KS 4 Chemistry Atomic Structure 1 of 49

KS 4 Chemistry • Atomic Structure 1 of 49 © Boardworks Ltd 2005

Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities 2 of 49 © Boardworks Ltd 2005

Discovery of atomic structure 3 of 49 © Boardworks Ltd 2005

Atoms – the building blocks All substances are made from very tiny particles called atoms. John Dalton had ideas about the existence of atoms about 200 years ago but only relatively recently have special microscopes (called electron microscopes) been invented that can ‘see’ atoms. The yellow blobs in this image are individual gold atoms, as seen through an electron microscope. 4 of 49 © Boardworks Ltd 2005

Elements – different types of atom Elements are the simplest substances. There about 100 different elements. Each element is made up of just one particular type of atom, which is different to the atoms in any other element. Copper is an element made up of copper atoms only. Carbon is an element made up of carbon atoms only. 5 of 49 © Boardworks Ltd 2005

How small is an atom? Atoms are extremely small – they are about 0. 00000001 cm wide. To make an atom the size of a football it would have to be enlarged by about 3, 000, 000 times. N X 3, 000, 000 If a football was enlarged by the same amount it would stretch from the UK to the USA. 6 of 49 © Boardworks Ltd 2005

The Amazing Atomic Zoom 7 of 49 © Boardworks Ltd 2005

Inside an atom Where are the electrons and nucleus found in an atom? 8 of 49 © Boardworks Ltd 2005

Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities 9 of 49 © Boardworks Ltd 2005

How heavy is an atom? A single grain of sand contains millions of atoms of silicon and oxygen. O Si O millions of these atoms join to form each tiny grain of sand Each atom must therefore have an extremely small mass. 10 of 49 © Boardworks Ltd 2005

Relative atomic mass Atoms are so small that their mass is not measured in grams but in atomic mass units. The atoms of each type of element have a relative atomic mass (RAM). The element carbon is the atom that the mass of all other atoms is compared to. Carbon has a RAM of 12. C 11 of 49 Relative atomic mass = 12 © Boardworks Ltd 2005

Relative atomic mass – examples The lightest atom is hydrogen. It has 1⁄12 the mass of carbon and so has a RAM of 1. 12 atoms x 1 = 1 atom x 12 H H HH H H H C Magnesium is twice as heavy as carbon. It has a RAM of 24. 1 atom x 24 = 2 atoms x 12 Mg Mg 12 of 49 CC © Boardworks Ltd 2005

Even smaller particles For some time people thought that atoms were the smallest particles and could not be broken into anything smaller. Scientists now know that atoms are actually made from even smaller subatomic particles. There are three types: proton neutron electron 13 of 49 © Boardworks Ltd 2005

Where are subatomic particles found? Protons, neutrons and electrons are NOT evenly distributed in an atom. The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus. The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells. 14 of 49 © Boardworks Ltd 2005

The atom: check it out! Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons. nucleus neutron 15 of 49 electron proton © Boardworks Ltd 2005

Properties of subatomic particles There are two properties of subatomic particles that are especially important: 1. Mass 2. Electrical charge Particle Mass Charge proton 1 +1 neutron 1 0 electron almost 0 -1 The atoms of an element contain equal numbers of protons and electrons and so have no overall charge. 16 of 49 © Boardworks Ltd 2005

How many protons? The atoms of any particular element always contain the same number of protons. For example: l hydrogen atoms always contain 1 proton; l carbon atoms always contain 6 protons; l magnesium atoms always contain 12 protons, The number of protons in an atom is known as its atomic number or proton number. It is the smaller of the two numbers shown in most periodic tables. 17 of 49 © Boardworks Ltd 2005

What’s the atomic number? What are the atomic numbers of these elements? 18 of 49 sodium iron tin fluorine 11 26 50 9 © Boardworks Ltd 2005

More about atomic number Each element has a definite and fixed number of protons. If the number of protons changes, then the atom becomes a different element. Changes in the number of particles in the nucleus (protons or neutrons) is very rare. It only takes place in nuclear processes such as: l radioactive decay; l nuclear bombs; l nuclear reactors. 19 of 49 © Boardworks Ltd 2005

Mass number Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus. The sum of the protons and neutrons in an atom’s nucleus is the mass number. It is the larger of the two numbers shown in most periodic tables. Atom Protons hydrogen 1 lithium 3 aluminium 13 20 of 49 Neutrons Mass number 0 1 4 7 14 27 © Boardworks Ltd 2005

What’s the mass number? Mass number = number of protons + number of neutrons What is the mass number of these atoms? Atom 21 of 49 Protons Neutrons helium 2 2 copper 29 35 cobalt 27 32 iodine 53 74 germanium 32 41 Mass number 4 64 59 127 73 © Boardworks Ltd 2005

How many neutrons? Number of neutrons = mass number - number of protons = mass number - atomic number How many neutrons are there in these atoms? Atom helium Atomic number 4 2 fluorine 19 9 strontium 88 38 zirconium 91 40 238 92 uranium 22 of 49 Mass number Number of neutrons 2 10 50 51 146 © Boardworks Ltd 2005

Building a nucleus 23 of 49 © Boardworks Ltd 2005

Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities 24 of 49 © Boardworks Ltd 2005

