a small positive charge in the atom (protons); this mass isn’t evenly spread out, meaning the positive charge must be concentrated in the centre
conclusions of the alpha scattering experiment (which formed the nuclear model)
- the mass of an atom was concentrated at the centre (nucleus)
- the nucleus was charged (positively), with a cloud of electrons surrounding it
- the atom is mainly empty space
how did Bohr adapt the nuclear model
he adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. the theoretical calculations of Bohr agreed with experimental observations
why did Bohr conclude that Rutherford’s cloud of electrons wouldn’t work
the electrons would be attracted to the nucleus, causing the atom to collapse
after Bohr’s adaptation of the nuclear model, what was discovered
later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge; the name ‘proton’ was given to these particles
what did James Chadwick do
his experimental work provided the evidence to show the existence of neutrons within the nucleus. this was about 20 years after the nucleus became an accepted scientific idea
number of protons in an atom =
number of electrons
define the atomic number
the number of protons in an atom of an element
atoms of different elements have different numbers of
protons
approximate radius of an atom
0.1nm (1x10⁻¹⁰m)
approximate radius of a nucleus I comparison to atom
less than 1/10000 of the atom’s radius (1x10⁻¹⁴m)
where is almost all of the mass of an atom located
in the nucleus
define the mass number
the sum of the protons and neutrons in an atom
