Download Atoms, Ions and Molecules

Welcome to University Life You are not in school any more! In school: At university: Your teachers/parents push you to study hard. You have to push yourself. If you do not study hard enough, your teachers/parents get cross. If you do not study hard enough, you fail. If you do not complete your work on Bme, you get punished. If you do not complete your work on Bme, you fail. There are right answers and wrong answers. You have to think for yourself. Everyone tells you what to do and when to do it. You are free to decide for yourself. What is Chemistry?   Chemistry is the science of atoms and molecules.   On the macroscale, chemistry is the study of the transformaBon of ma,er into different forms.   On the nanoscale, chemistry is the study of individual molecules and small clusters of atoms. Why Study Chemistry? If we understand chemistry, we can understand why materials behave the way they do. We can understand the transformaBon of materials. We can understand our environment, it’s problems and how to miBgate them. We can make new materials that produce new technologies and make the world a beLer place. Mixtures and Compounds Most of the substances you see in the world around you are mixtures or solu1ons. Mixtures are made from two or more pure substances and you can oOen separate the components of mixtures or soluBons by using physical separa1on techniques (note that some mixtures, like concrete or glass, cannot be separated by physical separaBon techniques). In the example below, we might separate sand from water by filtra1on or separate alcohol (ethanol) from water by dis1lla1on. Now we have separated the mixtures into separate pure compounds: water (H2O) and silica (SiO2) or water and ethanol (CH3CH2OH). However, we cannot separate compounds into different chemicals by physical separaBon techniques. Compounds and Elements If we use chemistry, we can break compounds up into other substances. For example, we can break water up into hydrogen and oxygen and silica into silicon and oxygen. However, hydrogen, oxygen or silicon cannot be made from any other chemicals. Hydrogen, silicon and oxygen are elements. All the material things we can see in the universe around us are made from pure elements or from combinaBons of elements. There are about 90 naturally occurring elements. An Analogy with Digital Pictures Mixtures, compounds and elements are a liLle like the parts of a digital picture. A mixture is like the picture itself, a combinaBon of coloured image pixels (an image pixel is one square of pure colour). A compound is like one of the image pixels that make up the picture. An element is like one of the screen pixels that make up the image pixels — there are only three colours of screen pixel: red, green and blue. All the pictures on your TV or computer screen ulBmately come from just red, green and blue and you cannot break these pixels into to any smaller pieces of other colours. Elements and the Periodic Table The elements are all related and the Periodic Table tells us about those relaBonships. Each element has a chemical symbol. Most, but not all, of the elements have symbols that come from their LaBn names. From E
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toms Gold (Au) is an element. What happens if we zoom in on a piece of gold? As we zoom in on the surface, we see more and more detail unBl we get to about 1 x 10–10 m. At this Bny scale, we can see that gold is made of Bny spheres. pure gold a gold crystal These spheres are the smallest possible par1cles of an element and are called atoms. A pure sample of an element contains only atoms of the same kind. Atoms combine to make huge, 3-­‐D arrangements called crystals or small discrete groups called molecules. Only the noble gases, such as helium, usually exist as individual atoms. an oxygen molecule oxygen gas a helium atom helium gas Inside a
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tom In chemistry we need to understand the structure of the atom so let’s take a very simple first look at atomic structure. This is a diagram of a helium atom but not to scale — in reality, the nucleus of a helium atom would be about the size of one blue pixel on the image below. electron electron cloud proton neutron nucleus a helium atom a helium nucleus An atom is composed of a nucleus surrounded by an electron cloud. The nucleus is composed of protons, with a posi1ve electric charge (protons are posi1ve), and neutrons with no electric charge (neutrons are neutral). The protons give the nucleus a posiBve charge. The electron cloud is a large volume where we find the electrons, which have a nega1ve electric charge (electrons are negaBve). The negaBvely charged electrons are strongly aLracted to the posiBve charge of the nucleus (opposites aLract). Atoms have equal numbers of protons and electrons. Elements and Atomic Numbers It is