atomic structure and the periodic table Flashcards
all substances are made of atoms…
an atom is the smallest part of an element that can exist
element
- a substance of only one type of atom
- represented by chemical symbol
- about 100 different elements in the periodic table
compound
- 2 or more elements combined chemically in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed
- can only be separated into elements
mixture
- consists of 2 or more elements or compounds not chemically combined together
- chemical properties of each substance in the mixture are unchanged
- can be separated by filtration, crystallisation, simple and fractional distillation and chromatography (physical processes, no new substances made, not chemical reactions)
chemical reactions
- represented by word equations or equations using symbols and formulae
how can mixtures be separated?
- by filtration, crystallisation, simple and fractional distillation and chromatography (physical processes, no new substances made, not chemical reactions)
simple distillation
- used to separate a solvent from a solution
- e.g producing pure water from seawater
- works as the dissolved solute has a much higher boiling point than solvent
- when solution is heated, solvent vapour leaves solution and is cooled and condensed
- remaining solution becomes more concentrated as the amount of solvent in it decreases
fractional distillation
- used to separate different liquids from a mixture of liquids
- used for liquids with similar boiling points
- e.g crude oil
- mixture is heated
- vapours rise through a column which is hot at the bottom and cool at the top
- vapours cool and condense when they reach a part of the column that is below the temperature of their boiling point
each liquid is led away from the column
chromatography
- separate mixtures of soluble substances often coloured substances e.g inks and dyes
- paper in water not touching pencil lines with dyes
- dye will spread up paper as it absorbs solvent
- stationary phase = very uniform, absorbent paper
- mobile phase = solvent that moves through paper carrying substances with it
filtration
- separate an insoluble solid from a liquid
- filter paper has tiny holes or pores in it large enough to let small molecules and dissolved ions through but not much larger particles
- filter paper in filter funnel, pour mixture
before the electron…
atoms were thought to be tiny spheres that could not be divided
crystallisation
- produces solid crystals from a solution
- solution is warmed in evaporating basin with bunsen burner, some solvent evaporates leaving crystals behind
- can be used to obtain copper sulfate crystals from copper sulfate solution
discovery of the electron led to…
JJ Thomson discovering the plum pudding model, which suggest an atom was a ball of positive charge with negative electrons embedded in it
alpha scattering experiment
- Ernest Rutherford designed an experiment to test the plum pudding model
- beam of alpha particles aimed at very thin gold foil and their passage was detected
- some alpha particles emerged from the foil at different angles and some came straight back
- positively charged alpha particles were being repelled an deflected by a small concentration of positive charge in the atom (nucleus)
alpha scattering conclusion
- mass of an atom was concentrated at the centre (nucleus) and the nucleus was charged
neil bohr suggested…
- electrons orbit the nucleus at specific distances
theoretical calculations of – bohr agreed with experimental observations
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, these were called protons
James Chadwick’s work
- provided evidence to show the existence of neutrons within the nucleus
- 20 years after the nucleus became an accepted scientific idea
relative electrical charges of the particles in atoms are
proton = +1
neutron = 0
electron = -1
atomic number
number of protons in an atom of an element
atom has an overall charge of 0 so…
number of protons = number of electrons
atoms of a particular element…
have the same number of protons
radius of atom
0.1nm
radius of nucleus
less than 1/10,000 if that if the atom, though holds almost all its mass
relative mass of particles
proton = 1
neutron = 1
electron = very small / 1/1840
mass number
sum of protons and neutrons in an atom
isotopes
atoms of the same element with different numbers of neutrons
relative atomic mass
an average value that takes account of the abundance of the isotopes of the element
carbon has 2 isotopes: carbon-14 with abundance 20% and
carbon-12 with abundance 80%. Calculate the relative atomic mass of carbon.
