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National 4/5 Summary Notes Sub –Topic 1 b.1 : Elements, Compounds, Mixtures, Atoms and the Periodic Table Elements Although there are so many different substances, they are actually made up of a few very basic substances. These basic substances are called elements. An element is a substance made of only one kind of atom. Its atoms are different from the atoms of other elements. Some elements exist as molecules – 2 atoms bonded together. Molecules consisting of 2 atoms are often described as diatomic. Elements existing naturally as diatomic molecules are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine. A diagram of an element could look like this or Uses of Elements Different elements have different properties. This means that different elements have different uses. Gold is used for jewellery because it does not corrode. Helium is used in balloons because it is lighter than air. Chemical Symbols Each element has a name of its own. Each element has its own symbol. It can be one letter (capital) e.g carbon is C It can be two letters (first capital, second lower case) e.g. magnesium is Mg It can be two letters based on its latin name e.g. iron is Fe Symbols for the elements are found on the Periodic table and in the data booklet. Compounds When atoms of different elements combine together compounds are formed. A compound is a substance formed when two or more elements have chemically joined together. A diagram of a compound could look like this :- see below Mixtures Mixtures When we put substances together they may or may not react together. If they react together, they form a compound. A mixture is a mix of substances which have not reacted together. A diagram of a mixture could look like this or It is not easy to separate the elements of a compound from each other because they have combined or joined with each other. The atoms are firmly attached to each other. It is much easier to separate the substances in a mixture because they have not combined with each other. Usually it is possible to separate the substances in a mixture by quite simple means. Separating Mixtures Mixtures can usually be separated, sometimes quite easily. Filtration is used to separate a solid from a liquid. Evaporation is used to separate a mixture of a dissolved solid in a liquid. Dissolving A solid which will dissolve in water is said to be soluble in water. A solid which will not dissolve in water is said to be insoluble in water. A solid substance dissolves in a liquid to form a solution. The solid which dissolves is called the solute. The liquid which does the dissolving is called the solvent. solute + solvent solution E.g. sugar + water sugar solution When a substance dissolves in a solvent to form a solution, no new substance is formed – solutions are mixtures. Concentration Of Solutions If there is not very much solute in the solution, we say that we have a dilute solution. If there is a lot of solute dissolved in the solution, we say that we have a concentrated solution. If there is so much solute dissolved in the solution that no more can dissolve we say that we have a saturated solution. If we add more solute to a saturated solution it will not dissolve. It will simply lie at the bottom of the beaker. Even stirring will not make any more dissolve. The solution can take no more of that solute. Elements Everything in the world is made up of about 100 simple substances called elements. Elements cannot be made into anything simpler. All the elements are found in the Periodic Table. They are listed in a special way and this allows us to find out about different elements. Each horizontal row of elements makes up a period. Each vertical column of elements makes up a group. Elements in the same group of the Periodic Table have similar properties, that is, their chemical behaviour is similar. The alkali metals are group 1 – these are very reactive metals The Halogens are group 7 – these are quite reactive non-metals. The Noble Gases are group 0 – these are very unreactive gases. The Transition Metals are the block in the middle – they are not in a group. group 0 group 1 group 7 Different Types Of Elements 1. Metals and Non-metals Elements can be classified as metal or non-metal. Metals usually shiny good conductors of electricity and heat Non-metals usually dull NOT good conductors of electricity and heat On the Periodic Table there is a black zig zag line on the chart. This line separates metal elements from non-metal elements. The metals are on the left of the line, the non-metals are on the right. Non metals Metals 2. Solids, Liquids & Gases Elements can be classified as solids, liquids and gases. 