Mendeleev arranged the elements known at the time, in a periodic table using properties of these elements and their compounds.
he could then use his table to predict the existence of other elements not yet discovered
elements are either metals or non metals depending on their place on the periodic table.
Structure of the Atom
An atom consists of a nucleus in the centre containing protons and neutrons, and electrons in shells (energy levels) surrounding the nucleus. the nucleus only takes up a minority of the atom.
atoms of a given element have the same number of protons in the nucleus (and therefore electrons) this number is unique to that element.
relative atomic masses:
The Modern Periodic Table
elements are classified as metal or non-metal according to their position in the preiodic table
the preiodic table is arranged so that the elements are arranged in order of increasing atomic number. they are also organised in rows called periods and columns called groups (similar properties).
different atoms of an elemet but with a different number of neutrons. this can affect the RAM as they may not always be whole numbers. you can claculate the RAM of an element by the relative masses and abundances of the isotopes.
the relative atomic mass of an element can be calculated by adding the fractions (as%) of each isotope:
lithium consists of 93% 6/3Li and 7% 7/3Li
therefore the RAM of lithium is-
(7/100)x6 + (93/100)x7
= 0.42 + 6.51= 6.93
there is a connection between the number of outer elctrons in the nucleus and its position in the periodic table (group1- 1 outer electron, group2- 2 outer electrons ect.)
the first shell can hold 2 electrons and the second and third can hold eight.
Noble gases are chemically unreactive because they have full outer shells of electrons, this is a very stable arrangement. all other atoms will bond together to achieve full outer shells.
ionic bonding occurs between metals and non-metals e.g sodium chloride
NA becomes NA+ the metal (a cation) and CL becomes CL- the non-metal (an anion)
Ionic compounds contain ions and you can work out the formula if you know the ions in it. the charges always balance.
contain more than one element.
Magnesium Nitrate is an ionic compound formed by Magnesium Mg+2 the cation and Nitrate NO3- the anion forming MG(NO3)2.
ide and ate compounds- end the compound name (ate shows there are oxygen atoms present).
ionic compounds have a lattice structure consisting of a regular arrangement of ions and held together by strong ionic bonds between oppositely charged ions
Ionic Compound Properties
Sodium chloride, magnesium oxide and other ionic substances do not conduct electricity as a solid but do when they are molten, which is why aluminium can be extracted from molten aluminium oxide through electrolysis. ionic substances will also conduct electricity when in an aqueous solution (dissolved in water).
two conditions must be met to conduct electricity. it must contain charged particles which are free to move, ionic compounds are only free to move when molten or in an aqueous solution.
ions have high melting and boiling points meaning they are usually solids at room temperature.
- sodium chloride (NaCl)- melting point in C- 801. boiling point in C- 1413.
- magnesium oxide (MgO)- melting point in C- 2852. boiling point in C- 3600.
Metal elements including transition metals exist as giant metallic lattice structures. there is a regular arrangement of positive ions surrounded by a sea of delocalised electrons. the bond is caused because the positive ions are attracted to the negaitive electrons, this gives the metals certain properties including:
- malleability (layers can slip over eachother)
- electrical conductors (delocalised electrons can move)
most of these metals are transition metals and therefore have high melting points and can form coloured compounds
A covalent bond is a pair of electrons shared between two atoms, it also resluts on the formation of molecules. it occurs between non-metals. you can use dot and cross diagrams to show this.
a hydrogen molecule (H2) methane (CH4)
hydrogen chloride (HCL) two pairs of shared electrons form a double bond:
Properties of Covalent Substances
- simple has low melting/boiling points (weak bonds between molecules)
- giant has high melting/boiling points (need lots of energy to break strong bonds)
simple molecular covalent substances- gases eg. hydrogen, methane, oxygen
giant molecular covalent substances- billions of atoms joined eg. diamond (hard) graphite (soft)
diamond and graphite have different properties as carbon atoms are arranged differently
- hard because joined with strong covalent bonds (used to make cutting tools)
- doesn't conduct electricty- no free electrons/charged particles to move around
- easily rubs away in layers (strong bonds within layers, weak forces between layers (lubricant)
- conducts electricity as there is an electron from each carbon atom which can move along the layers so can be used to make electrodes.