((14 x 20) + (12 x 80)) / 100
= 1240 / 100
= 12.4
electronic structure
- electrons occupy the lowest available energy levels (the shells closest to the central nucleus)
- electronic structure of an atom tells you how many electrons are in each shell
- 2, 8, 8, 8
periodic table
- elements are arranged in atomic number and elements with similar properties are in columns known as groups
- elements in the same periodic group have the same amount of electrons in their outer shell, which gives them similar chemical properties
John Newlands
- ordered his table in order of atomic weight
- realised similar properties occurred every 8 elements - ‘law of octaves’ but this did not apply after calcium
metals
- elements that react to form positive ions
- majority of elements
- found to the left and towards the bottom
Dmitri Mendeleev
- ordered his table in order of atomic mass, but not always strictly i.e in some places he changed the order based on atomic weights
- left gaps for elements he thought had not been discovered yet
- elements with properties predicted by mendeleev were discovered and filled the gaps
- knowledge on isotopes made it possible to explain why the order based on atomic weights was not always correct.
- when electrons, protons and neutrons were discovered in the early 20th century, elements were ordered in atomic number and elements were placed in their appropriate groups
non-metals
elements that do not form positive ions
- found towards the right and top
group 1
alkali metals
- soft
- relatively low melting points
- low densities
- metals react vigorously with water to create an alkaline solution and hydrogen
- all react with oxygen to create an oxide
- all react with chlorine to form a white precipitate
- reactivity of elements increase going down the group
lithium reactions
oxygen = burns with a strongly red-tinged flame and produces a white solid
water = fizzes steadily and gradually disappears
chlorine = white powder is produced and settles on the sides of the container
potassium reactions
oxygen = large pieces produce lilac flame, smaller ones make solid immediately
water = ignites with sparks and a lilac flame, disappears very quickly
chlorine = reaction id even more vigorous than with sodium
sodium reactions
oxygen = strong orange flame and produces white solid
water = fizzes rapidly, melts into ball and disappears quickly
chlorine = burns with bright yellow flame, clouds of white powder are produced and settle on sides of container
group 0
noble gases
- full outer shell = very stable
- unreactive and do not easily form molecules
- stable arrangement of electrons
- boiling points increase with increasing relative atomic mass (going down the group)
group 7
halogens
- non-metals and exist as molecules made of pairs of atoms (Cl2)
- react with metals to form ionic compounds in which the halide ion carries a -1 charge
- react with non metals to form covalent compounds (shared pair of electrons)
- reactivity decreases going down as halogens react by gaining an electron, the number of shells of electrons increases down the group so the element attracts electrons less
- can’t react as easily
- more reactive halogen can displace a less reactive one in an aqueous solution of its salt
transition metals compared to group 1
- harder and stronger
- have higher melting points and higher densities
- much less reactive and don’t react as vigorously with oxygen or water
transition metals typical properties
- ions with many different charges
- form coloured compounds
- useful as catalysts
manganese
hard and very brittle, difficult to fuse, easy to oxidise
ion charges = +2 +3 +4 +5
+6 +7
colours with compounds = +2(pink), +4(red), +6(green), +7(grey
uses as a catalyst = decomposition of hydrogen peroxide
chromium
lustrous, brittle, hard
ion charges = +2 +3 +4 +5
+6
colours with compounds = +2(blue), +3(green), +6 (yellow)
uses as a catalyst = x
iron
good conductor, rusts easily in air strong, ductile, malleable
ion charges = +2 +3 +4
+5 +6
colours with compounds = +2(pale green), +3 (pale yellow)
uses as a catalyst = for haber process
cobalt
brittle, hard, high melting point
ion charges = +2 +3 +4 +5
colours with compounds = +2(magenta) +3 (green)
uses as a catalyst = x
nickel
hard, malleable and ductile, fairly good conductor of heat and electricity
ion charges = +2 +3 +4
colours with compounds = +2 (green)
uses as a catalyst = manufacture of margarine
copper
highly ductile and conductive, malleable and soft
ion charges = +1 +2 +3
colours with compounds = +2 (blue)
uses as a catalyst = x
