C1 (atomic structure and the periodic table) Flashcards
describe the difference between:
- atoms
- elements
- compounds
atoms: all substances are made of atoms. an atom is the smallest part of an element that can exist.
elements: there are about 100 different elements, shown on the periodic table.
compounds: formed from elements by chemical reactions. always involve the formation of one or more new substances, and often involve a detectable energy change.
- contain two or more elements chemically combined. can only be separated back into elements through chemical reactions.
- the elements contained are always found in the same proportions
what is a molecule?
a group of two or more atoms, held together by chemical bonds
- can contain multiple different elements (e.g. water, carbon dioxide), or be two or more of the same element (e.g. oxygen)
- water and carbon dioxide are also compounds
what does the size of an atom depend on?
the element it forms
what is a mixture?
consists of two or more elements or compounds combined together. the chemical properties of each substance in the mixture are unchanged, as they haven’t reacted with each other
- can be separated by physical processes such as filtration, crystallisation, simple and fractional distillation, and chromatography.
- these processes do not involve chemical reactions and no new substances are made.
describe the history of the atom:
- atomic theory (democritus). everything is made up of the smallest particles, surrounded by empty space.
- ‘solid spheres’ (john dalton). different types of spheres make up different elements.
- plum pudding model (j.j. thompson). ball of positive charge containing negatively charged, discrete particles. experiments showed that atoms simply couldn’t be solid spheres
- nuclear model (earnest rutherford). instead of a general field of positive charge, there’s a compact nucleus, that contains the positive charge. it also has to have a cloud of negative charge around it.
- electrons (niels bohr). electrons orbit the nucleus in shells, which stops the atom from collapsing.
since then, ernest rutherford found the positive charge in the nucleus to be in small discrete particles (protons). james chadwick discovered neutral particles in the nucleus (neutrons).
how did ernest rutherford develop his nuclear model?
took positively charged alpha particles, and fired them at a thin sheet of gold. if the positive charge in the gold atoms was generally spread out, as j.j. thomson proposed, then the alpha particles should pass through the sheet, as the weak, spread out positive charge wouldn’t be enough to affect the particles.
- some of the alpha particles were either deflected out to the sides, or deflected back the way they’d come, and the rest simply went through.
what was the flaw with rutherford’s nuclear model?
there was nothing stopping the negative charge from rushing in on the positive nucleus. this means that the atom would automatically collapse, which we know it doesn’t.
describe the method of filtration:
- separates insoluble solids and liquids.
- place filter paper over a filter funnel, place in beaker to catch the liquid
- pour solution into funnel, the liquid drips through the filter paper (has lots of tiny holes that water can drip through but not solids), the solids stay in the funnel.
describe the method of evaporation:
- separates soluble (dissolved) solids and liquids
- a solution is placed in an evaporation dish and heated by a bunsen burner.
- the volume of the solution decreases because some of the water evaporates. solid particles begin to form in the basin as the solute becomes more concentrated
- this forms dry crystals of the solid in the basin, with all the solvent having disappeared
what are the positives and negatives of evaporation?
good: quick and easy method
bad: some solids decompose when heated (thermal decomposition), so we’d isolate it but break it down into something else. we can counter this by crystallising the solution instead, which is a slightly slower process
describe the method for crystallisation:
- to separate soluble solids (subject to thermal decomposition) from a solvent
- place the solution in an evaporating dish and heat it, but more gently (e.g. use a water bath)
- once some of the solvent has evaporated and crystals begin to form, stop heating the solution and leave it to cool
- as the solution cools, more crystals will start to form, as solids are less soluble in colder temperatures
- filter out these crystals from the remaining solution, using a filter paper and funnel
- dry out the collected crystals, by leaving them somewhere warm or warming them in the oven
what’s the difference between a solution and a mixture?
- putting an insoluble substance in a liquid forms a mixture
- putting a soluble substance in a liquid forms a solution, where the soluble substance is the solute and the liquid is the solvent
describe the method of chromatography:
- separates solutions with many different dissolved solutes (solids) in the liquid.
- ink or plant dye is dotted along a pencil line at the bottom of a strip of paper.
- as the paper is lowered into the solvent, some of the dye spreads up the paper.
- at the end, the paper will have absorbed the solvent, and the dye will have spread even further up the paper.
what is the difference between a solute, a solvent and a solution?
solute: dissolved solid
solvent: liquid in which a solid dissolves
solution: liquid containing a dissolved solid
describe the method of simple distillation:
- separates out a liquid from a solution (e.g. to separate pure water from sea water)
- heat the solution, until one of the liquids begin to evaporate.
