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
define an isotope
an atom of the same element with a different number of neutrons
describe the nuclear model of the atom
- the nucleus is in the centre of the atom, containing protons and neutrons
- protons are positively charged and neutrons have no charge (neutral) so the overall charge of the nucleus is positive
- electrons orbit the nucleus in distinct shells/energy levels
- electrons are negatively charged
define relative atomic mass
an average value that takes account of the abundance of the isotopes of the element
how to calculate number of neutrons in the nucleus
mass number (number on top) - atomic number (number on bottom)
why does the mass number of the atom not take into account electrons
the mass of the atom is concentrated in the nucleus - electrons have negligible mass
describe how the electrons are structured
the electrons in an atom occupy the lowest available energy levels (innermost available shells). the maximum number of electrons each shell can hold is 2,8,8
how can atoms become stable
by having a full outer shell
how are elements in the periodic table arranged
in order of atomic (proton) number and so that elements with similar properties are in columns (groups)
what is in between groups 2 and 3 on the periodic table
transition metals
why is it called the periodic table
because similar properties occur at regular intervals
describe the relationship between elements in the same periodic table group
they have the same number of electrons in their outer shell, and this gives them similar chemical properties and similar reactions
what happened before the discovery of protons, neutrons and electrons
scientists attempted to classify the elements by arranging them in order of their atomic weights
what was wrong with early periodic tables
they were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed (groups where they shared different properties), so these tables were not accepted
what did Mendeleev do at the start
he developed the first modern periodic table and overcame some of the problems of early periodic tables; Mendeleev started by arranging all elements in order of increasing atomic weight
the things that Mendeleev did that no one had done before
- he left gaps in is periodic table for elements that he thought had not been discovered; this allowed Mendeleev to predict the undiscovered elements’ properties
- he changed the order of specific elements
the things that Mendeleev did that no one had done before
- he left gaps in is periodic table for elements that he thought had not been discovered; this allowed Mendeleev to predict the undiscovered elements’ properties
- he changed the order of specific elements so that they were grouped according to their properties, rather than strictly following the atomic weight order
what caused Mendeleev’s periodic table to be accepted
because he left gaps in the table which allowed him to predict the undiscovered elements’ properties, when these elements were discovered to have the same properties he predicted, his ideas were confirmed and his table was widely accepted
what did the discovery of isotopes confirm
they explained why the order based on atomic weights was not always correct; this is because isotopes of the same element have different masses but the same chemical properties so occupy the same position on the periodic table but can appear too heavy or too light, distorting the order of atomic weights. this confirmed Mendeleev was correct
differences between the modern periodic table and Mendeleev’s
- it is ordered in order of atomic number rather than atomic weight (protons had not been discovered when Mendeleev made his table)
- the modern periodic table has group 0, the noble gases; these were not fully discovered when Mendeleev published his table
why is relative atomic mass used when referring to elements on the periodic table
because elements can have several isotopes, hence why some mass numbers are not whole numbers
what do periods (rows) tell you in the periodic table
how many electron shells the element has
define an ion
an atom formed by the loss or gain of electrons, which is now charged
what happens when metals react
they lose electrons to achieve a full outer shell and become stable; this los of electrons means that metals react to form positive ions because there are more protons than electrons when an electron is lost
why do metals lose electrons to form ions
because they have very few electrons in their outer shell so it is easier to lose electrons to gain a stable electronic structure (full outer shell) than gaining electrons; it requires less energy to lose than gain
what forms positive and what forms negative ions
positive ions -> metals
negative ions -> non-metals
the majority of elements are [..]
