Year 8 Chemistry Knowledge Organiser Topic 1: Periodic Table

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1 KPI 1.1: Identify, with reasons, differences between atoms, elements and compounds Key Terms Element Mixture Compound Elements Definitions A substance that contains only one type of atom A substance that contains 2 or more types of atom that are not chemically bonded together A substance that contains 2 or more elements that are chemically bonded together All 118 currently known elements are found on the periodic table. All elements are given a symbol. These must be written with a capital letter first and a lower case letter second. For example Au is the symbol for gold. Symbols to learn: Symbol Mg Cl Ar Au Ag Cu Pb Element Magnesium Chlorine Argon Gold Silver Copper Lead Symbol H O N He Fe S Na Element Hydrogen Oxygen Nitrogen Helium Iron Sulphur Sodium Mixtures Examples of mixtures are air, salt water and petrol. These can be easily separated using different techniques, for example distillation, chromatography and evaporation. Compounds A compound has at least two elements in it. Compounds form in chemical reactions. For example if iron and sulphur are heated up, they form a compound called iron sulphide. Compounds have a chemical formula. For example: H 2 O means 2 hydrogen atoms and 1 oxygen atom bonded together. Other examples of compounds include sodium chloride, carbon dioxide and methane. Compounds are very hard to separate because chemical bonds between atoms are strong. Compounds have different properties to the elements that started, for example iron is magnetic, iron sulphide is not. Chemical Reactions In a chemical reaction we start with reactants and we make products. We represent this using a word equation. Sodium + Chlorine Sodium Chloride Reactants Products We can also represent this reaction using a symbol equation 2Na + Cl 2 2NaCl

2 All the different elements are arranged on the periodic table. The elements are arranged in order of increasing atomic number. On the periodic table, we can see the metal elements on the left and non metal elements on the right. Key Terms Atom Proton Electron Neutron Atomic number Definitions Contains protons neutrons and electrons, and makes up all elements A sub atomic particle with a positive charge A sub atomic particle with a negative charge A sub atomic particle with a neutral charge The number of protons in an atom Atomic Number and Mass Number This is the total of protons + neutrons The section in the middle of the periodic table is known as the transition metals. This is the number of protons Structure of the Atom Therefore sodium has 11 protons, 11 electrons and 23-11=12 neutrons. An atom is made up of three subatomic particles: protons, electrons and neutrons. Every element has equal numbers of protons and electrons. Protons and neutrons are found in the nucleus of the atom (in the centre). Electrons are found orbiting the nucleus in shells (also known as energy levels). Electrons occupy the shells, following some very strict rules: A maximum of 2 electrons can go in the first shell The first shell MUST be full before the second shell is filled up The second shell can take a maximum of 8 electrons. This too must be full before the third shell can be filled. The third shall can take a maximum of 8 electrons. Structure of the atom and the Periodic Table Elements on the periodic table are arranged by increasing atomic number (number of protons). Hydrogen has an atomic number of 1 as it has only one proton, it is therefore the first element in the periodic table. Whereas gold has an atomic number of 79 and is therefore a much larger atom and is found much further down the periodic table. Hydrogen Gold

3 Structure of the Atom As mentioned previously, atoms carry different number of protons, electrons and neutrons. All atoms have a neutral charge (overall zero charge) as the number of negative charges (electrons) is the same as the number of positive charges (protons). The opposite charges cancel out each other. Example of positive ion Lithium ion Lithium has an atomic number of 3. This means it has 3 protons and 3 electrons. There are 2 electrons on the first shell and 1 electron on the second shell. To achieve a full outer shell, it is easier for lithium to lose 1 electron than to gain 7 electrons onto the second shell. After losing 1 electron, a lithium ion is formed. It still has 3 protons (positive charges) while it only has 2 electrons (negative charges). Therefore, the overall charge is +3-2 = +1. There are 6 electrons, i.e. 6 negative charges in carbon. At the same time, there are 6 protons, i.e. 6 positive charges. The overall charge will be -6+6 = 0. Structure of the ion An ion is formed when electrons are gained or lost by an atom, in order to gain a full outer shell. In a full outer shell, there would be 2 or 8 electrons. Atoms will try to achieve a full outer shell using the simplest method. Metal atoms lose electrons and form positive ions. Non-metal atoms gain electrons and form negative ions. Example of negative Chloride ion Chlorine has an atomic number of 17. This means it has 17 protons and 17 electrons. There are 2 electrons on the first shell, 8 on the second and 7 on the outer shell. To achieve a full outer shell, chlorine tends to gain one 1 electron onto the outer shell. After gaining an electron, a chloride ion is formed. The chloride ion still has 17 protons (positive charges) but now it has 18 electrons (negative charges). Therefore, the overall charge is = -1.

