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Physical Change vs Chemical Change
Physical Change vs Chemical Change
In our daily life we encounter many changes in the materials that surround us such as metals rusting or food being cooked. All these changes can be described as either a chemical or physical changes.
A physical change is a change that affects the physical traits of a substance without changing the internal structure of that substance. e.g. cutting wood. When you cut wood you take a large piece and split it into many smaller pieces. However you do not change the fact that it is wood. Physical changes are often easily reversible.
A chemical change is a change that affects the internal structure of that substance. It always involves a chemical reaction. e.g. wood burning. When you burn wood it changes from wood to charcoal. This change is in the chemical nature of the wood and results in it having different physical and chemical traits.
Chemical vs Physical Change - Question PDF
Chemical vs Physical Change - Experiment PDF
Chemical vs Physical Change - Experiment 2 PDF
Chemical changes are always accompanied with a chemical reaction. This is normally shown by the production of a gas (fizzing), a change in colour, heat given off, and a variety of other factors. This is because in a chemical reaction new substances are formed. For example, when Octane burns in your car engine it reacts with oxygen gas to produce carbon dioxide and water. This chemical reaction can be represented by what we call a word equation. On the left side of a word equation are the reactants, these are the chemicals that react together. On the right side are the products, these are the things produced in the reaction.
The word equation for this reaction would be:
Octane + Oxygen gas ⟶ Carbon Dioxide + Water
Octane + Oxygen gas ⟶ Carbon Dioxide + Water
Have some practice with writing word equations.
Checkpoint
Checkpoint
physical changes do not result in a new chemical, they are changes in the physical properties of an object like shape, size.
chemical changes make new chemicals and result in a change of chemical properties.
Word equations have the reactants (what is reacted together) on the left side and the products (what is produced after) on the right side. An arrow shows the direction of the reaction.
Practice some simple questions to check you know the basics here!
Atoms and the Periodic Table
Atoms and the Periodic Table
All matter is made up of atoms. Atoms are the smallest whole unit of matter. Just like cells are the building blocks of living things, atoms are the building blocks of matter. A substance made entirely of the same type of atom is called an element. For example, a diamond is made entirely of carbon atoms. There are just over 100 different elements known but they make up the millions of different things in the world. The elements are arranged by scientists onto a table known as the Periodic Table.
Every element on the periodic table has a name (which can be different in different languages) and a symbol of one or two letters (which is the same in every language). Although the symbol is often based on the English name e.g. O for oxygen, sometimes it is based on another language. If you were a student in Germany, Na for natrium, which we call sodium, makes perfect sense but O seems to be a strange symbol for the element you call sauerstoff. The first 20 Elements are shown below. You will need to know the symbols and names of these elements.
All atoms are made up of protons, neutrons and electrons. Each element has a unique number of protons that cannot be changed. This number is called the Atomic Number and is the smallest number displayed besides the element on the periodic table. The number of neutrons and electrons can change without changing the element. Chemical reactions only ever involve changing the number of electrons. Reactions that change the number of protons or neutrons are called Nuclear reactions because both the protons and neutrons are found in the nucleus of the atom (the very middle). For example, when certain types of solar radiation hit nitrogen atoms in the atmosphere they change a proton into a neutron. This decreases the number of protons by one which turns the nitrogen atom into a carbon atom. The electrons fly around the nucleus in what we call shells or orbitals.
In a neutral atom (an atom that has not lost or gained any electrons) the number of electrons is always equal to the number of protons (the Atomic number). The other number given is called the Atomic Mass (sometimes called the Mass Number). This number tells us the number of nucleons the atom has. Nucleons are things found in the nucleus so this number tells us how many protons and neutrons there are.
Lithium has an atomic number of 3 and an atomic mass of 7. This means Lithium has 3 protons and 3 electrons and 7 nucleons in total. Since we know 3 of the 7 nucleons are protons that means there are 4 neutrons. So Lithium has 3 protons, 3 electrons, and 4 neutrons. The protons and neutrons are all inside the nucleus in the middle of the atom. However as said before the electrons are in shells around the outside. These shells have specific configurations that they allow which is why we call it the periodic table.
Each shell can take a maximum number of electrons and they fill up from the first (innermost) shell outwards. The outermost shell is called the valence shell. The first shell can only take up to 2 electrons. The second and third shell can take up to 8 each. The fourth shell can take up to 18.
Going back to our example of Lithium, it has 3 electrons so 2 of the 3 go in the first shell filling it up fully. The last and final electron goes into the second shell. The atomic structure of Lithium can be seen to the left. You will need to be able to draw these diagrams for the first 20 elements.
Here is a draw on exercise for some of the elements. Draw the correct number of electrons for each element in the correct configuration.
Checkpoint
Checkpoint
Atoms contain protons (positive) and neutrons (neutral) in the nucleus while electrons (negative) orbit around the nucleus.