metals
properties of metals
- strong
- can be bent and hammered into different shapes (malleable)
- great conductors of heat and electricity
- high melting and boiling points so usually solid at room temperature (except mercury)
properties of non-metals in comparison to metals
- duller and more brittle than metals
- lower boiling and melting points so are sometimes gases at room temperature
- poor conductors of heat and electricity
- often have a lower density than metals
why are group 0 gases very unreactive
because they have a full outer shell of electrons (except helium which has 2) so they do not need to gain or lose any electrons via a reaction, because they’re already stable
why are noble gases used in lightbulbs
because of their stable electronic structure, they’re unreactive, meaning they’re good for use in light bulbs because they’re non-flammable so are less likely to ignite in the heat
what happens as you go down group 0
relative atomic mass increases. as this increases, the boiling points increase
group 1 aka
group 0 aka
group 7 aka
1 - alkali metals
0 - noble gases
7 - halogens
describe how group 1 elements react with oxygen
they react rapidly with oxygen in oxidation reactions whereby metal oxides are formed e.g. lithium + oxygen -> lithium oxide
describe how group 1 elements react with chlorine
they react rapidly with chlorine to form metal chlorides e.g. lithium + chlorine -> lithium chloride
describe how group 1 elements react with water and what you would see when adding UI
they react rapidly and vigorously with water to form metal hydroxides and hydrogen; there is effervescence as hydrogen is produced
since metal hydroxides are alkali, if we placed UI in the reaction between alkali metals (and any metal, in fact) and water, it would go from green (pH7) top purple (alkaline pH>7)
reactivity with reacting alkali metals with water
the further down group 1 (the more reactive), the more violent the reaction; hence francium + water is very explosive, but sodium + water is only a little bit of fizzing
how should alkali metals be handles and why
they should be stored in oil and handled with forceps, because alkali metals are very reactive
why do groups have similar reactions
because they all have the same number of electrons in their outer shell
why does the reactivity of alkali metals increase as you go down the group
as you move down group 1, the outermost electron is further away from the nucleus, because the atoms get larger and have more shells. this means that there are weaker electrostatic forces of attraction between the negative outer electron and the positive nucleus, so it is easier to lose this electron during a reaction; hence reactivity increases
what things weaken electrostatic forces of attraction
- the shielding effect from the internal energy levels
- increased distance of electrons from the nucleus
what decreases/increases as you go down group 1
- reactivity increases
- melting and boiling points decrease
- increased relative atomic mass
what decreases/increases as you go down group 0
- reactivity stays the same
- melting and boiling points increase
- increased relative atomic mass
what are halogens
non-metals that consist of molecules made of pairs of atoms; they have coloured vapours
why do all halogens form diatomic molecules
each atom needs to gain 1 electron to have a stable electronic configuration, so if it bonds to another atom by a covalent bond, both atoms in the molecule become stable
how does boiling and melting point increase as you go down group 7
- the relative formula mass of the molecules increases
- due to increased relative formula mass, the molecules get larger meaning there are stronger intermolecular forces which require more energy to overcome
- because of this, they have increasing melting and boiling points as you go down the group (hence why top few halogens are gases but bottom few are solid)
only exception to the rule of ‘melting and boiling points increase as you go down the group’
group 1 - alkali metals
how does reactivity decrease as you go down group 7
as you go down the group, the outer shell of electrons gets further away from the nucleus. this increased distance, together with the increased shielding effect of inner shells, weakens the electrostatic forces of attraction between the positive nucleus and the outer shell, so it is harder to gain electrons in reactions as you go down the group
what happens when you put two different halogens in an aqueous solution
the more reactive halogen can displace the less reactive halogen from an aqueous solution of its salt e.g. with sodium bromide + fluorine, fluorine is more reactive than bromine, so will displace it
what decreases/increases as you go down group 7
- reactivity decreases
- boiling and melting point increase
- relative atomic mass increases
what are the transition metals
metals with similar properties which are different from those of the elements in group 1
compare melting points of transition metals vs alkali metals
group 1 metals have relatively low melting points, whilst transition metals have relatively high boiling points (apart from mercury)
compare densities of transition metals vs alkali metals
group 1 metals have a relatively low density (Li, Na and K are less dense than water) whereas transition metals have a high density
compare strength of transition metals vs alkali metals
group 1 metals are soft and can be cut with a knife, whereas transition metals are hard and strong e.g. iron
compare reactivity with oxygen, halogens and water of transition metals vs alkali metals
group 1 metals all react very rapidly with oxygen halogens and water, whereas transition metals are much less reactive than group 1 metals
compare ions of transition metals vs alkali metals
transition metals can form ions with different charges e.g., iron can form Fe2+ or Fe3+, but group 1 metals can only form 1+ ions
what are transition elements like and how can they be used
they have ions with different charges; they can form coloured compounds; they are useful as catalysts
