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Unit 23 Inside Atoms On completing this unit you should be able to: explain why scientists use models of atoms describe different models of the atom list the properties of sub-atomic particles explain what makes each chemical element unique read information off the Periodic Table of the elements. Model A: Space-filling Scientists use models to describe the internal structure of atoms because they cannot look inside atoms with existing microscopes. Only recently have we been able to see images of whole atoms, such as the gold atoms sitting on a bed of carbon atoms shown in the image above. When scientists originally discovered that matter was made of atoms, they thought atoms were solid indivisible spheres, rather like billiard balls. Space-filling Model Coloured Balls There is an international colour code that is used when making or drawing space-filling models of atoms or molecules. Colour Code for Atoms Hydrogen atom Although this model of the atom was found to be incorrect, the idea is useful for understanding how atoms are arranged in molecules (groups of bonded atoms) and in the lattices of crystals. Oxygen atom In the space-filling model, different-coloured spheres are used to represent individual atoms. Carbon atom The spheres give a good idea of the relative size of the atoms, and building a model of a molecule gives you a picture of how the atoms are arranged. Chlorine atom Sulfur atom Model B: Mini Solar System As scientists investigated atoms more closely, they discovered that atoms were actually made of smaller sub-atomic particles. The three important sub-atomic particles that you need to know about are protons, electrons and neutrons. The protons and neutrons are packed into the nucleus (central core) of the atom. The electrons travel at high speed around the nucleus. Atoms are largely empty space except for the dense nucleus. Solar System Model: Helium Atom orbiting electrons The New Zealand scientist Ernest Rutherford thought that the electrons orbited the nucleus rather like the planets orbited the Sun. In his model atoms were considered to be like miniature solar systems. But electrons are not found in simple orbits like planets. 92 Atoms & Reactions central nucleus stationary proton stationary neutron Helium has a central nucleus with two protons and two neutrons. Two electrons orbit the nucleus. Nitrogen atom Sub-atomic Particles Scientists have also compared the mass and electrical charge of the three sub-atomic particles. Electrons are very light, negatively charged particles travelling at high speeds around the nucleus. Protons and neutrons are heavier particles that are closely packed in the nucleus. Protons have a positive charge but neutrons have no charge. Atoms are largely empty space except for the dense nucleus. The attraction that the positively charged protons have for the negatively charged electrons keeps those electrons flying around the nucleus. Sub-atomic Particle Location Charge Relative Mass proton nucleus +1 1 neutron nucleus neutral 1 electron in orbit –1 0.0005 Atomic Number An element consists of one type of atom only. There are 92 different naturally occurring elements, so there must be 92 different kinds of atoms. What makes them different? The atoms forming a particular element all have the same number of protons, e.g. oxygen atoms all have 8 protons. Neutral Atoms The number of protons in an atom is its atomic number. The atomic number distinguishes different kinds of atoms. Each proton has a positive electrical charge and each electron has an equal size but negative electrical charge. If an atom has equal numbers of protons and electrons, then it is said to be neutral. The overall positive charge of the protons is balanced by the overall negative charge of the electrons. Carbon Atom (neutrons not shown) A carbon atom has 6 six orbiting electrons protons and 6 electrons. The 6 positive charges balance the 6 negative charges, giving six a neutral atom. Model C: Energy Shells As further discoveries were made, scientists refined their model of the atom. Electrons are thought to be confined to energy shells as they vibrate around the nucleus as waves. Energy Shell Model Electrons in a shell all have the same amount of energy and only a certain number of electrons can occupy a shell. third shell second shell The innermost shell can accommodate up to two electrons, the next shell can accommodate up to eight electrons. stationary Atoms can central nucleus protons gain or lose electrons. This makes them charged atoms or ions. The ions will be either positively or negatively charged. inner shell Whether an atom has a filled or partly filled outer energy shell affects how it reacts with other atoms. Reading a Periodic Table Science Skill • Each element is different because it consists of a unique kind of atom. Sulfur is a unique element because it consists of sulfur atoms only. Reading information off a Periodic Table: • As there are 92 different kinds of atoms which have been found in nature, there are 92 naturally occurring types of elements. 2. In the box find the Atomic Number (16 – a sulfur atom has 16 protons and therefore 16 electrons). • Each element is given a unique chemical symbol. Sometimes it is a single-letter code, but more often a two-letter code (first letter a capital, Atomic Number second letter lower case, e.g. Al). 13 • Atoms of each element are unique, Al Aluminium because of the number of protons they possess. This number is called the Atomic Number. • The elements are arranged on a chart called the Periodic Table. They are arranged in order from the lowest Atomic Number across rows. Each row relates to electron energy shells – the first row has two elements, the second row has eight elements, the third row has eight elements, etc. • A column of elements is called a group and elements in a group have similar properties. 1. Locate the name of the element (e.g. Sulfur) – the box will give you the chemical symbol (S). 3. From its position along a row you can tell whether it has a full or partly full shell (sulfur has six electrons in its outer shell – two short). 