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KS4 Chemistry. Atomic Structure. Contents. Atomic Structure. Introducing atoms. Atomic number and mass number. Electron configuration. Isotopes. Summary activities. Discovery of atomic structure. Atoms – the building blocks.
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KS4 Chemistry Atomic Structure
Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities
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.
Elements – different types of atom Elements are the simplest substances. There are 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.
How small is an atom? N X3,000,000,000 Atoms are very small – they are about 0.00000001cm wide. To make an atom the size of a football it would have to be enlarged by about 3,000,000,000 times. If a football was enlarged by the same amount it would stretch from the UK to the USA!
Inside an atom Where are the electrons and nucleus found in an atom?
Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities
How heavy is an atom? O Si millions of these atoms join to form each tiny grain of sand O A single grain of sand contains millions of atoms of silicon and oxygen. Each atom must therefore have an extremely small mass.
Relative atomic mass Relative atomic mass = 12 C 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.
Relative atomic mass – examples 12 atoms x 1= 1 atom x 12 C H H H H H H H H H H H H H H H H H H H H H H H H 1 atom x 24= 2 atoms x 12 Mg Mg C C The lightest atom is hydrogen. It has 1⁄12 the mass of carbon and so has a RAM of 1. Magnesium is twice as heavy as carbon. It has a RAM of 24.
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
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.
The atom: check it out! nucleus electron neutron proton Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons.
Properties of subatomic particles The atoms of an element contain equal numbers of protons and electrons and so have no overall charge. There are two properties of subatomic particles that are especially important: 1. Mass 2. Electrical charge 1 +1 1 0 -1 almost 0
How many protons? The number of protons in an atom is known as its atomic numberor proton number. The atoms of any particular element always contain the same number of protons. For example: • hydrogen atoms always contain 1 proton; • carbon atoms always contain 6 protons; • magnesium atoms always contain 12 protons, It is the smaller of the two numbers shown in most periodic tables.
What’s the atomic number? What are the atomic numbers of these elements? sodium iron tin fluorine 26 50 9 11
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: • radioactive decay; • nuclear bombs; • nuclear reactors.
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. 0 1 1 7 4 3 27 13 14
What’s the mass number? Mass number = number of protons + number of neutrons What is the mass number of these atoms? 2 2 4 29 35 64 27 32 59 74 53 127 41 32 73
How many neutrons? = mass number - atomic number Number of neutrons = mass number - number of protons How many neutrons are there in these atoms? 2 10 50 51 146
Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities
How many electrons? 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. 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. 2 2 2 35 29 29 53 53 74
Calculating the number of electrons What are the missing numbers? 6 5 5 11 5 20 19 19 39 19 24 28 24 24 52 80 121 80 80 201 18 18 18 40 22
How are electrons arranged? 1st shell 2nd shell 3rd shell Electrons are not evenly spread but exist in layers called shells. The arrangement of electrons in these shells is often called the electron configuration.
How many electrons per shell? 1st shell holdsa maximum of2 electrons 2nd shell holdsa maximum of8 electrons 3rd shell holdsa maximum of8 electrons Each shell has a maximum number of electrons that it can hold. Electrons will fill the shells nearest the nucleus first.
Summary: the atom so far Electrons are: The nucleus is: • Dense – it contains nearly all the mass of the atom in a tiny space. • Made up of protons and neutrons. • Positively charged because of the protons. • Thinly spread around the outsideof the atom. • Very small and light. • Negatively charged. • Found orbiting the nucleus in layers called shells. • Able to be lost or gained in chemical reactions.
Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities
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
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.
Isotopes of carbon 6 protons 6 neutrons 7 electrons 6 protons 6 neutrons 8 electrons 6 protons 6 neutrons 6 electrons Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14.
Isotopes of hydrogen 1 proton 0 neutrons 1 electron 1 proton 1 neutron 1 electron 1 proton 2 neutrons 1 electron Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist. hydrogen deuterium tritium
Isotopes of chlorine 17 protons 18 neutrons 17 electrons 17 protons 20 neutrons 17 electrons About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37.
Isotopes of oxygen What are the particle numbers in each isotope? Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18. 8 8 8 8 8 10
Isotopes and RAM 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 Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this.
Calculating RAM 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) Bromine contains 50.5% bromine-79 and 49.5% bromine-81. What is the RAM of naturally-occurring bromine?
Contents Atomic Structure Introducing atoms Atomic number and mass number Electron configuration Isotopes Summary activities
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.
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.
