revision pack chemis · pdf file 2019-01-18 · cc3 atomic structure atoms are made...
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Revision Pack CHEMISTRY
CC1 States of Matter
CC2 Separating Mixtures CC3 Atomic Structure
CC4 Elements and Periodic Table CC5 Ionic Bonding
CC6 Covalent bonding
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CC1 States of Matter The three states of matter are solid, liquid and gas, described below. State of matter Particle arrangement Particle movement Solid Neatly arranged in rows
Tightly packed Particles vibrate
Liquid Closely packed and touching Not organised into rows
Particles can move past each other
Gas Particles separate and only touching when they collide Particles free to move
Gas particles have the most energy whilst solid particles have the least. If solid particles are given enough energy, they will begin to move more vigorously and start to separate into a solid. The same will happen with a liquid turning into a gas. When substances change from one state to another, this is called a physical change, not a chemical change. CC2 Separating Mixtures A pure substance only has one type of substance present. A pure substance will have an exact melting/boiling point, whereas mixtures contain more than one type of substance, and melt over a range of temperatures. Mixtures can be separated using a range of methods, dependant on the mixture
Mixture Separation Technique Example Solid and liquid Filtration Sand and water Dissolved solid and liquid Crystallisation Salt and water Liquids of different boiling points Distillation Alcohol and water Different colours in ink Paper chromatography Red and green inks Immiscible liquids Separating funnel Oil and water Pure water can be obtained by a) sedimentation to let solid lie at the bottom, b) filtration to remove the solid and c) chlorination to kill bacteria present.
CC3 Atomic Structure Atoms are made up of three subatomic particles
Subatomic particle Mass Charge Location
Proton 1 +1 Nucleus Electron 1/1840 (negliable) -1 Outer shells Neutron 1 0 Nucleus Nearly all the mass of an atom is found in the nucleus, as this is where the protons and neutrons are. Whilst an atom has a neutral charge (is not an ion), it will have the same number of protons and electrons, as they are oppositely charged. On the periodic table, the atomic mass of each element is the larger number. This tells you the total number of protons and neutrons. The atomic number tells you the number of protons. The number of protons matches the number of electrons. The neutrons can be calculated by subtracting the atomic number from the atomic mass. Isotopes are versions of atoms that have the same number of protons, but a different number of neutrons. For example, these are the three isotopes of Hydrogen: The relative atomic mass of each element takes into account the prevalence of each isotope, and creates a weighted average for each element. For example the relative atomic mass of Chlorine is calculated like this: 35Cl (75.78%) and 37Cl (24.22%). ( 0.7578 × 35) + (0.2422 × 37) = 35.48
CC4 Elements and Periodic Table Originally, Mendeleev arranged the known elements into groups based on the properties of elements and their compounds. By looking at patterns in properties, Mendeleev predicted the presence of elements that hadn’t yet been discovered. Mendeleev thought he had arranged elements in order of increasing relative atomic mass, however this was not always true, because of the relative abundance of isotopes of some pairs of elements in the periodic table. Elements are now arranged in order of atomic number (how many protons are in the nucleus of each atom) Each row is called a period Each column is called a group The group of metals in the middle are called the transition metals. Elements in groups have similar properties. Each period matches to a new shell of electrons around the nucleus The line between Al and Si, and down to Lv and Uus separates the metals (on the left) and non-metals (on the right). Is an atom is neutral (not a charged ion), the number of electrons equals the number of protons. Electrons are arranged in shells around the nucleus. 2 electrons can fit onto the first shell 8 electrons can fit onto the second and third shell Once one shell is full, the electrons are then drawn on the next shell. Electrons are drawn as small dots or crosses on circular shells. The number of electrons in each shell is called the electronic configuration. This is written with numbers and commas, for example, the electronic configuration of Oxygen is 2,6 for calcium it is 2,8,8,2.
CC5 Ionic Bonding Electrons can transfer from one atom to another. They may do this so that the atoms have a full outer shell of electrons and are therefore stable. If an atom gains electron(s) it becomes more negative. If it loses electron(s) it becomes more positive. If an atom gains 2 electrons, it has a charge of 2- If it loses 3 electrons, it has a charge of 3+ (etc). Once an atom has a positive or negative charge, it is called an ion. Atoms in group 1 have 1 electron in their outer shell, and therefore easily form 1+ ions by losing this electron. This gives them a full outer shell. Atoms in group 7 however, have 7 electrons in their outer shell, and easily form 1- ions by gaining an electron to form a full outer shell. Atoms in group 6 readily form 2- ions by gaining two electrons. Atoms can get these electrons from other atoms that are gaining/losing electrons. Metals (which easily lose electrons) and non-metals (gaining electrons) therefore bond together by donating electrons. A positive metal ion then attracts to a negative non-metal ion, to form an ionic bond.
Once atoms have bonded to form ionic compounds, the endings ‘ide’ and ‘ate’ are used in naming them. (ate is used if oxygen is also present) i.e. Sodium + Chlorine à Sodium chloride Potassium + Sulfur à Potassium sulfide Magnesium + Sulfur (+oxygen) à Magnesium sulfate Ionic compounds have a neutral overall charge, so the charge of each ionic part can be used to work out the formula of the compounds. For example: Chlorine à Cl-
Sodium à Na+
Magnesium à Mg2+
A chlorine ion has a 1 negative charge, and a sodium has a one positive. Therefore you need one of each to balance them out. The formula is therefore NaCl A magnesium ion however has a 2+ charge. To balance this with Chlorine one negative, you would need 2 chlorine ions. The formula would therefore be MgCl2 Groups of atoms can also have a charge, for example ‘NO3’ has a 1- charge. Ionic compounds can form a lattice structure; where many ions are organised into a regular arrangement. They are held together with strong electrostatic forces (ionic bonds) between positive and negatively charged ions. Ionic compounds have high melting points and high boiling points, as the electrostatic forces between ions are very strong. Ionic compounds do not conduct electricity when solid, as the electrons are not free to move, however they do conduct electricity when molten or dissolved in an aqueous solution.
CC5 Covalent Bonding Atoms can also share electrons with other atoms to fill their outer shell. Pairs of electrons are shared between two atoms to form molecules. The shared electrons are drawn in an overlap between the two outer shells. Just the outer shells can be drawn. Examples include: H2 H2O Cl2 CH4
Covalent molecules have low melting points and low boiling points, as the forces between molecules are weak. They are also poor conductors of electricity.