1. Atoms, Ions & Molecules

2. Recall from Sec 1… Recall what it means to be a model We have explored the particulate model However, one of your classmates have given an example: When water is freezing (from 4 to 0 °C), it expands instead of contracting The particulate model cannot explain this! We need a new model to supplement the particulate model….

3. Recall from Sec 1… This model is called the Atomic-Molecular Model (Mr Wong’s own notation) Many many years ago, Ancient Greeks proposed that everything on Earth is made up of 4 elements: Earth, Fire, Air, Water They believe that all other elements (e.g. animals, people etc.) are composed of some combination of these 4 elements. Ancient China/Japan has 5 elements: Wood, Fire, Earth, Metal, Water

4. Recall from Sec 1… By today, Scientist have found that instead of 4 or 5 elements, there are 91 naturally occurring elements Scientists have further managed to manufacture 27 man-made elements, bringing the list to a total 118 elements. (note: this number may change in the future)

5. Recall from Sec 1… All matter is composed of elements That means that we can break down everything in the universe into one of the 91 naturally occurring elements Some common elements you have already heard of: oxygen, carbon, hydrogen, iron, silver, gold

6. Element Composition of Human Body

7. Recall from Sec 1… The concept of the atom was proposed by the ancient Greek philosophers They thought: we can always cut something in half, and that thing in half, and that thing in half, etc. Will we ever reach a point where we can no longer cut something apart? The smallest particle which cannot be cut apart is called an Atom (Greek: Atomos) meaning “indivisble”

8. Recall from Sec 1… Back then, Greek believed in the 4 elements, so they thought that when we cut down substances small enough, we will get tiny particles of Earth, Fire, Water or Air But now, we know that there are 91 elements, when we cut down something small enough, we will get one of 91 types of particles. Each small particle is called an Atom

9. Recall from Sec 1… How atoms interact with their neighbouring atoms is key to understanding how the atomic- molecular model works (and in general, how Chemistry works) It turns out that there are some substances which are made of a fixed ratio of elements. For example, we can split water up into 2 elements: Hydrogen and Oxygen. But when we do so, we notice that there is twice as much hydrogen as oxygen

10. Recall from Sec 1… Scientists propose that a single water particle is a combination of 2 hydrogen atoms and 1 oxygen atom. This is expressed as H 2 O (this is called the chemical formula) Substances which are made of such a combination of elements are called compounds Some other common compounds: salt (sodium chloride, NaCl), glucose sugar (C 6 H 12 O 6 ), sucrose sugar (C 12 H 22 O 11 )

11. Two kinds of particles One H 2 O particle is called a molecule One H 2 O molecule is made of 3 atoms – two hydrogen atoms and one oxygen atom Note that the terms “compound” and “element” is not the same as “molecule” or “atom”. They do not refer to particles. 1 kg of water is one compound, but it contains many many molecules of water! Similarly 1 kg of iron is one element, but it contains many many atoms of iron!

12. NOW FOR SEC 2 SCIENCE….

13. Topics Looking Inside an Atom Electronic Configuration Molecules & Chemical Formulae

14. LOOKING INSIDE AN ATOM

15. Electrical Charges Like mass and volume, charge is a physical quantity of an object Similar to North Pole and South Pole of a magnet, there are two kinds of charge. Scientists decided to call them positive and negative charge Just like N and S poles, unlike charges attract each other, while like charges repel each other

16. Subatomic Particles Recall in Sec 1 we said an atom was the smallest possible particle and we cannot divide it further Turns out we were wrong! (lol trolled) Atoms are actually made of 3 smaller particles: protons, electrons and neutrons Scientists call these subatomic particles (i.e. particles smaller than atoms)

17. Subatomic Particles The proton has a positive charge, the electrons have a negative charge while neutrons are neutral (no charge) All 3 subatomic particles are very small, but the smallest is the electron The proton and neutron are the same size, 2000 time larger than an electron Protons and Neutrons are concentrated in the center of the atom (called the Nucleus) while electrons orbit around the Nucleus.

18. Carbon Atom

19. Subatomic Particles In a normal atom, the number of protons = number of electrons, and the charges balance out How many protons inside an atom determine what element that atom is E.g. if an atom has 1 proton, it is a Hydrogen atom. If it has 6 protons, it is a Carbon atom.

20. Subatomic Particles Recall that the proton and neutron are much heavier than the electron Hence, when considering the mass of an atom, we only count the number of protons and neutrons, and ignore the electrons. We call this the mass number E.g. a carbon atom with 6 protons and 6 neutrons has a mass number of 12

21. Subatomic Particles The number of neutrons in each atom is not fixed Atoms which have the same number of protons but different number of neutrons are called isotopes E.g. most carbon atoms have 6 protons & 6 neutrons (i.e. carbon-12), but there exist carbon atoms with 7 and 8 neutrons!

