Starter: Write Starter: Write some some

information information about the atomabout the atom

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The structure of the atomThe structure of the atom

ELECTRON – negative,

mass nearly nothingPROTON –

positive, same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

I did some experiments in 1808 that proved this and called these particles ATOMS. Most of an atom is empty space and the nucleus is actually very small so this diagram is wrong:

Dalton

What particles are atoms What particles are atoms made of?made of?

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 particles. There are three types:

How are these particles arranged inside the atom?

What is the structure What is the structure of an atom?of an atom?

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.

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Mass and atomic numberMass and atomic number

Particle Relative Mass Relative Charge

Proton 1 +1

Neutron 1 0

Electron Very small -1

MASS NUMBER = number of protons + number of neutrons

SYMBOL

PROTON NUMBER = number of protons (obviously)

21/04/23Atomic mass in more detailAtomic mass in more detail

SYMBOL

PROTON NUMBER = number of protons (obviously) – this

number is always the same for an element

RELATIVE ATOMIC MASS, Ar

(“Mass number”) = number of protons + number of neutrons

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Mass and atomic numberMass and atomic numberHow many protons, neutrons and electrons?

Atomic structure worksheetAtomic structure worksheet21/04/23

Use a periodic table to help you complete the table below.Element SymbolProton number

Mass number

Number of

protons

Number of

neutrons

Number of

electrons

 

carbon

 

C  

 

12       

aluminiu

m

     

 

13    

 

chlorine        

 

18  

 

  

 

33         

caesium    

 

133       

   

 

36         

titanium           

How are electrons How are electrons arranged?arranged?

Electrons are not evenly spread but exist in layers called shells. (The shells can also be called energy levels). The arrangement of electrons in these shells is often called the electron configuration..

Note that this diagram is not drawn to scale – the atom is mostly empty space. If the electrons are the size shown, the nucleus would be too small to see.

1st shell2nd shell

3rd shell

How many electrons per How many electrons per shell?shell?Each shell has a maximum number of electrons

that it can hold. Electrons will fill the shells nearest the nucleus first.

1st shell holdsa maximum of2 electrons

2nd shell holdsa maximum of8 electrons

3rd shell holdsa maximum of8 electrons

This electron arrangement is written as 2,8,8.

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Electron structureElectron structureConsider an atom of Potassium:

Potassium has 19 electrons. These electrons occupy specific energy levels “shells”…

Nucleus

The inner shell has __ electrons

The next shell has __ electrons

The next shell has __ electrons

The next shell has the remaining __ electron

Electron structure

= 2,8,8,1

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Electron structureElectron structureDraw the electronic structure of the following atoms:

Electron structure

= 2,8,8,2

Nucleus

Electron structure

= 2,8,2

Nucleus

Electron structure

= 2,5

Nucleus

Electron shells worksheetElectron shells worksheet21/04/23

21/04/2321/04/23IsotopesIsotopesAn isotope is an atom with a different number of neutrons:

Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.

Notice that the mass number is different. How many neutrons does each isotope have?

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Strange atomic massesStrange atomic massesWhen you look at a periodic table

sometimes the atomic mass is not a whole number. Consider chlorine, for example:

How can an atom have a decimal for its mass?This is because out of every four naturally occurring chlorine atoms, 3 have a mass of 35 and 1 has a mass of 37 so the average atomic mass is:

(3 x 35 + 1 x 37) / 4 = 35.5

Q. Magnesium is often found as 24Mg or 26Mg. If 79% of magnesium is 24Mg what is the average atomic mass?

(79 x 24 + 21 x 26) / 100 = 24.4

Calculating relative

atomic mass

Where are r.a.m. values Where are r.a.m. values found?found?

The values of relative atomic mass (r.a.m.) are usually given in a data book or found in the periodic table, so you don’t have to work them out or remember them all!

When looking up r.a.m in the periodic table, remember that it is always the larger of the two numbers given.

atomic number

symbol

relative atomic mass

Why isn’t r.a.m. always a whole Why isn’t r.a.m. always a whole number?number?

Relative atomic mass (r.a.m.) is not always a whole number. For example, the r.a.m. of chlorine is 35.5.

Chlorine has two isotopes:chlorine-35 (75%) and chlorine-37 (25%).

The standard r.a.m. value of each element is actually the average relative atomic mass, which takes all the isotopes of each element into account.

average r.a.m. of chlorine= (35 × 75%) + (37 × 25%)= (35 × 0.75) + (37 × 0.25)= 26.25 + 9.25= 35.5

What 2 pieces of information do you need in order to calculate the r.a.m.?

1.The abundance of the isotopes (%)2.The relative mass of the isotope.

average r.a.m. of chlorine = (35 × 75%) + (37 × 25%)= (35 × 0.75) + (37 × 0.25)= 26.25 + 9.25= 35.5

Calculating average r.a.m. Calculating average r.a.m. from isotopesfrom isotopes

What is the average r.a.m. of naturally-occurring bromine?

Naturally-occurring bromine is composed of two isotopes:bromine-79 (50.5%) and bromine-81 (49.5%).

average r.a.m. = (79 × 50.5%) + (81 × 49.5%)= (79 × 0.505) + (81 × 0.495)= 39.895 + 40.095= 79.99

This figure can be rounded up.

To calculate the average r.a.m. of a mixture of isotopes, multiply the percentage of each isotope by its relative

atomic mass and then add these together.

Past paper questions…Past paper questions…

Plenary…Plenary…

• What are isotopes?• Tell me something you have learnt about

electron shells

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