1. Isotopes and RAM
Noadswood Science, 2016

2. Isotopes and RAM To understand isotopes and relative atomic mass
Thursday, April 11, 2019
To understand isotopes and relative atomic mass
1 proton; 0 neutrons; 1 electron
1 proton; 1 neutron; 1 electron
1 proton; 2neutrons; 1 electron

3. Precise Learning
The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes.

4. The Periodic Table
Periodic table shows all the known elements (with spaces left for undiscovered ones)
Amongst other information it shows key information about an element’s atomic mass and atomic number…

5. Number of protons + neutrons
Atomic Mass & Number
Atomic mass
Number of protons + neutrons 12 C
Atomic number
Number of protons (therefore number of electrons too in a neutral atom) 6
6 protons
6 neutrons
6 electrons

6. H B O Na Cl U Atomic Mass & Number
How many protons, neutrons and electrons are there in the following… 1 H 11 B 16 O 1 5 8
1 protons 0 neutrons 1 electrons
5 protons 6 neutrons 5 electrons
8 protons 8 neutrons 8 electrons 23 Na 35 Cl
238 U 11 17 92
11 protons 12 neutrons 11 electrons
17 protons 18 neutrons 17 electrons
92 protons 146 neutrons 92 electrons

7. Nucleus

8. Isotopes
An atom is made from a nucleus surrounded by electrons – the nucleus contains protons and neutrons
Isotopes are atoms that have the same number of protons, but different numbers of neutrons – the nuclei of some isotopes are unstable, emitting radiation and breaking down to form smaller nuclei…

9. Isotopes
Isotopes are the atoms of an element with different numbers of neutrons – they have the same proton number, but different mass numbers…
Look at the isotopes of hydrogen: -
1 proton; 0 neutrons; 1 electron
1 proton; 1 neutron; 1 electron
1 proton; 2neutrons; 1 electron

10. Radioactive
The nuclei of some isotopes are unstable – they can split up or ‘decay’ and release radiation
Such isotopes are called radioactive isotopes or radioisotopes
When a radioactive isotope decays, it forms a different atom with a different number of protons

11. Carbon
Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14

12. Carbon Dating
What is carbon dating, and how does it work?

13. Carbon Dating
Carbon-14 is created at a constant rate in the upper atmosphere by cosmic rays acting on nitrogen
The carbon-14 which is formed is radioactive and decays producing nitrogen again – there is therefore a fixed amount of carbon-14 in the environment which is a balance between the rate at which it is formed and the rate at which it decays
All living things take carbon into themselves: plants take in carbon during photosynthesis; and animals take in carbon when they eat their food
All living things therefore have carbon-14 in them at the same amount which is present in the environment – this amount is small (1 in 850 billion are carbon-14, the rest are mainly carbon-12 which are not radioactive)

14. Carbon Dating
When a living thing dies, it stops taking in carbon from its environment., so he amount of carbon-14 in it will start to decrease as the carbon-14 slowly decays
The further back in time that something died, the less carbon-14 it will have – measuring the amount of carbon-14 can tell you how long ago the thing died and therefore the "age" of the sample

15. Carbon Dating – Limitations
Carbon dating can be used on anything which used to be alive, e.g. animal substances (remains including skin, fur and bone); and plant substances (remains including wood, seeds, pollen, cloth, rope etc…)
Some fossils can be dated this way if they still contain some of the original carbon of the plant or animal
Carbon dating cannot be used on things which have never lived or for substances older than around 50’000 years (as the carbon-14 sample is very small after 9/10 half-lives)
The method of carbon dating uses an assumption that the amount of carbon-14 present in the past is the same as that present in the environment today

16. Relative Atomic Mass
Many elements have more than one isotope (such as carbon seen previously). This means when referring to the masses of elements, relative atomic mass (Ar) is used
This is an average mass taking into account the different masses of isotopes that make up the element and how abudnent each isotope is
Relative atomic mass (Ar) = sum of (isotope abundance x isotope mass number)
sum of abundances of all the isotopes

17. Relative Atomic Mass
Relative atomic mass (Ar) = sum of (isotope abundance x isotope mass number)
sum of abundances of all the isotopes
For example copper has two stable isotopes: Cu-63 (69.2% abundance) and Cu-65 (30.8% abundance) – what is its relative atomic mass?
Relative atomic mass (Ar) = (69.2 x 63) + (30.8 x 65)
Relative atomic mass (Ar) = 63.6

18. Relative Atomic Mass
Relative atomic mass (Ar) = sum of (isotope abundance x isotope mass number)
sum of abundances of all the isotopes
What would the Ar be for magnesium in the following case?
Magnesium has three stable isotopes: Mg-24 (79.0% abundance), Mg-25 (10.0% abundance) and Mg-26 (11.0% abundance) – what is its relative atomic mass?
Relative atomic mass (Ar) = (79.0 x 24) + (10.0 x 25) + (11.0 x 26)
Relative atomic mass (Ar) = 24.3

19. Practice Questions – Application
What are isotopes?
What is relative atomic mass?
Which is the following is a chemical symbol for an isotope of Cl?
Cl Cl Cl Cl
The relative atomic mass of copper is Explain why this value is not a whole number.
92.5% of lithium atoms are Li-7 and the remaining 7.5% are Li-6. Calculate the relative atomic mass of lithium.
Boron has two stable isotopes - 10B and 11B. 10B has an abundance of 19.9%. Calculate the relative atomic mass of boron. 35 17 37 35 36 17 17 16 16 35

20. Answers
Isotopes are different forms of the same element, which have the same number of protons but different numbers of neutrons
An average mass of an element, taking into account the different masses of the isotopes, along with their relative abundance
Cl – same number of protons but different number of neutrons
Copper has more than one isotope and the relative atomic mass is an average that takes into account the different masses of these isotopes
Lithium Ar = 6.9
Ar = (92.5 x 7) + (7.5 x 6)
Abundance of 11B = 100 – 19.9 = 80.1% so Boron Ar = 10.8
Ar = (19.9 x 10) + (80.1 x 11) 37 17