Soluble In Water:
- common sodium, pottassium, ammonium salts
- all nitrates
- most chlorides
- most sulfates
- sodium, pottasium, ammonium hydroxide
Insoluble In Water:
- silver, lead chloride
- lead, barium, calcium sulfate
- most carbonates
- most hydroxides
precipitation reactions occur when the reactants are soluble but at least one of the products is insoluble
lead nitrate + potassium iodide -> lead iodide (insoluble) + potassium nitrate
- the ions from the soluble salts have swapped
- potassium nitrate forms when potassium and nitrate ions meet
As all nitrates are soluble, the potassium nitrate does not form a precipitate
Barium Meals (barium sulfate-an insoluble salt) are given to X-Ray patients as it is
- opaque to X-Rays
- insolubilty stops it from entering blood as barium salts are toxic
Flame tests are used to test for Na+, K+, Ca2+, Cu2+ in solids or solutions
certain anions identified by precipitation tests, to test for Cl-(aq) a few drops of nitric acid are added and shaked, then a few drops of silver nitrate are added. white precipitate of silver chloride forms if it contains chloride ions.
to identify aqueous sulfate ions, SO4 2-(aq), add a few drops of hydrochloric acid to the solution and shaken, then a few drops of barium chloride solution are added. a white precipitate of barium sulfate forms if there are sulfate ions present.
when an acid is added to a substance containing carbonate ions, CO3 2-, carbon dioxide is given off, and then passed through limewater, which will turn milky if carbon dioxide is present.
scientists use spectroscopy to detect the presence of very small amounts of elements, this has led to the discovery of new elements including rubidium and caesium.
Miscible or Immiscible
immiscible liquids- liquids that do not mix completely with each other
miscible liquids- two liquids that dissolve in eachother
immiscible liquids are separated using a separating funnel (lower liquid runs out of funnel first)
miscible liquids are separated using fractional distillation, eg. the fractional distillation of liquid air to produce nitrogen and oxygen
paper chromatography can be used to separate and identify components of mixtures, including colouring agents and foodstuffs
the paper with the separated components on it is called a chromatogram
the Rf value is the distance the compound has risen divided by the distance the solvent has risen
Rf = distance compound has risen
distance solvent has risen
chromatography can be used to separate and identify food colourings, helping to keep food safe. the police also use it to analyse blood samples to work out if suspects have been at a crime scene
Alkali Metals (Group 1)
alkali metals are found in group one of the periodic table as they all have one electron in their outer shell, are solids at room temperature but have low melting points compared to other metals. they are all soft and the atoms are held together by metallic bonding.
if you react lithium, sodium or potassium with water, they form alkaline hydroxides and hydrogen gas.
the reactivity of the alkali metals increases as you go down the group as they are more unstable.. the elements at the bottom of the group have more electrons/electron shells so the outer electrons are further from the nucleus . so there is less of a magnetic pull. eg. Caesium is more reactive than Lithium
Halogens (Group 7)
Fluorine and Chlorine are gases
Bromine is a brown liquid at room temperature
Iodine is a grey solid
they become less reactive as you go down the group
each halogen needs one more electron to complete the outer shell and metals need to get rid of electrons to have a full outer shell, therefore they react, forming metal halides.
they form ions with a charge of -1
halogens react with hydrogen to produce hydrogen halides, these dissolve in water to form acidic solutions.
exothermic change/reaction- heat energy is given out
endothermic change/reaction- heat energy is taken in
the breaking of bonds is endothermic and the making of bonds is exothermic
the overall heat energy change for a reaction is:
- exothermic if more heat energy is released making bonds in the products than is required to break bonds in the reactants
- endothermic if less heat energy is released making bonds in the products than is required to break bonds in the reactants.
Rates of Reaction
the rates of chemical reactions vary from very fast, explosive reactions to very slow reactions
changes in temperature, concentration and surface area of a solid have effects on the rate of reaction.
reactions can occur when particles collide and the rate of reaction can be increased by increasing the frequency/enrgy of collisions.
the more concentrated the solution, the more solute particles there are in a given volume. meaning it is more likely that reactant particles will collide with eachother, and the more collisions, the higher the rate of reaction.
the higher the temperature, the faster the particles move, meaning they collide with more energy and are more likely to react, they also collide more frequently, increasing the temperature increases the rate of reaction.
not all collisions lead to a reaction, especially if particles collide with low energy
catalysts are substances that speed up the rate of reaction without being used up in the reactions.
catalytic converters in cars:
- have a high surface area to increase the rate of reaction of carbon monoxide and unburnt fuel from exhaust gases with oxygen in the air to produce carbon dioxide and water
- work best at high temperatures.
the yield is the mass of the product obtained in a reaction
the theoretical yield is often more than the actual yield obtained in the reaction as it is a calculation.
precentage yield = actual yield x100
reactions do not give the theoretical yield due to:
- incomplete reactions
- practical losses due to the preparation
- competing, unwanted reactions
Waste and Profit
many reactions produce waste products which:
- are not commercially useful
- can present economic, environmental and social problems with disposal
chemists in industry work to find the economically most favourable reactions where:
- the percentage yield is high
- all the products of th reaction are commercially useful
- the reaction occurs at a suitable speed