the number of protons that determines which element an atom belongs to. Hydrogen is the simplest atom with only one proton and one electron, this is why it is the most abundant element in the universe. Helium atoms have two protons, oxygen atoms have 8, and gold atoms have 79. The number of protons in an atom is known as the atomic number. Elements and Isotopes The number of neutrons does not change which element an atom belongs to. Atoms of the same element that have different numbers of neutrons are known as isotopes. Isotopes have idenBcal chemical properBes but have different weights and certain other different physical properBes. The number of protons in an atom added to the number of neutrons gives us the atomic mass units of the atom. For example, hydrogen has only one proton so it has 1 atomic mass unit. Carbon usually has six protons and six neutrons so these atoms have 12 atomic mass units. 1H 2H 3H When discussing isotopes, we add the atomic mass number to the name of the element, e.g. carbon-­‐12, carbon-­‐13 and carbon-­‐14 or chlorine-­‐35 and chlorine-­‐37. Some common isotopes: 1H (2H) 12C (13C) (14C) 35Cl 37Cl Ions Each proton in the nucleus has an electric charge of +1 and each electron in the electron cloud has an electric charge of –1, so atoms have a total charge of 0. Atoms can lose or gain electrons. When this happens, the atom gets an electric charge equal to (no. of protons) – (no. of electrons). Atoms (and molecules) with an electric charge are called ions. For instance, hydrogen atoms can easily lose one electron. In this case we are leO with only one proton so the electric charge is 1 – 0 = +1 hydrogen atom hydrogen ion (H+) = a proton oxygen atom oxygen ion (O2–) Oxygen atoms easily gain two electrons. In this case the oxygen ion has 8 protons and 10 electrons so the overall charge is 8 – 1 0 = –2. Remember that neutrons have no electric charge so the number of neutrons (usually 8 for oxygen) does not maLer. We will learn later that different elements have different preferences for how many electrons they gain or lose. For example, hydrogen easily makes H+ but does not oOen make H–. Ionic compounds Ions with opposite charges sBck together with electrosta1c interac1ons (also called ionic bonds or Coulombic interac1ons). In a solid, they form massive 3-­‐dimensional crystal la=ces. Cl– chloride ions (chlorine atoms + 1 electron => –1 electric charge) Na+ sodium ions (sodium atoms – 1 electron => +1 electric charge) A sodium chloride (Na+Cl–) crystal lamce From Atoms to Molecules In molecules, atoms share a pair of electrons to make chemical bonds with each other. Molecules made from only one kind of atom are molecules of an element. Molecules made from different kinds of atoms are molecules of a compound. Molecules can be separate from each other, as in a gas, or sBck to each other, as in liquids and solids. H hydrogen gas (H2) oxygen gas (O2) water (H2O) carbon dioxide (CO2) O H water (H2O) ethanol (CH3CH2OH) Pure, SoluBon, Mixture Now we can understand the chemical difference between mixtures, compounds and elements. A pure sample of a molecular compound contains molecules of only one kind. Mixtures and solu1ons contain molecules and/or ions of more than one kind. pure liquid water helium/oxygen gas mixture water/methanol soluBon a mixture of solids Mixtures, Compounds, Elements A) i) Which element has an atomic number of 45? ______ ii) Which element has a relaBve atomic mass of approximately 45? ______ iii) Which of these elements has atoms with the highest total number of electrons? ______ B) Label the following substances as being ‘mixture’, ‘compound’ or ‘element’. i) wood: ___________________ ii) copper: ___________________ iii) air: ___________________ iv) iron chloride: ___________________ [2] [2] Mixtures, Compounds, Elements C) i) Label the following diagram of an atom. The atom is neutral and all parBcles are visible. ii) IdenBfy the element: ___________________ iv) This diagram is a simplificaBon of an atom. Name one way this diagram is simplified, when compared to a real atom. ______________________________________________________________________ [6] Answers A) i) Which element has an atomic number of 45? Rh ii) Which element has a relaBve atomic mass of approximately 45? Sc iii) Which of these elements has atoms with the highest total number of electrons? 45 B) Label the following substances as being ‘mixture’, ‘compound’ or ‘element’. i) wood: mixture ii) copper: element iii) air: mixture iv) iron chloride: compound [2] [2] Answers C) i) Label the following diagram of an atom. The atom is neutral and all parBcles are visible. neutron proton electron ii) IdenBfy the element: Be (beryllium) iv) This diagram is a simplificaBon of an atom. Name one way this diagram is simplified, when compared to a real atom. The diagram is not to scale/the electrons follow simple orbits/the diagram is 2D. [6]