Download Basic chemical concepts: mixture, pure substance, atom, molecule

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Basic chemical concepts: mixture, pure substance, atom, molecule, element, compound
Review of atomic structure: electrons, protons, neutrons, mass number, atomic number
1.
Evidence for ions (conductivity and electrolysis experiments)
The conductivity of certain solutions and liquids can be explained by the movement of
charged particles (ions) between electrodes (which can be seen with coloured ions). In a solid
ions are fixed and so cannot move)
During electrolysis the atom to ion process is reversed. Electrical energy is passed through a
conducting solution (electrolyte) and positive ions (cations) migrate towards the cathode
and negative ions migrate towards the anode (positive). At the cathode cations pick up
electrons to become neutral atoms while at the anode anions lose electrons to become
neutral atoms.
2.
Electron shells / arrangement and formulas of ionic compounds
Electrons are arranged in ‘shells’ (2,8,8). The outer (valence) electrons are gained or lost
when ions are formed to give a full shell which is very stable c.f. noble gases.
Examples: NaCl, MgO. The ionic bond is the electrostatic attraction between ions of
opposite charge. Another possibility exists in which non-metals share pairs of electrons to
get a full outer shell (the covalent bond) e.g. H2, F2, N2, O2
Ionic vs. covalent compounds: Ionic compounds exist as ions, they are high melting and
conduct electricity in solution or when molten. They are made from a metal and a nonmetal. Covalent compounds melt at low temperatures and do not exist as ions so do
not conduct electricity in solution. They are made from two or more non-metals
joined together;
3.
Mass balance in chemical reactions
Chemical reactions can be represented by an equation with reactants, products and state
symbols. The law of conservation of mass stipulates that matter cannot be created or
destroyed during a chemical reaction. For this reason chemical equations must be ‘balanced’
with equal numbers of atoms on left hand side (LHS) and RHS of the equation. Students need
plenty of practice to understand the difference between coefficients, subscripts and
parentheses. Only coefficients change during the balancing of a chemical equation.
4.
Ion tests and ionic equations
Reactions between solutions of ionic compounds often give a precipitate. The formation
of the precipitate is due to strong ionic attractions, often between doubly charged ions.
Water is unable to interfere with these strong attractions and the substance is therefore
insoluble. These reactions can be used as the basis of tests for different anions and
cations.
What you should be able to do for the exam in February 2011:

(Use the ‘useful links’ on the website to practice key points)

To understand and be able to define the terms: element, atom and compound
To be able to explain (or draw) the basic structure of an atom including the
electron shell structure of all elements up to calcium (atomic number 20)
To have some quantitative idea of the size of atoms (10-10m) and explain why they
are neutral.
To be able to find atomic number, mass number, number of electrons or number
of neutrons given the other three facts.
To understand and be able to define the term ‘ion’
To be able to explain (or draw) the difference between a solid, a liquid and a
solution in terms of particles – revision
To understand that an electric current is composed of moving charges (electrons,
ions or even protons) – revision
To be able to explain the conductivity of ionic compounds in solution in terms of
the movement of the ions from which they are composed.
To be able to identify the main sources of danger in a chemical laboratory and
know how to avoid them.
To be able to explain ion formation as the consequence of elements wishing to
obtain a complete outer shell of electrons (noble gas structure is very stable)
To be able to draw the electron shell structure of simple ions given the electron
arrangement of the parent element
To be able to deduce the formula of an ionic compound from the charges of the
ions it contains
To improve a variety of manipulative skills including weighing, measuring
conductivity and systematic rinsing of equipment.
To practice using spreadsheet software to produce graphs and analyse data from
the investigation
To write a scientific report in the light of multiple hypotheses
 To understand and be able to explain which ions move to the cathode and which
to the anode during electrolysis
Be able to draw and label a simple electrolytic cell
To explain electrolysis observations in terms of ion migration
To understand that during chemical reactions new substances are formed revision
To understand that all chemical reactions can be represented by an equation
involving reactants, products and state symbols.
To understand the difference between coefficients in equations and subscripts in
formulas
To be able to balance simple chemical equations
 Be able to decide if an atom prefers to lose or gain electrons to form ions (metals
lose, non-metals gain) Group 4 elements bond by sharing electrons
Be able to find the charge on the ion of an element from its group number
Be able to work out a chemical formula of a compound from the charge on the
ions it contains (drop and swap)
To assemble a report on one ionic compound
 Develop practical skills by working on a small scale
Practice analytical skills in identifying an unknown compound
To be able to identify tests for specific ions including Cl-, CO3
2-, SO42-, Cu2+, Fe2+,Fe3+