How many electrons? Atoms have no overall electrical charge and are neutral. This means atoms must have an equal number of protons and electrons. The number of electrons is therefore the same as the atomic number. Atom Protons Neutrons Electrons helium 2 2 2 copper 29 35 29 iodine 53 74 53 Atomic number is defined as the number of protons rather than the number of electrons because atoms can lose or gain electrons but do not normally lose or gain protons. 25 of 49 © Boardworks Ltd 2005

Calculating the number of electrons What are the missing numbers? Atom Protons Neutrons Electrons Atomic Mass number boron 5 6 5 5 11 potassium 19 20 19 39 19 chromium 24 28 24 24 52 80 80 201 mercury 80 121 argon 26 of 49 18 22 18 18 40 © Boardworks Ltd 2005

How are electrons arranged? Electrons are not evenly spread but exist in layers called shells. The arrangement of electrons in these shells is often called the electron configuration. 1 st shell 2 nd shell 3 rd shell 27 of 49 © Boardworks Ltd 2005

How many electrons per shell? Each shell has a maximum number of electrons that it can hold. Electrons will fill the shells nearest the nucleus first. 1 st shell holds a maximum of 2 electrons 2 nd shell holds a maximum of 8 electrons 3 rd shell holds a maximum of 8 electrons 28 of 49 © Boardworks Ltd 2005

Calculating electron configurations 29 of 49 © Boardworks Ltd 2005

Properties of the nucleus and electrons 30 of 49 © Boardworks Ltd 2005

Summary: the atom so far The nucleus is: l Dense – it contains nearly all the mass of the atom in a tiny space. l Made up of protons and neutrons. l Positively charged because of the protons. Electrons are: l Thinly spread around the outside of the atom. l Very small and light. l Negatively charged. l Found orbiting the nucleus in layers called shells. l Able to be lost or gained in chemical reactions. 31 of 49 © Boardworks Ltd 2005

Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities 32 of 49 © Boardworks Ltd 2005

What is an isotope? Elements consist of one type of atom, but sometimes these atoms can be slightly different. Although atoms of the same element always have the same number of protons, they may have different numbers of neutrons. Atoms that differ in this way are called isotopes. mass number is different atomic number is the same 33 of 49 © Boardworks Ltd 2005

Properties of isotopes The isotopes of an element are virtually identical in their chemical reactions. This is because they have the same number of protons and the same number of electrons. The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density. Natural samples of elements are often a mixture of isotopes. 34 of 49 © Boardworks Ltd 2005

Isotopes of carbon Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14. 35 of 49 6 protons 6 neutrons 6 electrons 7 electrons 8 electrons © Boardworks Ltd 2005

Isotopes of hydrogen Hydrogen-1 makes up the vast majority of the naturallyoccurring element but two other isotopes exist. hydrogen 36 of 49 deuterium tritium 1 proton 0 neutrons 1 neutron 2 neutrons 1 electron © Boardworks Ltd 2005

Isotopes of chlorine About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37. 37 of 49 17 protons 18 neutrons 20 neutrons 17 electrons © Boardworks Ltd 2005

Isotopes of oxygen Almost all of naturally-occurring oxygen is oxygen-16 but about 0. 2% is oxygen-18. oxygen-16 8 8 8 38 of 49 What are the particle numbers in each isotope? protons neutrons electrons oxygen-18 8 8 10 © Boardworks Ltd 2005

Isotopes and RAM Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this. To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass and add them together. For example, chlorine exists as two isotopes: chlorine-35 (75%) and chlorine-37 (25%). RAM of chlorine = (75% x 35) + (25% x 37) = (0. 75 x 35) + (0. 25 x 37) = 26. 25 + 9. 25 = 35. 5 39 of 49 © Boardworks Ltd 2005

Calculating RAM Bromine contains 50. 5% bromine-79 and 49. 5% bromine-81. What is the RAM of naturally-occurring bromine? RAM of bromine = (50. 5% x 79) + (49. 5% x 81) = (0. 505 x 79) + (0. 495 x 81) = 39. 895 + 40. 095 = 79. 99 = 80 (the RAM is usually rounded to the nearest whole number) 40 of 49 © Boardworks Ltd 2005

Summarizing atomic structure 41 of 49 © Boardworks Ltd 2005

Atomic structure word check 42 of 49 © Boardworks Ltd 2005

Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities 43 of 49 © Boardworks Ltd 2005

Glossary (part 1) atom – The smallest particle that can exist on its own. atomic number – The number of protons in the nucleus of an element, also known as the proton number. electron – Negative particle that orbits the nucleus of an atom. =element – Substance made up of only one type of atom. isotopes – Different atoms of the same element. They have the same number of protons and electrons, but a different number of neutrons. 44 of 49 © Boardworks Ltd 2005

Glossary (part 2) nucleus – The dense positive centre of an atom, made up of protons and neutrons. neutron – A neutral particle, with a mass of 1. It is found in the nucleus of an atom. mass number – The number of protons and neutrons in an atom. proton – A positive particle, with a mass of 1. It is found in the nucleus of an atom. relative atomic mass (RAM) – The mass of an element compared to the mass of 1⁄12 of the mass of carbon-12. 45 of 49 © Boardworks Ltd 2005

Anagrams 46 of 49 © Boardworks Ltd 2005

Atomic structure word search 47 of 49 © Boardworks Ltd 2005

Properties of subatomic particles 48 of 49 © Boardworks Ltd 2005

Multiple-choice quiz 49 of 49 © Boardworks Ltd 2005