In this presentation you will:

1. In this presentation you will: Chemical Reactions and Valence Electrons Cl- Na+ • explore how chemical reactions depend on valence electrons Next >

2. Introduction During a chemical reaction, chemicals combine to create different chemicals. These reactions can produce very different substances. For example, if a liquid and a solid were added together, a liquid and a gas might be produced. Next >

3. Valence Electrons Each atom that takes part in a reaction has a nucleus, with one or more electrons orbiting that nucleus. Valence electron The electrons exist in different orbits called shells. Ni The more electrons an atom has, the more shells are occupied. Valence electron The electrons in the outermost shell are called the valence electrons. Next >

4. Valence Electrons Since the valence electrons are the outermost electrons, they are the nearest part of the atom to other atoms. The valence electrons are also furthest from the nucleus, so the force holding them to the nucleus is the weakest. Valence electrons If these electrons gain just a small amount of energy, they can move from the outer shell of one atom to the outer shell of another. Next >

5. Valence Electrons Each shell has a maximum number of electrons it can contain. Each shell is divided into a number of sub-shells. p sub-shell A full outer shell or sub-shell is a very stable configuration. Shell 2 Neon s sub-shell s sub-shell For example, the atoms of inert gases, like neon, have full outer sub-shells and are very unlikely to react with other atoms. p sub-shell Next >

6. Valence Electrons When electrons move between shells they try and achieve this stable configuration. An element with only one electron in its outermost sub-shell is likely to easily lose this electron in a reaction with other atoms. The single electron would be most likely to go to an incomplete sub-shell. Next >

7. Valence Electrons and the Periodic Table The Periodic Table can show us how many electrons each element has. Group 1 elements, the alkali metals, have one valence electron. Group 2 metals, the alkaline earth metals, have two valence electrons. Group 18 elements, the noble gases, have eight valence electrons. Group 17 elements, the halogens, have seven valence electrons. Next >

8. Valence Electrons and the Periodic Table Group 1 and 2 metals are highly reactive because they can lose their valence electrons easily. Magnesium – Group 2 Group 18 elements are almost non-reactive (inert). They have a full outer shell of electrons. Neon – Group 18 Fluorine – Group 17 Group 17 elements are very reactive because they have 1 space in their outermost shell. They can easily capture an extra electron to fill the shell. Next >

9. Valence Electrons and Charge Normally, the sum of charges in an atom of an element is zero. This is because the number of protons is the same as the number of electrons. 28 electrons 28 protons A proton carries a single positive charge, and an electron carries a single negative charge of the same value. Ni If an electron is gained or lost, then the balance of charges is no longer the same. The atom is positively or negatively charged. Next >

10. Valence Electrons and Charge If the atom has lost an electron, it becomes positively charged. 17 photons 18 electrons 11 photons 10 electrons Positive ion Negative ion If the atom has gained an electron, it becomes negatively charged. Next >

11. Valence Electrons and Charge These charged atoms, now called ions, will be attracted to oppositely charged ions and repelled by similarly charged ions. The attraction force forms an ionic bond and creates a new chemical. Attraction Repulsion Next >

12. Valence Electrons and Charge Alternatively, the valence electrons may not totally move across and join another atom. Instead the atoms may share each other’s valence electrons. This sharing of valence electrons is called a covalent bond. Next >

13. Examples Sodium and chlorine bond ionically to form sodium chloride. The single valence electron from sodium will join the outer shell of chlorine and fill that sub-shell. There is then an electrostatic attraction between the two ions. Next >

14. Examples Four hydrogen atoms (each with one valence electron) and one carbon atom (4 valence electrons) share their electrons. This creates methane, CH4, which has four covalent bonds. This sharing gives all the atoms a full shell of electrons. Next >

15. Question 1 Which electrons are involved in bonding? A) Valence electrons B) Innermost electrons C) Most energetic electrons D) Shell electrons Next >

16. Question 1 Which electrons are involved in bonding? A) Valence electrons B) Innermost electrons C) Most energetic electrons D) Shell electrons Next >

17. Question 2 What is a positive ion? A) An atom with an equal number of protons and electrons B) An atom that has lost an electron C) An atom that has more electrons than protons D) An atom that has gained an electron Next >

18. Question 2 What is a positive ion? A) An atom with an equal number of protons and electrons B) An atom that has lost an electron C) An atom that has more electrons than protons D) An atom that has gained an electron Next >

19. Question 3 Two chemicals can react to form a new chemical when: A) an electron of one atom changes place with the electron of another atom B) all the atoms lose their outermost electrons in ionic bonding C) electrons of one element change places with the protons of the other element D) an atom of one element gives up its electron to join the atom of the other element Next >

20. Question 3 Two chemicals can react to form a new chemical when: A) an electron of one atom changes place with the electron of another atom B) all the atoms lose their outermost electrons in ionic bonding C) electrons of one element change places with the protons of the other element D) an atom of one element gives up its electron to join the atom of the other element Next >

21. Question 4 Covalent bonding is when: A) atoms of different elements bond by sharing an outermost electron B) atoms of different elements bond by one element giving up an outermost electron to the other C) electrons of one element change places with the protons of the other element D) an atom of one element gives up its electron to join the atom of the other element Next >

22. Question 4 Covalent bonding is when: A) atoms of different elements bond by sharing an outermost electron B) atoms of different elements bond by one element giving up an outermost electron to the other C) electrons of one element change places with the protons of the other element D) an atom of one element gives up its electron to join the atom of the other element Next >

23. Summary In this presentation you have seen: • how chemical reactions depend on valence electrons End >