Sheet 5.4 Ions - Science for the NZ Curriculum

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Sheet 5.4 Revision Ions
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Sheet 5.4 Revision
Ions
Atomic structure
The model of the atom used most frequently today consists of two parts: a small, positively
charged nucleus and the orbiting negatively charged electrons. The nucleus is made up of two
types of sub-atomic particles called neutrons and protons. The nucleus contains nearly all the
mass of the atom and all of the positive charge. The protons are positively charged and the
neutrons have no charge (are neutral). Protons and neutrons have a similar mass, with the
neutron having very slightly more mass. Electrons have a negative charge. They orbit the
nucleus at a great distance (if the nucleus was as large as an orange, the electrons would be
orbiting over 20 km away). Unlike protons, electrons can leave an atom or join another atom.
An atom with an unequal number of protons and electrons is called an ion. Electrons are
responsible for creating the bonds between atoms that form chemical compounds.
The atoms of each element can be represented by a symbol (letters) and two numbers.
The symbol describes the name of the element. Note that not all symbols have the same
letters as the name of the element. This is because these elements were known by different
names in the past, or the letters represent the name of the element in a language other than
English. For example, Au is the symbol for gold. The Latin word for gold is aurum.
Similarly, copper has the symbol Cu. This is from the Latin word cuprum.
Atoms can lose or gain electrons
Atoms are electrically neutral. This means they contain an equal amount of positive and
negative charge (they have an equal number of positively charged protons and an equal
number of negatively charged electrons). Many atoms, however, have a tendency to either
lose or gain electrons. When they do this they are no longer neutral - they become ions.
Figure 11.36: A Lithium atom losing an electron, an oxygen
– atom gaining two electrons
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
Sheet 5.4 Revision Ions
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Metals have a tendency to lose electrons. This means they will then have a greater
number of protons than electrons, so the ions formed will have an overall positive charge.
Non-metals tend to gain electrons, forming negatively charged ions.
The number of electrons lost or gained by an atom is determined by the number of
electrons in the outer shell of an atom. Shells that are completely full or empty are more
stable. For example: • Lithium has one electron in its outer shell. To achieve an outer shell
that is full it can gain seven more electrons or donate the one outer electron. Lithium always
does the latter, forming a lithium ion Li+.
• Oxygen has six electrons in its outer shell. To achieve an outer shell that is full it gains two
more electrons or donates the outer six. Oxygen always tries to gain two electrons, forming
an oxygen ion O2+.
• When oxygen and lithium react, electrons are transferred from two lithium atoms to a
single oxygen atom. In this way lithium oxide (Li2O) is formed.
Note that charge numbers are written in superscript, whereas molecular formulas use
subscript to denote numbers of atoms making up one molecule in a compound.
Negative ions tend to have special names. For example, peroxide (O-), oxide (O2-) and
chloride (Cl-). Molecules can be ionic too. For example, OH- (hydroxide), SO4 2- (sulfate) and
NH4 + (ammonium).
The names of the negative ions can be confusing, but there are some simple rules:
• If a negative ionic molecule contains oxygen, the name ends in –ate (e.g. SO4 2- = sulfate)
• If a negative ionic molecule contains only one type of element, the name ends in -ide (e.g.
S2- = sulfide).
An exception to this is OH- (hydroxide).
Ionic bonds
When an ionic bond is made, one atom loses electrons to another. This means they are no
longer neutral atoms, but oppositely charged ions. Opposite charges attract, so a new
compound is formed as the ions are attracted to one another. Because they are held together
by electrical charge, ionic compounds can be separated using electricity. Separation of ions
by this method is called electrolysis.
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
Sheet 5.4 Revision Ions
3
Figure 11.38: Separation of ions in copper chloride
by electrolysis
cation A positively charged ion that is attracted to the negatively charged cathode in electrolysis
cathode A negatively charged electrode
anion A negatively charged ion that is attracted to the positively charged anode in electrolysis
anode A positively charged electrode
Positive ions are called cations – they are attracted to the negative cathode during
electrolysis. Similarly, negative ions are called anions – they are attracted to the positive
anode during
electrolysis. In the example shown in Figure 11.38, copper metal can be seen
forming on the negative cathode, and bubbles of chlorine gas can be seen forming on the
positive anode.
Ions tend to form compounds with no overall charge. For example, sodium and chlorine
react to form sodium chloride (table salt). Sodium forms ions with a charge of 1+ and
chlorine forms ions with a charge of 1-. So one ion of sodium and one ion of chlorine will
have an overall charge of zero, giving the molecular formula NaCl.
Similarly, magnesium and chlorine react to form magnesium chloride. Magnesium forms
ions with a charge of 2+, so two Cl- ions are needed to balance this. The molecular formula
for magnesium chloride is MgCl2.
Since there are only two parts of an ionic compound, one positive and one negative, it is very
straightforward to work out the molecular formula of their molecules. Remember that the
molecule must have an overall charge of zero.
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
Sheet 5.4 Revision Ions
4
Example 3
Step 1: Identify the positive and negative ions in the compound.
e.g. Magnesium chloride is made from positive magnesium ions (Mg2+) and negative
chloride ions (Cl-).
Step 2: Add more negative or positive ions so that the overall charge is zero.
e.g. Mg2+ requires two negative charges to balance, so two Cl- ions will be needed.
Step 3: Write the molecular formula for the molecule using subscripts to show if there is
more than one.
e.g. MgCl2 (there is no need to put a 1 by magnesium)
This technique works for all ionic molecules, but be careful with complex ions such as nitrate
(NO3 -). In these instances treat the whole molecule as one ion (it sometimes helps to put
brackets around it so you know to treat everything as one ion). When a compound contains
more than one complex ion, put brackets around the ion and the subscript number and
indicate how many of these ions there are with another subscript number on the outside of the
brackets.
Example 4
Step 1: Identify the positive and negative ions in the compound.
e.g. Magnesium nitrate contains Mg2+ and (NO3-) ions.
Step 2: Add more negative or positive ions so that the overall charge is zero.
e.g. Mg2+ requires two negative charges to balance, so two (NO3-) ions
will be needed.
Step 3: Write the molecular formula for the molecule using subscripts to show how
many of each ion there are.
e.g. Mg(NO3)2
Activity 11.17: the ion jigsaw
Making the jigsaw pieces
1 Use the table of ions from Activity 11.16 to make some ion jigsaw pieces. Make
single ions 2 cm × 4 cm and double ions 4 cm × 4 cm (see diagram below).
2 Label each piece with the name of the ion, the symbol and charge.
3 Colour positive ions red and negative ions blue.
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
Sheet 5.4 Revision Ions
4 Make three of each type of ion.
Using the jigsaw pieces
1 Form ionic compounds by placing positive and negative jigsaw pieces together.
2 Each jigsaw must have positive ions on the left and negative ions on the right.
3 Each complete ionic compound must form a rectangle.
4 Write the molecular formula of the compound you have created by writing the number of
pieces of each ion type in subscript next to the symbol for the ion
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
5
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