3. Natural and Man-made Elements Most elements occur naturally in the world. A few do not – they did not exist until they were made by scientists. These elements are manmade or synthetic. All the elements with Atomic Number greater than 92 (Uranium) are man-made. More About Atoms All elements have just one kind of atom in them. Atoms are built of three kinds of even smaller particles – protons, electrons and neutrons. Name proton electron neutron Symbol p e n Charge +1 -1 0 Mass (amu) 1 1/2000 1 Protons are positively charged and found in the nucleus. Neutrons have no charge (neutral) and are found in the nucleus. Electrons are negatively charged and are found outside the nucleus. An atom is neutral overall since the number of positive protons equals the number of negative electrons. Electron Arrangement We know that electrons are found outside the nucleus. They are arranged around the nucleus in different levels of energy. The level closest to the nucleus has the lowest energy. Electrons always go to the lowest energy level where there is space. The electron arrangement of any atom can be worked out if we know how many electrons can fit into each level. Level Number Energy 1 2 3 lowest higher higher still Maximum number of electrons 2 8 8 any extra electrons go in here Lithium has atomic number 3. It has 3 electrons. The first 2 electrons go into the 1st level. The next electron goes into the 2nd level. The electron arrangement is 2,1. Carbon has atomic number 6. It has 6 electrons. The first 2 electrons go into the 1st level. The next 4 electrons go into the 2nd level. The electron arrangement is 2,4. Chlorine has atomic number 17. It has 17 electrons. The first 2 electrons go into the 1st level. The next 8 electrons go into the 2nd level. The next 7 electrons go into the 3rd level. The electron arrangement is 2,8,7. The electron arrangements of the first 20 elements are : Why The Group Is Important Elements in the same group have the same number of outer electrons. Atomic Number The Atomic Number of an element tells us the number of protons in the nucleus of its atoms. It is the number of protons that make an element what it is. Since an atom is neutral it also tells us the number of electrons. Atomic number = number of protons = number of electron Elements from the same Group all behave like each other. They behave like each other because they have the same number of electrons in their outer energy levels. It is the number of electrons in the outer shell that makes an element behave the way it does. The Noble gases have filled outer energy levels They are also very unreactive. This suggests that a filled outer energy level is the most stable arrangement. Atomic Number The Atomic Number of an element tells us the number of protons in the nucleus of its atoms. It is the number of protons that make an element what it is. Each element has a different atomic number, therefore each element has a different number of protons in its nucleus. Since an atom is neutral it also tells us the number of electrons. Atomic number = number of protons = number of electrons. Mass Number The mass number of an element is the number of protons PLUS neutrons in the nucleus of an atom. Mass number = number of protons + number of neutrons National 5 Notation There is a special way of writing the symbol of an element to show the atomic number and the mass number. mass number symbol atomic number This atom has 23 Na 11 11 protons (the same as the atomic number) 11 electrons (the same as the number of protons) 12 neutrons (mass number - atomic number) Ions Atoms sometimes lose or gain electrons to try and have a full outer energy level of electrons. When this happens, the number of electrons does not equal the number of protons and the particle now has a positive or negative charge. These particles are called ions. An ion can be formed when an atom :1. Gains electrons – to form a negative ion. e.g. 15 8 2- Protons Electrons Neutrons 2. Loses electrons – to form a e.g. 27 3+ Protons Al Electrons 13 Neutrons =8 = 10 =7 positive ion = 13 = 10 = 14 Atoms form ions in order to achieve a noble gas electron arrangement. This means they get a full outer shell of electrons. A full outer shell of electrons is very stable. E.g. A sodium atom has an electron arrangement of 2,8,1. If it loses one electron it will have a full outer shell. It now has an electron arrangement of 2,8. It is now an ion and would now be written as Na electron. + as it has lost one E.g. A chlorine atom has an electron arrangement of 2,8,7. If it gains 1 electron it will have a full outer shell. It now has an electron arrangement of 2,8,8. It is now an ion and would be written as Cl- as it has gained one electron. Atoms And Isotopes Atoms with the same number of protons but a different number of neutrons are called isotopes. Isotopes have the same atomic number but different mass numbers. Most elements occur as a mixture of isotopes. E.g Relative Atomic Mass is the average mass of all the isotopes of an element. It is rarely a whole number since it is a calculated average mass of the isotopes.