4. From the colour code on the chart identify whether it is a metal or a non-metal (sulfur is a non-metal) Periodic Table of the First 20 Elements 1 H 2 He Hydrogen 3 4 Li Be Na Mg 19 20 K Ca Lithium Beryllium 11 12 5 B Boron 6 C Carbon 7 8 9 N O F Nitrogen Oxygen Fluorine Helium 10 Ne Neon 13 14 15 16 17 18 Al Si P S Cl Ar Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Metal Elements Non-Metal Elements Potassium Calcium Atoms & Reactions 93 ? 4 Decide whether the following statements are true or false. a) Atoms cannot be broken down into smaller particles. Assessment Activities 1 Match up the words with their meanings. model atoms molecule space-filling model sub-atomic particle protons electrons neutrons nucleus solar system model electrical charge neutral atom ion element Atomic Number electron shell energy shell model chemical symbol Periodic Table a) table of all the elements arranged to show trends b) model in which atoms are viewed as solid balls c) negatively charged particles orbiting the nucleus d) central core of an atom e) can be either negative, positive or neutral f ) substance made of only one type of atom g) basic building blocks of matter h) neutral particles found in the nucleus i) neither negatively nor positively charged j) can be filled by a fixed number of electrons k) positively charged particles in the nucleus of an atom l) model where electrons occupy shells m) code name for an element n) group of atoms bonded together o) equal to the number of protons in an atom p) particle such as an electron, proton or neutron q) model where electrons orbit the nucleus like planets r) a way of visualising objects we cannot see s) atom which has lost or gained electrons 2 Identify the molecules using the names in the box. b) In a space-filling model, atoms are represented by solid balls. c) Three important sub-atomic particles are the electron, proton and nucleus. d) The nucleus of an atom contains electrons and protons. e) Electrons and protons have opposite charges. f ) Attraction between neutrons and electrons is what holds atoms together. g) Neutrons and protons are much heavier than electrons. h) A neutral atom has equal numbers of protons and electrons. i) The number of neutrons an atom has determines which element it belongs to. j) An element contains only one type of atom. a) b) c) d) 5 The Atomic Number of a neutral atom gives the number of protons or electrons. The Mass Number gives the total number of protons and neutrons in the nucleus. Use these definitions to complete the following table. e) f) g) h) Atom Atomic Mass Number of … Number Number electrons protons neutrons j) i) l) k) • ammonia • nitrogen • oxygen • hydrogen chloride • • carbon monoxide • water • sulfur trioxide • • nitrogen dioxide • sulfur dioxide • hydrogen • • carbon dioxide • chlorine • Note: mono = one, di = two and tri = three 3 Copy and complete the table below. Sub-atomic Particle Location Charge proton nucleus 94 Atoms & Reactions -1 0 Relative Mass Carbon 6 12 Oxygen 8 16 Lithium 3 7 6 6 Fluorine 9 Chlorine 17 35 Aluminium 13 6 Complete the diagram of a sodium atom. g) How many protons will it have? a) b) h) How many electrons are in each shell? i) What’s its Atomic Number? c) 6 10 14 d) } e) f) 7 Complete the diagrams to show the number and location of the electrons in these electrically neutral atoms. Name the atoms involved. Periodic Table of the First 20 Elements 1 b) a) 1 0 Copy and complete this Periodic Table of the first 20 elements. Then answer the questions below. H 3 4 Beryllium 11 12 Magnesium 19 K d) c) 20 5 B 13 Al 6 7 Carbon Nitrogen 14 15 Silicon P Phosphorus 8 O 16 Sulfur 2 9 Helium 10 Fluorine Neon 17 18 Cl Metal Elements Non-Metal Elements Calcium a) How many non-metals are shown on this Periodic Table? b) Where are the non-metals found? c) How many metals are shown on this Periodic Table? d) Where are the metals located? e) Which elements have chemical symbols of one letter only? f ) Which elements have completely filled electron shells? g) Which elements are one electron short of a complete outer shell? 8 Name the different models of the atom illustrated below. a) b) c) h) Which elements have only one electron in their outer shell? i) Which elements are found in the first group on the Periodic Table of elements? j) Which elements are found in the last group of the table? k) Why are elements in the far right group unreactive? l) What happens to the number of protons as you go along a row of the Periodic Table from left to right? m)How many elements are not shown on this table? n) Is hydrogen a metal or a non-metal? 9 Read the scientific article opposite, then answer the questions below. a) Why have scientists had to rely on indirect evidence for the existence of atoms in the past? b) How many nanometres are there in a metre? c) How big are the iodine atoms (the pink and blue bumps) in the image? d) How far apart are the iodine atoms? e) What does a scanning tunnelling microscope use to observe atoms? f ) How does the STM scan the surface contours? g) Why is the image not an actual photograph? h) Can a scanning tunnelling microscope observe inside atoms? Seeing and Feeling Atoms • Atoms are extremely small objects. The iodine atoms shown in the image are less than 1 nanometre in size. A nanometre is one billionth of a metre. • Scientists believed we would never see atoms. But in 1988 physicists Binnig and Rohrer published images of atoms made by a small device called a scanning tunnelling electron microscope (STM). • In the STM an extremely fine needle moves back and forward over a surface of atoms. More current flows through the needle when it is close to an atom and less when it is further away. • The height of the needle tip above the surface of the atoms is adjusted so that the current flow is regular. • The vertical (up and down) movements of the needle are used to produce a contour map of the surface of the substance. The ‘hills’ represent atoms. • Current variations are also used to produce artificially coloured images of the atoms, which gives a threedimensional appearance. The atoms look solid. • Scientists are working on a robotic manipulator, which would allow your hand to feel the bumps of atoms as the needle moves across a surface. Atoms & Reactions 95