CHEMICAL SUBSTANCES AND ATOM BONDING.

As we have seen in the previous section, in the periodic table the elements are organised according to their properties. These properties and the position in the periodic table are related to the characteristics of the atom’s shell. The atom’s shell splits into different levels with different number of electrons in each level. The outer electron shell (the farthest from the nucleus) is called the valence shell. All the elements in the same group (column) have an equal number of electrons in their valence shell and, for that reason, they have the same chemical properties.

In nature, we find all the substances made up of groups of atoms, except noble gases. Atoms bond to become more stable, that means they have less energy as a group than as separated atoms. The force of attraction that keeps two or more atoms together is called a chemical bond.

But, why and how atoms create a chemical bond? Are all chemical bonds equal? And, why do the atoms in noble gases not bond? The reason is related to their valence shell; noble gases have 8 electrons in their valence shell so they are stable despite of the fact of being isolated.

Chemist observed that when atoms bound acquire the electronic configuration of the noble gas nearest to them, that is, they obtain 8 electrons in their valence shell. This fact is explained by the octet rule, which means that a neutral atom can lose or gain the specific number of electrons needed to have 8 electrons in their valence shell. Some atoms are far from a noble gas and would have to lose or gain an excessive number of electrons, in this case, atoms can share electrons with one another to comply with the octet rule. There are exceptions to the octet rule because there are some atoms with less than 8 electrons, such as Hydrogen, Lithium or Beryllium. Those elements need to acquire the electronic configuration of Helium, which means having 2 electrons in their valence shell.

The elements are classified into metals, non-metals, semimetals and noble gases depending on their properties, that is, on their valence shell. In general terms, metals need to lose electrons, non-metals want to gain electrons and semimetals can do both things. When some atom gains, or loses electrons it creates an ion, negative or positive respectively, so metals create positive ions, called cations, and non-metals create negative ions, called anions.

We define three types of chemical bond depending on which elements form it and how is the octet rule complied. We summarize the three types of chemical bond and the properties of the compounds formed in each case in the next mindmap.

ATOM BONDING by BARBARA PLAZA VIDAL

I encourage you to do another mind map but from macro structures to micro (atoms), I mean, you must separate the matter into the substances we find in nature (gases, liquids and solids) and you have to split each state into the different compounds depending on their chemical bond. You do not need to write all the properties but you must use them to analyse the type of substance in each case.

Sources of information:

     - Chemical bonds: www.khanacademy.org

- Covalent bonding-single bonds: www.chemguide.co.uk

- Física y química. 4ºESO. Grupo Anaya, 2015.

THE PERIODIC TABLE

     By the middle of the 19^th century, a great number of chemical elements had been discovered and scientists needed a more precise way to classify them. As new elements continued to be discovered, scientists observed that some elements had similar properties, such as chlorine, bromine and iodine or calcium, strontium and barium.

     In 1817, J.Döbereiner revealed the similarities between the properties of certain groups of three elements, with a gradual variation of these properties from the first element to the last one. These groups of three elements were called triads. To classify them, Döbereiner explained that the average atomic weight of the weights of the extreme elements, is like the element in the middle of the triad.

     Later, in 1864, J.A.R. Newlands listed the elements increasing their atomic weight. He noticed that similar physical and chemical properties recur at intervals of eight and he grouped known elements with similar properties into groups of eight. He compared this grouping to octaves in music. Initially, his work was not accepted by the scientific community, but was later recognised as the first attempt to classify the elements.

     In 1869, D. Mendeleev and L. Meyer independently published a classification of the elements considering the increase of the atomic mass of the elements and grouping elements together according to their properties. This classification presented several limitations which were improved by H. Moseley in 1913. The Moseley table orders the element by their atomic number (Z), that is, using the number of protons of the elements. Moreover, the position of each element in the periodic table can be related to the characteristics of the atom’s shell. The chemical elements are arranged into:

-   18 columns, called groups or families: all elements in the same group have an equal number of electrons in their valence shell and therefore they have similar chemical properties.

-      7 horizontal rows, called periods: all elements in the same period have the same number of electron shells.

ACTIVITIES: 

     The following activities are presented in different blocks related to different areas. You can choose which area or activity you prefer working on or, you can choose another area for your research. Make a little report with the information you consider relevant and give your opinion about it. Enjoy it!

Focus on English!  Look for the idioms related to the word “table”.

Physics and chemistry:

1. Interactive Periodic Table

You can play and learn with this interactive periodic table and discover the hidden information behind the elements. You can learn more about the uses of each element, It´s up to you! (Element Uses)

2. Build an atom

In this simulation, you can build an atom out of protons, neutrons, and electrons, and see how the element, charge, and mass change. Then play a game to test your ideas!

3. Isotopes and atomic mass

Use the simulation to learn about isotopes and how abundance relates to the average atomic mass of an element.

4. Radioactive elements: search information about one or more radioactive elements and their uses in our society.

Maths:

5. Choose the element you prefer to work with and look for the information you need to calculate their average atomic mass (weighted mass).

Medicine:

6. Look for the elements used in different kind of therapies like radiotherapy or chemotherapy.
7. Search for information about which elements are used to make medicines, implants, pharmacology…

Biology:

8. Search the necessary elements for a balanced diet.
9. Look for which elements are relevant for animals and plants and make a table with their similarities and differences.
10. Look for which elements are dangerous for life (animals and plants).
11. Which animals have poisoned protection? Which elements are involved in it?

Geology:

12. Look for elements used in jewellery.
13. Search which elements are relevant in geology, or which are the most abundant elements in Earth.

History:

14. Which elements are used to know the age of the different discoveries in archaeology?

Technology/Engineering:

15. Look for the elements needed for doing a watch, laptop, iPhone, tablet, battery, etc.
16. Which elements are useful in aeronautics, vehicles, buildings, energy systems…?

Music/Art:

17. Look for information about which elements are the best for doing different instruments like a flute, saxophone, etc.
18. Search information about the elements used for doing different colours or for the conservation of the artwork, etc.

At home or in your school: 

19. Look for the elements we use in our daily life or we have around us at home or at school. 

Sources of information:

- Physics and chemistry. 3th Secondary Education. Grupo Anaya, 2015.

- The periodic table of the elements, in pictures and words. Keith Enevoldsen  http://elements.wlonk.com/ElementsTable.htm

- Periodic table in https://en.wikipedia.org/wiki/Periodic_table

- Phet Simulations in https://phet.colorado.edu/