The atomic number states the number of protons and electrons an atom of that element has.
The atomic mass states the number of things in that atoms nucleus (protons + neutrons). You can calculate the number of neutrons by substracting the atomic number from the atomic mass.
Electrons orbit the atom in electron shells. Up to 2 electrons can orbit in the first shell, 8 in the second, and 8 in the third.
Test your atomic knowledge with this quiz.
Ions
Ions
Elements form ions to increase the stability of their outermost (valence) shell. A valence shell is the most stable when it is either completely full or completely empty. Atoms can gain or lose electrons (by giving them away to other atoms or taking them from other atoms) to get to this state.
Lithium Atom (Li) 2,1
Lets go back and look at Lithium again. With 3 electrons Lithium will have 2 in the first shell and one electron in the second shell (shown on the left). Now to complete the outer shell Lithium would have to gain 7 electrons. However it could also lose its outermost electron making that shell completely empty and this is what Lithium does (shown on the right). This makes it have 3 protons and 2 electrons. Since protons carry positive charge and electrons carry negative charge the loss of one electron makes the Lithium ion positively charged. We label this Li+ .
Lithium Ion (Li+ ) 2,
Fluorine Atom (F) 2,7
Consider Fluorine with an electron configuration of 2,7 (on the left). It has 7 valence electrons. There are two possible routes to stability: gain 1 electrons to become 2, 8 or lose 7 to become 2. Fluorine could do either, but it is incredibly hard to lose 7 electrons. So fluorine instead gains one electron.
Since it has gained one electron it now has 10 electrons in total but only 9 protons. This means it has one extra negative charge and so becomes the F- ion to indicate it has a charge of -1 (on the right).
Fluorine Ion (F- ) 2,8
The names of the positive ions formed from single atoms are the same as the names of the metals (and hydrogen) that form them. Negative ions can form from single atoms, in which case we put "ide" on the end of the element name: oxide, sulfide, chloride etc.
Other negative ions are formed by special, multiple atom groups called polyatomic ions. Although there are lots of different ones of these, the only ones of these you need to know about contain oxygen and we distinguish them by putting "ate" on the end of their name: sulfate, carbonate, nitrate.
There is only one positive polyatomic ion you need to know about, formed by four hydrogen and one nitrogen atom. It is called the ammonium ion. Below is a table of all the ions you need to know. We organise this table by grouping the ions together based on their net charge. Note that the table you are given in the exams DOES NOT have the names of the ions on, so you need to memorise these to be able to write names or formula. It will look something like the one below:
Ionic compounds are made up of atoms that have become ions together. Lets take the example of Lithium and Fluorine. Lithium would like to lose one electron to become stable. Fluorine would like to gain one electron to become stable. When they are mixed together the lithium gives up its one electron to the fluorine. However this leaves them negatively and positively charged. This charge is what creates the ionic bond holding them together. Opposite charges attract so the positively charged Lithium is attracted to the negatively charge Fluorine. Together they would form the ionic compound Lithium Fluoride (we call the Fluorine ion Fluoride).
Naming Compounds Practice - PDF Naming Compounds Practice - Google Doc
The image to the right shows how ionic bonds are formed. Sodium (Na) gives electrons to empty its valence shell to the Fluorine to fill its valence shell. This makes the Sodium a positive ion and the Fluorine a negative ion. They are then attracted to each other and form an ionic bond holding them together. We would write the symbol for this equation as NaF.
The positive cation always is written first. Then follows the negative anion.
However, some atoms lose or gain more than one electron for example Calcium forms the ion Ca2+ by losing two electrons. Now obviously Calcium can lose these two electrons to any atom that would like to gain 2 electrons such as oxygen. This would form Calcium Oxide or CaO. But Calcium can also form ionic bonds with ions that do not gain 2 electrons. Calcium Chloride is formed when calcium loses its two electrons and gives them to two chlorine atoms. Each chlorine atoms gains one electron makes a full outer shell. These three ions bond together the same way. This would have the formula CaCl2 as it contains 2 chloride ions to every 1 calcium ion.
These ionic bonds form all around the atoms. Positive ions are surrounded by negative ions and vice versa. This creates a structure which is called a Lattice.
Ionic Formula Practice - PDF Ionic Formula Practice - Google Doc
Ionic Formula Mistakes - PDF Ionic Formula Mistakes - Google Doc
Naming and Formula - PDF Naming and Formula - Google Doc
Checkpoint
Checkpoint
Ions are atoms that have gained or lost electrons. A gain in electrons makes a negative ion (anion) while a lose of electrons makes a positive ion (cation).
Ions gain or lose electrons to make their outer electron shell either full or empty. e.g. an atom with 2,2 would lose electrons 2 electrons to become 2,0. An atom that is 2,6 would gain 2 electrons to become 2,8.