- the vapour from the solution rises and the pressure forces it down a condenser, causing it to cool and condense back into liquid form, which will run down the condenser and collect in the beaker
describe the setup for simple distillation:
- flask containing solution/liquid mixture
- beneath the flask is some sort of heating device, e.g. bunsen burner
- the flask is sealed at the top with a bung, so no gas can escape
- put a thermometer through the bung to measure the temperature inside the flask
- a condenser is attached to the neck of the flask, which consists of the main pipe, surrounded by a water jacket, which contains a stream of continually flowing cold water
- at the end of the condenser is a beaker to catch the pure liquid
describe the method of fractional distillation:
- separates multiple liquids with different boiling points (a mixture)
- place solution in a flask, and heat from below with a bunsen burner. place a fractionating column on top of the flask, connected to a condenser.
- the solution is heated in the flask, and the temperature is controlled carefully using a thermometer. the liquid with the lower boiling point evaporates first, and is condensed and collected.
- this is continued for any other liquids, until there’s only one liquid left in the original flask.
what is the point of the glass rods in the fractionating column?
some of the other liquids will try and evaporate when it’s not their time to leave, and so when they come into contact with the glass rods, which are cooler than their boiling point, they’ll condense back into liquid form and fall back into the flask
what are the key features of a fractionating column?
- full of little glass rods, providing a large surface area
- the column, due to its height, is cooler at the top than at the bottom
why is fractional distillation used for alcohol?
when separating water and ethanol, it can purify or strengthen alcohol.
how can you check that the liquid produced is fully pure?
boil the liquid again, keeping in mind its boiling point. if it boils at a different boiling point than expected, it isn’t fully pure, and still has other liquids inside it.
what are the relative masses of electrons, protons, and neutrons?
electron: almost 0
proton: 1
neutron: 1
what are the relative charges of protons, electrons and neutrons?
protons: +1
electrons: -1
neutrons: 0
- the number of electrons is equal to the number of protons. atoms therefore have no overall charge.
- the number of protons in an atom of an element is its atomic number. all atoms of a particular element have the same number of protons.
- the sum of the protons and neutrons in an atom is its mass number.
what is the size of an atom?
has a radius of about 1 x 10^-10 metres
what is the size of the nucleus of an atom compared to the entire atom?
the radius of a nucleus is less than 1/10,000 the radius of an atom, but almost all of the atom’s mass is concentrated in the nucleus.
why do the electrons closest to the nucleus have the least energy?
because more protons are present in the nucleus, the force of attraction between the protons and electrons is stronger. this means the electrons closer to the nucleus have less energy.
what are isotopes?
atoms of the same element can have different numbers of neutrons, these atoms are called isotopes of that element.
- the other definition is atoms that have the same atomic numbers but different mass numbers
do all isotopes still react in the same way?
yes, as even though the neutron numbers vary, they don’t affect how the atom reacts
describe the three isotopes of hydrogen:
protium: 1 proton, 0 neutrons. 99.98% of hydrogen isotopes are protium. it’s used in fuel cells and the production of plastics.
deuterium: 1 proton, 1 neutron. 0.02% of hydrogen isotopes are deuterium. it’s used in nuclear fusion.
tritium: 1 proton, 2 neutrons. very rare. used in thermonuclear fusion weapons.
why do elements in the same group react in the same way?
because they all have the same number of electrons in their outer shells. e.g. noble gases react in similar ways because they’re all stable (all have 8 electrons in their outer shells).
what is meant by a stable/unstable atom?
stable: full outer shell (2/8)
unstable: incomplete outer shell, so wants to react with other atoms to form molecules/compounds to gain a full outer shell
describe dobereiner’s triads:
- discovered that elements with similar chemical properties often occurred in groups of threes (triads).
- e.g. lithium, sodium, potassium (all react rapidly with water).
how did dmitri mendeleev organise the periodic table?
- arranged elements in increasing atomic weight
- switched the order of some elements to fit the patterns of other elements in the same group
- realised some elements had not yet been discovered, and left gaps in the table
- predicted the properties of the undiscovered elements. scientists then believed him.
describe the periodic table:
elements arranged in order or atomic number and so that elements with similar properties are in columns (groups). called a periodic table because elements with similar properties occur at regular intervals. elements in the same group have the same number of electrons in their outer shells, so they have similar properties.
- elements that react to form positive ions are metals. the majority of elements are metals, found towards the bottom of the periodic table.
- elements that react to from negative ions are non-metals. these are found towards the right and top of the periodic table.
what are the differences between mendeleev’s periodic table and the modern day table?
- in the modern periodic table, elements are arranged in order of atomic number (at mendeleev’s time, protons had not yet been discovered. ordering them by atomic weight is inaccurate, due to the presence of isotopes)
- modern periodic table has group 0 (noble gases) - these were not fully discovered at mendeleev’s time.
what do the atoms in each period have in common?
all have the same number of shells
describe the noble gases:
- group 0/8.
- they’re very unreactive (they’re stable, with a full outer shell). exist as single atoms, due to their unreactivity
- not flammable
- they have very low boiling points (makes sense, they’re gases).
- the boiling points increase as you go down the group.
