2. The Structure of the Atom The atom is composed of a negative, positive and neutral charge Na Nucleus Electrons They are very, very tiny so the atom is mainly empty space. Electrons have a negative charge Shells The first shell always contains two electrons when full whereas the other shells have a maximum of eight electrons The nucleus contains protons which have a positive charge. It also contains neutrons which have no charge *Na being sodium with this particular atomic structure. Back to Contents

3. How atoms combine Ionic bonding Back to Contents

4. W HY DO COMPOUNDS FORM IN THE FIRST PLACE ? Scientists found that elements in Group 8 were very non-reactive. They also noticed that those in Groups 1,2,6 and 7 were extremely reactive. They also noticed that metallic substances had several properties that were very different from other elements. They could not at first understand why. Eventually they discovered that it had to do with ELECTRON CONFIGURATIONS and STABILITY Prepared by JGL 8/21/2009 3

5. E LECTRON CONFIGURATION AND STABILITY Scientists’ research showed that in compounds, elements will combine so that the valence or outermost electrons will have the same electron configuration as the nearest noble gas (in Group 8) Prepared by JGL 8/21/2009 4

6. H OW CAN ELEMENTS COMBINE TO ACHIEVE THIS ? There are three (3) ways An element can lose electrons to another element to have the same electron configuration as the nearest noble gas. Once an atom loses one or more electrons, it forms a positively charged particle known as a CATION. An element can gain electrons from the element it combines with to have the same electron configuration as the nearest noble gas. Once an atom gains one or more electrons, it becomes a negatively charged particle known as an ANION An element can share valence electrons with another element to have the same electron configuration as the nearest noble gas. Prepared by JGL 8/21/2009 5

7. Y OU MAY WELL BE ASKING WHAT DOES THIS MEAN ? Prepared by JGL 8/21/2009 6

8. L ET ’ S TAKE A LOOK AT SOME EXAMPLES TO UNDERSTAND THIS CONCEPT MORE FULLY Prepared by JGL 8/21/2009 7

9. 8 Sodium’s atomic number is Z=11. Its electron configuration is therefore 2,8,1 Neon’s atomic number is Z=10. Its electron configuration is 2,8. It is the nearest noble gas to sodium. Sodium will combine with another element so that it can change its electron configuration from 2,8,1 to 2,8. To do this, it must lose 1 electron and give it to the element with which it combines. Let’s take sodium as an example

10. 9 Chlorine’s atomic number is Z=17. Its electron configuration is therefore 2,8,7 Argon’s atomic number is Z=18. Its electron configuration is 2,8,8. It is the nearest noble gas to chlorine. Chlorine will combine with another element so that it can change its electron configuration from 2,8,7 to 2,8,8. To do this, it must gain 1 electron from the element with which it combines. Let’s take chlorine as an example

11. Ionic bonding It only occurs between a metal and a non-metal It is formed from an attraction between positively charged and negatively charged ions Ionic bonding involves the complete transfer of electrons from one atom to another This means that elections will try to fill an empty shell of an atom using the minimum amount of energy. So if there are 7 in one outer shell and 1 in the other, the atom with 1 will lose its electron and the one with 7 will gain it Remember that when an atom loses an electron it becomes positively charged [e.g. 1+] whereas the atom which gains an electron becomes negatively charged [e.g. 1-] Back to Contents

12. Ionic bonding example e.g. Sodium chloride. Remember to draw all the shells in an exam unless it asks for just the outer shell as it may lose you marks Na Cl 1+ 1-

13. IONIC BONDING OF SODIUM CHLORIDE In order to form the compound sodium chloride, there are three (3) steps. First, the sodium atom loses one electron to form a positive sodium ion. (cation) Then the chlorine atom accepts the electron from the sodium atom to form a negative chloride ion (anion). Then the sodium cation and chloride anion become attracted to each due to their different charges, forming an ionic bond Prepared by JGL 8/21/2009 12 Source: www.revisionworld.co.uk

14. I ONS DEFINED An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.atommolecule electronsprotonselectrical charge Prepared by JGL 8/21/2009 13

15. R EVIEW – ELECTRON CONFIGURATIONS What is an electron configuration? How do we represent electron configurations? Definition: Electron configuration is the arrangement of electrons in an atom, molecule or other body.electronsatommolecule By using Bohr-Rutherford diagrams 11 p 10 n Or electron configuration notation 2,8,1 Prepared by JGL 8/21/2009 14

16. L ET ’ S CONTRAST – FLOURINE AND NEON Element symbol F Group 17 Atomic Number Z = 9 Mass number A = 19 Electron configuration: 2,7 Bohr-Rutherford diagram Element symbol Ne Group 18 Atomic Number Z = 10 Mass number A = 20 Electron configuration: 2,8 Bohr-Rutherford diagram FluorineNeon 9 p 10 n 10 p 10 n Prepared by JGL 8/21/2009 15 REMEMBER – “CONTRAST” MEANS “LOOK AT THE DIFFERENCES”

17. L ET ’ S CONTRAST – SODIUM AND NEON Element symbol Na Group 1 Atomic Number Z = 11 Mass number A = 23 Electron configuration: 2,8,1 Bohr-Rutherford diagram Element symbol Ne Group 18 Atomic Number Z = 10 Mass number A = 20 Electron configuration: 2,8 Bohr-Rutherford diagram SodiumNeon 10 p 10 n 11 p 12 n Prepared by JGL 8/21/2009 16

18. C OMPARE AND CONTRAST ALL 3 ELEMENTS F and Ne have the same number of electron shells Different atomic numbers (Z) and therefore protons Different mass numbers (A) and therefore different neutrons F needs to gain 1 electron to have the same number of electrons as Ne Na needs to lose 1 electron to have the same number of electrons as Ne SimilaritiesDifferences Scientists found that when elements from Group 1 and Group 7 combine, they lose or gain an electron to have the same number of electrons as the nearest Noble Gas. i.e. F and Na form ions that are ISO- ELECTRONIC with Ne Prepared by JGL 8/21/2009 17 Remember – “Compare” means “Look at

19. I N GENERAL To become ISO- ELECTRONIC with the nearest Noble Gas (either within the same Period or the Period just above) 1. Group 1 elements lose 1 e- 2. Group 2 elements lose 2 e- 3. Group 3 elements lose 3 e- This only happens when combining or reacting with another element(s) from Groups 15,16 or 17 To become ISO- ELECTRONIC with the nearest Noble Gas (either within the same Period or the Period just above) 1. Group 15 elements gain 3 e- 2. Group 16 elements gain 2 e- 3. Group 17 elements gain 1 e- This only happens when combining or reacting with another element(s) from Groups 1,2 or 3 Groups 1, 2 and 3Groups 15, 16 and 17 Prepared by JGL 8/21/2009 18