IGCSE Double Award Extended Coordinated Science

Transcription

1 IGCSE Double Award Extended Coordinated Science Chemistry Ions and Ionic Bonds Ions You need to know what ions are and how they can be formed. An ion is a charged atom, or a molecule - Caused by loss or gain of electrons - This causes the proton number electron number An ion is formed so that atoms will lose or gain electrons to form a full valence shell (full outer shell) - Full valence shell provides stability to the atom, so this is why they do it. There are two types of ions, a cation and an anion. - Cation, pronounced cat-ion - Is a positively charged ion, formed by loss of electrons - Anion, pronounced ann-ion - Is a negatively charged ion, formed by gain of electrons - Remember: cat s are awesome, so they are positive. - So a cat ion is positive. For example, take the sodium atom s formation of sodium ion (cation). - Sodium atom has 1 electron in its 3rd shell. - It can either: - Gain 7 electrons to form a full 3rd valence shell or - Lose 1 electron to form a full 2nd valence shell (there are no electrons in the 3rd shell) - Losing one electron is easier than gaining 7 electrons, so the sodium atom does just that. - Now that it has lost one electron, it has 11 protons and 10 electrons. - The charges do not balance out - There is one more proton, so the overall charge of the sodium ion will be 1+ - The charge is written as a superscript, like so: Na +

2 We can also look at an anion formation, like for oxygen. - Oxygen has 6 electrons in its 2nd shell (valence shell) - Since it is easier for the atom to gain 2 electrons rather than lose 6 electrons, - It will gain 2 electrons - Now it has 8 protons and 10 electrons - Charges do not balance out, since there are two more electrons - So the overall charge of the oxide ion is 2- - If the charge is more than one, the number comes before the negative/positive sign - Forming O 2-, the oxide ion. You need to know some rules of ion formation - Since all metals have low numbers of electrons in their valence shell (groups I, II, III) - All metals form cations by losing electrons to form a full valence shell [ e.g. Na +, Mg 2+, Al 3+ ] - And all metal ions are named by adding the word ion after the metal name - Sodium ion, magnesium ion, aluminium ion etc. - Since all nonmetals have high numbers of electrons in their valence shell, (groups IV, V, VI, VII) - All nonmetals form anions by gaining electrons to form a full valence shell [ e.g. Cl -, O 2-, N 3- ] - Nonmetal ions are not named simply by adding the word ion - The name ending changes to -ide - Chlorine atom -> chloride ion - Oxygen atom -> oxide ion - Nitrogen atom -> nitride ion - Group VIII, or 0, called the noble gases, do not form ions - They already have a full valence shell and do not need to lose or gain any electrons - So their stable atom is their natural state, where it is neutral. - it is logical to notice that all the elements in the same group form ions with same charges - All group I metals form ions with charge 1+ - All group VII nonmetals form ions with charge 1- You need to know that since these ions are electrically charged, - Like charges will repel : so cations will repel cations, anions will repel anions - Opposite charges will attract : so cations and anions will be attracted to each other

3 Ionic Bonding You need to know what ionic bonding is. Ionic bond is the bonding between a metal and a nonmetal formed by the transfer electrons to form ions. To form an ion, a metal has to lose electrons and a nonmetal has to gain electrons - So in ionic bonding, the metal s electrons are transferred to the nonmetal - This forms ions of both, a cation and an anion - The opposite charged will electrostatically attract each other - This attractive force is the ionic bond - The force causes the two element to chemically join and become a compound - In this case, the formed compound is Sodium Chloride (takes the name of the ions) You need to understand dot and cross diagrams. Dot and cross diagrams are used to show the movement of electrons when elements bond. - For simplicity, only the valence shell is drawn. - One element s electrons are drawn as crosses, the other element s electrons are drawn as circles - This is to distinguish where they came from - The elements electron arrangements for before the transfer of electrons and after is drawn - The movement of electron is shown by an arrow For the case of the ionic bonding in sodium chloride, one electron from sodium goes to chlorine. - Sometimes it may not be this simple

4 - Taking magnesium chloride as an example, - Magnesium is in group II, so it needs to lose two electrons to be stable - Chlorine is in group VII, so it only needs one electron to be stable. - In this case, two chlorine atoms receive one electron each from the one magnesium atom. - In a dot-and-cross diagram, it will look something like this: - From this we can determine the chemical formula of magnesium chloride - For magnesium ion, the bond needs two chloride molecules, so it is MgCl 2 - Using the same idea, we can look at compounds like - potassium oxide, K 2 O - sodium phosphide Na 3 P Ionic Lattice You need to know what ionic lattices are. Ionic bonding occurs between lots of ions in a huge structure called the giant ionic lattice They do not exist in simple molecules (one of each ion) - But in huge numbers of cations and anions forming a continuous and regular structure by attraction - There is no way of finding out the total number of each ion, - So we can write the ratio between the two, called the empirical formula - Sodium chloride is one to one, so NaCl - Magnesium chloride is one to two, so MgCl 2 - And so on

5 The syllabus says you should be able to, (SO check if you can): - Describe the formation of ions by electron loss or gain. - Describe the formation of ionic bonds between metals and non-metals as exemplified by elements from Groups I and VII. - Explain the formation of ionic bonds between metallic and non-metallic elements. - Describe the lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions, exemplified by the sodium chloride structure.