Atomic Structure Introducing atoms

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? Atoms are extremely small – they are about cm wide. N X3,000,000,000 If a football was enlarged by the same amount it would stretch from the UK to the USA. To make an atom the size of a football it would have to be enlarged by about 3,000,000,000 times.

The Amazing Atomic Zoom

Inside an atom Where are the electrons and nucleus found in an atom?

Atomic Structure Atomic number and mass number

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

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

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

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

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

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

ParticleMassCharge proton1+1 neutron10 electronalmost 0 Properties of subatomic particles There are two properties of subatomic particles that are especially important: 1. Mass 2. Electrical charge The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

How many protons? The atoms of any particular element always contain the same number of protons. For example: 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. hydrogen atoms always contain 1 proton; carbon atoms always contain 6 protons; magnesium atoms always contain 12 protons,

What’s the atomic number? What are the atomic numbers of these elements? 11 sodium 26 iron 50 tin 9 fluorine

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.

AtomProtonsNeutronsMass number hydrogen101 lithium347 aluminium 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.

127 What’s the mass number? What is the mass number of these atoms? Mass number = number of protons + number of neutrons AtomProtonsNeutronsMass number helium22 copper2935 cobalt2732 iodine5374 germanium3241

How many neutrons? How many neutrons are there in these atoms? Atom Mass number Atomic number Number of neutrons helium42 fluorine199 strontium8838 zirconium9140 uranium Number of neutrons = mass number - number of protons = mass number - atomic number

Building a nucleus

Atomic Structure Electron configuration

AtomProtonsNeutronsElectrons helium222 copper iodine 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. 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.

AtomProtonsNeutronsElectronsAtomic number Mass number boron56 potassium1920 chromium2428 mercury80121 argon1822 Calculating the number of electrons What are the missing numbers?

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

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. 3 rd shell holds a maximum of 8 electrons 2 nd shell holds a maximum of 8 electrons 1 st shell holds a maximum of 2 electrons

Calculating electron configurations

Properties of the nucleus and electrons

Thinly spread around the outside of 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. Summary: the atom so far The nucleus is: Electrons are: 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.

Isotopes

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

Properties of isotopes The isotopes of an element are virtually identical in their chemical reactions. 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. This is because they have the same number of protons and the same number of electrons.

Isotopes of carbon Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon protons 6 neutrons 7 electrons 6 protons 6 neutrons 8 electrons 6 protons 6 neutrons 6 electrons

Isotopes of hydrogen Hydrogen-1 makes up the vast majority of the naturally- occurring element but two other isotopes exist. hydrogendeuteriumtritium 1 proton 0 neutrons 1 electron 1 proton 1 neutron 1 electron 1 proton 2 neutrons 1 electron

Isotopes of chlorine About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine protons 18 neutrons 17 electrons 17 protons 20 neutrons 17 electrons

What are the particle numbers in each isotope? oxygen-16oxygen-18 protons neutrons electrons Isotopes of oxygen Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18.

Isotopes and RAM Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this. For example, chlorine exists as two isotopes: chlorine-35 (75%) and chlorine-37 (25%). To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass and add them together. = (0.75 x 35) + (0.25 x 37) = = 35.5 RAM of chlorine = (75% x 35) + (25% x 37)

Calculating RAM Bromine contains 50.5% bromine-79 and 49.5% bromine-81. = (0.505 x 79) + (0.495 x 81) = = = 80(the RAM is usually rounded to the nearest whole number) RAM of bromine = (50.5% x 79) + (49.5% x 81) What is the RAM of naturally-occurring bromine?

Atomic structure word check

Atomic Structure 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.