Ions bond together to form ionic compounds. We write the formula of the compound as the simplest ratio of ions that can make it. E.g. Zn (Zinc) is a 2+ ion, Cl (chloride) is a 1- ion. To balance the charges we need 2 chloride ions for every 1 zinc ion. The formula would therefore be ZnCl2
Sometimes we need to use brackets in formula. When ions have multiple different elements in them like hydroxide which is OH- we use brackets when we need multiple hydroxide ions. e.g. Zinc hydroxide would be Zn(OH)2 as if we wrote ZnOH2 this only indicates we need 2 H not 2 of the whole hydroxide ion.
Test your knowledge of ions using this quiz!
Chemical Reactions
Chemical Reactions
We have looked previously at writing word equations for chemical reactions. In this section we will focus on writing the symbol equations instead of the word equations. It is easiest to write the symbol equation with a word equation done. Lets start with a reaction you would be familiar with. When Magnesium burns it is reacting with oxygen in the air to produce an ionic compound Magnesium Oxide.
The Word Equation for this would be:
Magnesium + Oxygen Gas ⟶ Magnesium Oxide
Now for the symbols. Magnesium has the chemical symbol Mg. Oxygen has the chemical symbol O but because it is Oxygen gas it is actually O2 . These combine to give us Magnesium Oxide. The magnesium ion is 2+ as it loses two electrons. These two electrons are taken by the oxygen to become the oxide ion 2-. Since Magnesium loses 2 electrons and Oxygen gains 2 electrons they bond at a 1:1 ratio. Therefore the formula for Magnesium Oxide is MgO.
This gives us the symbol equation:
Mg + O2 ⟶ MgO
Word and Symbol Equation Practice
However, this equation needs some work. We can see each side of the equation has one magnesium but there are two oxygen atoms on the left side of the equation and only one oxygen atom on the right. We need to make this equation make sense as you cannot some how lose an oxygen atom. This is called balancing the equation. To balance the equation you place numbers at the front of the symbols to show more than one of this whole ingredient is reacting or produced. Lets first of all try to balance the oxygen.
Mg + O2 ⟶ 2MgO
By placing the two infront of the MgO I have balanced the Oxygen atoms as there are now 2 on each side. However this has unbalanced the magnesium atoms as we start with one but somehow end up with two Magnesium atoms. We now need to balance the Magnesium atoms.
2Mg + O2 ⟶ 2MgO
This equation is now balanced as we have equal amounts of each element on both sides of the equation.
Here are some practice questions on balancing symbol equations.
Patterns of Reactivity
Patterns of Reactivity
Reactivity refers to how reactive an element is e.g. how easily it burns or carries out other chemical reactions. There are patterns to this reactivity, although they are only simple for the alkali metals and the halogens.
Reactivity in the alkali metals increases downwards, as the video above demonstrates.
Reactivity of the halogens (fluorine, chlorine, bromine, iodine) decreases downwards.
The reason for this has to do with the fact that electrons are negatve. In the alkali metals, it becomes increasingly easier to 'kick off' the electron in the valence shell the further it is from the nucleus. The halogens have the opposite problem: they need to attract electrons, and this becomes increasingly difficult as the valence shell is further away from the positive (and therefore attractive) nucleus.
The patterns of reactivity are due to the similar electron arrangements of the elements in the same period. The Alkali Metals for example all have electron arrangements that result in one valence electron.
Li - 2,1 Na - 2,8,1 K - 2,8,8,1
This means they all form 1+ ions as they all lose one electron to become stable. This causes the pattern of reactivity for the alkali metals
Specific Learning Outcomes
Specific Learning Outcomes
By the end of this unit you should be able to:
Describe the difference between chemical change and physical change and be able to identify changes as chemical or physical
Identify reactants and products in a chemical reaction. Construct simple word equations to describe chemical reactions
Describe possible changes that indicate a chemical reaction
Know that a particular chemical reaction may only occur under certain conditions, such as in aqueous solution (in water) or with heat, light or electricity to start them off or keep them going.
Describe the structure of the atom (protons, neutrons and electrons) including electron arrangements of neutral atoms of atomic number 1-20
Explain how atoms can form ions in terms of gain or loss of electrons and identify the charge of ions
Relate the formation of simple ions to the electron arrangement of the atom which forms it
Explain the arrangement of the periodic table (rows, columns) terms of atomic number, number of electrons and atomic mass
Explain the atomic structure of carbon
Describe covalent bonding and explain why carbon forms so many covalent compounds
Describe trends in relative reactivity of the alkali metals (group 1) and the halogens (group 7/17) and be able to explain why these exist
Explain why metals form positive ions and non-metals form negative ions
Describe what an ionic bond is and explain why it works
Be able to correctly work out chemical formula of simple compounds
Be able to construct balanced chemical equations from word equations
Be able to explain Molar mass and calculate Moles using n=m/M
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