4 Groups and Periods Elements are arranged on the periodic table in groups and periods. Horizontal rows are called periods and vertical columns are called groups. Key Terms Group Period Definitions The vertical groups of elements in the periodic table The horizontal groups of elements in the periodic table Metals and Non-Metals Metals are found on the left hand side of the periodic table, the majority of elements are metals. Some elements are known as amphoteric meaning they have the properties of metals and non metal Properties of metals are, high density, high melting point (except mercury) and good conductors of heat and electricity. Only three metals are magnetic (iron, cobalt and nickel). Groups are labelled 1-7 from left to right, with last group being called either group 8 or 0. Elements in the same group have similar properties, because of this we can make predictions about the elements reactivity (see the chemical reactions topic). The History of the Periodic Table Throughout history scientists have tried to classify substances and many scientists attempted to construct a periodic table. One of the first attempts was by a scientist called John Dalton, which looks very different from today s Periodic Table. In 1864 a scientist call John Newlands arranged the elements by atomic mass and he noticed a pattern in the properties every 8 elements. Although he made some mistakes it was an important step in making the modern periodic table. Mendeleev Following Newland s work, in 1869 Dimitri Mendeleev a Russian scientist, published his periodic table, it was slightly different to those that had been before. He still arranged elements by atomic weight but he also left gaps for where he predicted elements would be. He very accurately predicted the properties of elements that were not discovered until many years later e.g. Gallium. The modern periodic table looks different to the one Mendeleev made. We now have very advanced instruments that can give us lots of information about elements. We have now discovered the elements that were left blank by Mendeleev, as well as other elements that were not discovered at the time of Mendeleev. In new elements were added to the periodic table, making 118 in total.

5 Topic 4: Chemical Reactions KPI 4.1: Represent chemical reactions as word equations and apply this to the idea of conservation of mass Chemical Change vs Physical Change Physical Change In a physical change, the matter's physical appearance is changed, but no chemical bonds are broken or formed. For example, when water is heated from liquid water to gaseous steam, only the appearance of water is changed both steam and liquid water have the chemical formula H 2 O. Chemical Change A chemical change involves a change in the chemical composition. Different elements or compounds are present at the end of the chemical change. Bonds of the reactants are broken down; new bonds are formed after the chemical change to produce new compounds. A chemical change usually is indicated by: 1. A colour change 2. Emission of a gas 3. An increase or decrease in mass 4. Formation of a new solid Key terms Physical change Chemical change Conservation of mass Definition A physical change usually refers to a change of state. No chemical bonds are broken or formed in a physical change A chemical change involves the breaking and forming of bonds. Usually a new chemical (product) is formed afterwards Matter involved in a physical or chemical change is the same before and after the change. Mass is the same before and after a physical change; the number of atoms in the reactants of a chemical reaction should stay the same after the chemical change Using state symbols in chemical equations When looking at reactions we also need to include state symbols to explain what is happening to the elements involved in the reaction. (s) shows that the element or compound is a solid (l) shows that the element or compound is a liquid (g)- shows that the element or compound is a gas (aq)- shows that the element or compound is aqueous. This means dissolved in water. A example of how we show the state symbols is, Sodium(s) + Water(l) Sodium Hydroxide(aq) + Hydrogen(g) Chemical Reactions and Conservation of Mass In a chemical reaction, there is the breaking of chemical bonds and the formation of new chemical bonds. In a chemical reaction we start with reactants and we make products. We represent this using a word or symbol equation. For example: Sodium + Chlorine Sodium Chloride Reactants Products We can also represent the reaction using a symbol equation. The numbers indicate the number of atoms involved. The number of each type of atom must be the same before and after the reaction. 2Na + Cl 2 2NaCl

