Recap: Chemical and Physical Change

1. Recap: Chemical and Physical Change • Different states of a substance are different physical ways of packing its component particles • A physical change is associated with a change of state. • A chemical change involves changing one substance into another • Physical properties are the properties that a substance shows by itself, without changing or interacting with another substance • Chemical properties are the properties of a substance that result in the formation of a new substance

2. Structure of Atoms – History 1808 J Dalton Atomic Theory All matter consists of atoms - are tiny indivisible particles of an element that cannot be created or destroyed. Atoms of one element cannot be converted into atoms of another element. Atoms of an element are identical and are different from atoms of any other element. http://www.humantouchofchemistry.com/john-dalton.htm

3. Structure of Atoms 1897 J J Thomson studied cathode rays • Negatively charged particles. • All metals produced the same particles. • ~ 1000 times lighter than a hydrogen atom Atoms are divisible! • Cathode rays were later renamed electrons.

4. Structure of Atoms – History 1897 J J Thomson Plum Pudding model of the atom

5. Structure of Atoms – History 1909 R A Millikan measured the charge of an electron 1.6 x 10-19 C  m(e-) = 9.1  10-31 kg Fig 2.6 Silberberg

6. Structure of Atoms – History 1909 E Rutherford

7. Structure of Atoms – History 1909 E Rutherford • Atoms are mostly empty space occupied by electrons. • All the positive charge and essentially all the mass lies in a tiny region in the centre – the nucleus. • The nucleus is made of positively charged particle, protons, and uncharged neutrons.

8. Atomic Structure Protons and neutrons are also called nucleons.

9. Elements What determines the element type? How do atoms of different elements differ? The number of protons in the nucleus of the atoms. • This is the atomic number, Z. • Atomic number is also equal to the charge of the nucleus.

10. Atomic Symbol • The atomic symbol, X, is uniquely related to the number of protons (atomic number, Z). e.g. 2 protons (Z = 2) = helium = He 8 protons (Z = 8) = oxygen = O 26 protons (Z = 26) = iron = Fe (ferrum) 92 protons (Z = 92) = uranium = U • Atoms have no overall charge so the atomic number is also equal to the number of electrons

11. Forces in Atoms • Nucleons are held together by the Strong Nuclear Force • Only effective over very short distances • Strong enough to overcome the repulsion between the protons Silberberg

12. Structure of Atoms – History 1932 Chadwick The nucleus is more than just protons – neutrons (uncharged). Overhead 14 Figure 2.8 Silberberg

13. Writing an Element mass number: A = p+ + n0 X A Z atomic number:Z = p+ = e- chemical symbol Silberberg

14. Cations and Anions • In atoms: • number of protons = number of electrons = Z • Atoms can gain electrons to form anions: • e.g. Add 1 electrons to F gives F- • Add 2 electrons to O gives O2- • Add 3 electrons to N gives N3- • Atoms can lose electrons to form cations: • e.g. Take 1 electron from Na gives Na+ • Take 2 electrons from Mg gives Mg2+ • Take 3 electrons from Al gives Al3+

15. Learning Outcomes: • By the end of this lecture, you should: • know that atoms are made from protons, neutrons and electrons • know the relative masses and charges of these particles • appreciate the relative size of the nucleus and the atom • know how the atomic number and mass number are related to the number of protons, neutrons and electrons • know how to write and read the information on an atomic symbol for a neutral atom, cation and anion • be able to complete the worksheet (if you haven’t already done so….)

16. Questions to complete for the next lecture: • What is the name of the element with atomic symbol K? potassium, 19 • What is the atomic symbol of the element sodium? Na, 11 • What is the name of the element with atomic symbol Ar? Argon, 18 • What is the atomic symbol of the element silver? Ag, 47 • What is the atomic symbol of the element iodine? I, 53

17. Questions to complete for the next lecture: 6. What is the composition of 8X? 16 • Nitrogen with 8 protons and 16 neutrons • Nitrogen with 8 protons and mass of 16 • Oxygen with 8 protons and mass of 16 • Oxygen with 16 protons • Nitrogen with 8 neutrons and 8 protons

18. Questions to complete for the next lecture: • 19 • 6. What is the composition of 9X? • Potassium with 9 protons and 10 neutrons • Potassium with atomic no. 19 and 9 protons • Fluorine with 10 neutrons and mass of 19 • Fluorine with 10 protons and 9 neutrons • Neon with 10 protons and mass of 19