- colourless gases
what colours are the halogens?
- halide salts (e.g. KCl, Kbr, Ki) are colourless
- bromine water is orange
- chlorine water is colourless
- iodine water is brown
describe group 1 metals:
- alkali metals.
- group 1 metals are soft
- all group 1 metals react rapidly with oxygen, chlorine and water, and the reactions become more extreme as you go down the group.
- relatively low melting and boiling points, which decrease as you go down the group
- low density (li, k, na are less dense than water)
how do group 1 metals react with water?
creates a metal hydroxide and a gas
- as you go down the group, they become more reactive, and so produce more energy. from potassium downwards, it produces so much energy that it reacts with the hydrogen gas, forming flames
how do group 1 metals react with oxygen?
creates a metal oxide.
- at room temp, oxygen reacts with surface of the metal, forming a white oxide which covers the surface. the metal below the surface doesn’t react.
what can lithium react with oxygen to form?
lithium oxide (Li2O)
what can sodium react with oxygen to form?
- sodium oxide (Na2O)
- sodium peroxide (Na2O2)
what can potassium react with oxygen to form?
- potassium peroxide (K2O2)
- potassium superoxide (KO2)
how do group one metals react with chlorine?
produces metal chlorides.
- often form ionic compounds with non-metals as they lose an electron so easily that non-metals can pick up
- they’re then attracted together due to their opposite charges, forming an ionic compound
- these ionic compounds tend to be white solids (salts) and dissolve in water to form colourless solutions
why are elements more reactive as they move down the group?
- radius of the atoms increases - greater distance between nucleus and electron, meaning the negative electron is less attracted to the positive nucleus, and so can leave more easily.
- outer electron is repelled by other electrons (shielding), decreasing the attraction between the nucleus and the outer electron. as you move down group 1, there are more internal electron shells, meaning shielding increases.
compare metals and non-metals:
metals:
- metallic bonding: very strong, responsible for metals’ physical properties
- malleable
- good conductors of heat and electricity
- nearly all have high boiling and melting points
- shiny
- sonorous (create ringing sound when hit)
non-metals:
- dull in colour
- brittle
- low melting and boiling points (many are gaseous at room temperature)
- poor conductors of electricity
- less dense than metals
describe group 7 elements (the halogens):
- every group 7 element forms a molecule consisting of two atoms joined by a covalent bond. DIATOMIC MOLECULES
- can also form covalent bonds with other non-metals (simple molecular structures)
- melting and boiling points increase as you move down the group
- reactivity decreases as you go down the group.
- a more reactive halogen can displace a less reactive halogen
why does reactivity decrease as you go down the halogen group?
the outermost shell gets further and further away from the nucleus, so the attractive force needed to pull in another electron gets weaker. if the halogen can’t attract an electron to complete its outer shell, it can’t react
how do the halogens change their names when they form ions?
become halides
what compounds do halogens typically form?
ionic compounds with group 1 metals
describe each of the halogens:
- fluorine: poisonous yellow gas, very reactive
- chlorine: poisonous green gas, less reactive
- bromine: reddy-brown volatile liquid, poisonous
- iodine: dark grey solid that can form poisonous purple vapours. also forms an antiseptic, which has saved millions of lives
does an atom’s mass change on the moon?
no, the mass stays constant across all environments
how does an atom’s electron arrangement change when it absorbs or emits electromagnetic radiation?
absorbs electromagnetic radiation: electrons move to a higher energy level further away from the nucleus
emits electromagnetic radiation: electrons can drop to a lower energy level, closer to the nucleus
what is the relative atomic mass of an element?
average value that measures the abundance of the isotopes of the element.
describe the nature of the compounds formed when chlorine, bromine and iodine react with metals.
chlorine:
reacts vigorously with many metals to form ionic compounds, called metal chlorides. these are generally soluble in water, conduct electricity when dissolved or molten (ionic).
bromine:
forms metal bromides. properties similar to metal chlorides.
iodine:
least reactive among the halogens, reacts slowly with metals. forms metal iodides. shares some similarities with metal chlorides and bromides, but is less soluble in water.
describe the nature of the compounds formed when chlorine, bromine and iodine react with non-metals.
chlorine:
reacts with non-metals, including other halogens, to form covalent compounds. exist as diatomic molecules.
bromine:
forms covalent compounds.
iodine:
forms covalent compounds.
what are the transition metals?
metals with similar properties which are different from those of the elements in group 1. found in the centre of the periodic table
share some properties with all metals:
- conduct electricity in solid and liquid states.
- shiny when freshly cut.
most transition metals have, compared to other metals:
- higher melting points
- higher densities
- greater strength
- greater hardness
many transition metals can form multiple different ions with different charges, form coloured compounds, and are useful as catalysts.
name some examples of transition metals as catalysts:
- iron is used as a catalyst in the Haber process
- nickel is used as a catalyst in the hydrogenation of alkenes