6 Topic 4: Chemical Reactions Chemical and physical properties Elements in different groups have their own properties. Physical properties refer to physical characteristics such as how their colour and their states. Chemical properties refer to how the elements react when they form new bonds. Group 1 (Alkali metals) Group 7 (Halogens) Group 0 (Noble Gases) State of matter at room temperature Soft, low density solids Chlorine and fluorine are gases. Bromine is a brown liquid. Iodine is a purple solid. All of them are gases. Melting point and boiling point Melting point and boiling points of group 1 metals are higher than those of group 7 and zero. Melting point and boiling points decreases down the group. Low melting and boiling point, exist as pair (Cl 2 ). Melting point and boiling point increases down the group as the atoms become bigger. Low melting point/boiling point Eight electrons in outer shell (except helium). Melting and boiling point increases down the group as the atoms become bigger. Electrical conductivity Group 1 metals are all electrical conductors. Group 7 gases are not electrical conductors. Group 8 gases are not electrical conductors. KPI 4.2: Explain how an elements position in the periodic table links to its properties and reactivity. Key terms Electrical conductors Density Definition A substances that allows electricity to pass through. How tightly particles are packed in a fixed volume. Group 1 Element in the same group share similar chemical properties as they have similar electronic structure on their outer shells; however their reactivity changes down the group. Group 1 elements they all have 1 electron on their outer shell, therefore they all react similarly by losing 1 electron. Also they also react similarly with water. For example: Lithium + Water Lithium Hydroxide + Hydrogen Sodium + Water Sodium Hydroxide + Hydrogen + Water Hydroxide + Hydrogen Group 1 elements becomes more reactive down the group as atomic size increases down the group, meaning that there will be greater shielding effect between the nucleus and the outer shell electrons larger group 1 elements are more likely to lose their outer shell electron and react. Increasing atomic size Increasing reactivity

7 Topic 4: Chemical Reactions KPI 4.2: Explain how an elements position in the periodic table links to its properties and reactivity. Group 7 Group 7 elements they all have 7 electrons on their outer shell, therefore they all react similarly by gaining 1 electron. Group 7 elements becomes less reactive down the group as atomic size increases down the group. Shielding effects increases down the group and weakens the ability of the nucleus to gain an electron. As a result, fluorine is the most reactive while iodine is the least reactive. Since group 7 elements have different reactivity, the more reactive one can displace the less reactive one. This is called displacement reaction. For example: Sodium + Chlorine Sodium Chloride Sodium Bromide + Chlorine Sodium Chloride + Bromine Increasing atomic size Increasing atomic size Key terms Atomic size Reactivity Displacement reaction Definition Displacement Reaction of Halogens Atomic size refers to how big the atom is. Reactivity is the likeliness of an atom to gain or lose electrons. In a displacement reaction, a more reactive element displaces (take the place) of a less reactive metal. Halogens, or group 7 elements, became less reactive down the group. Fluorine is the most reactive, while iodine is the least reactive. When a more reactive halogen is mixed with a less reactive halogen bonded to another element, e.g. chlorine is mixed with potassium iodide, the more reactive halogen will displace the less reactive one. In the case mentioned above, when chlorine is mixed with potassium iodide, chlorine displaces iodide and form potassium chloride and iodine. We can also represent the reaction using a word and symbol equation: Chlorine + potassium iodide potassium chloride + iodine Cl 2 + KI KCl + I 2 The table below shows the displacement reactions between halogens and their compounds. Chloride Bromide Chlorine X chloride + bromine Iodide chloride + Iodine Bromine No reaction X bromide + Iodine Iodine No reaction No reaction X