22. Nuclear Notation An atom can be represented using the nuclear notation, which consists of two numbers and the chemical symbol

23. Test Yourself Express the following in nuclear notation: 1.Hydrogen (1 proton) 2.Hydrogen (1 proton & 1 neutron) 3.Nitrogen (7 protons & 7 neutrons) 4.Nitrogen (7 protons & 8 neutrons)

24. Electronic Configuration As we move across the periodic table, atoms have more and more electrons All the electrons do not orbit at the same radius When the number of electrons increases, they form different layers of orbits – we call these “shells” The shells closer to the nucleus must “fill up” first before the outer shells fill up

25. Electronic Configuration The first shell (n = 1) has a capacity of 2 electrons The second shell (n = 2) has a capacity of 8 electrons The third shell (n = 3) has a capacity of 8 electrons E.g. carbon has 6 electrons. 2 electrons goes into the n=1 shell, while the remaining 4 goes into the n=2 shell. There is still 4 vacancies remaining in the n=2 shell.

26. Test Yourself What is the electronic configuration of the following? 1.Oxygen (8 electrons) 2.Neon (10 electrons) 3.Sodium (11 electrons) 4.Chlorine (17 electrons)

27. Some Observations (part 1) What do these 3 elements have in common: Helium (2 electrons), Neon (10 electrons) and Argon (18 electrons) Can you locate these 3 elements in the periodic table? What did you observe?

29. Noble Gases These 3 elements are all in the same group in the periodic table Elements in these group are known as “noble gases” These elements are extremely inert (does not form chemical reactions easily), and when they are in gas form, exist as monoatomic molecules. Why do you think these elements are so inert? When a the outermost electron shell (i.e. valence shell) is full, the atom is said to have an duplet (helium) or octet (neon, argon) structure.

30. Some Observations (part 2) What do these 3 elements have in common? Lithium (3 electrons), Sodium (11 electrons), Potassium (19 electrons) Each of these elements are very reactive (they react with water vigorously, some even explode) Can you locate these elements in the periodic table? What did you observe? Why do you think these elements are so reactive?

32. Group I Metals These elements are called Group I metals, or alkali metals (they form alkali when they react with water) Notice that each of these elements have exactly one electron more than a duplet/octet structure Since atoms like to be in a duplet/octet structure, these elements tend to want to lose their (one) outermost electron, to form a duplet/octet structure When they do so, they will form a 1+ positive ion

33. Positive Ions When an atom loses electrons, it becomes a positive ion (i.e. it has more protons than electrons) Na → Na + + e - What do you think happens when a Magnesium atom (12 electrons) tries to form an octet structure?

34. Transition Metals Group I metals often form 1+ ions Group II metals often form 2+ ions However, there is a group of metals (in the middle of the periodic table) which are capable of forming multiple ions E.g. Iron can form either Fe 2+ or Fe 3+ ions. These metals are called “transition metals” We use roman numerals to indicate how many electrons they have lost, e.g. Iron (II), Iron (III).

35. Test Yourself Can you write the chemical equation for the following formation of ions? 1.Lithium metal to Lithium ion 2.Calcium metal to Calcium ion 3.Lead metal to Lead(II) ion 4.Lead metal to Lead(IV) ion Qn: you have heard that your body needs iron and calcium to be healthy. Do you think it refers to iron and calcium metal or ions?

36. Some Observations (part 3) What do these 2 elements have in common: Fluorine (9 electrons), Chlorine (17 electrons) Can you locate these elements in the periodic table? What did you observe? Can you predict what kind of ions these elements would form?

38. Halogens (Group 7) Elements in Group 7 are called halogens They are elements which are just missing one electron to form an octet structure Hence, they want to accept one more electron Doing so would form a 1- ion

39. Negative Ions When an atom accepts electrons, it becomes a negative ion (i.e. it has more electrons than protons) Cl + e - → Cl - What do you think when an Oxygen atom (8 electrons) form an ion? Group 7 elements form 1- ions Group 6 elements form 2- ions

40. CHEMICAL FORMULAE

41. Molecules Recall that molecules are made up out of 2 or more atoms chemically bonded together For elements in gaseous form, many of them form diatomic molecules (e.g. H 2, O 2, Cl 2, etc.) For compounds, molecules contain a fixed number of atoms joined together (e.g. H 2 O, CO 2, C 6 H 12 O 6 ) The symbols in red are called chemical formulae

42. Chemical Formula Note that in each chemical formula, there is a subscript number at the bottom right of an element symbol This number represents the number of atoms in that molecule. If there is no number present, then there is only 1 atom.

45. BALANCING CHEMICAL EQUATIONS

46. A simple math question… A bowl of fruit salad requires half an apple and one orange. What is the minimum number of bowls of fruit salad I can make without wasting any fruits? How many apples and how many oranges do I need?

47. Another simple math question… One molecule of ZnO requires half a molecule of O 2 (i.e. one atom) and one atom of Zn to form. What is the minimum number of ZnO molecules I can form without “wasting” any O 2 atoms? (fill in Q1 in Pg 8 of your notes)

48. Balancing Chemical Equations There are several ways of thinking to solve balancing equation problems. Here is one suggested method: 1) Put 1 molecule each at the left hand side of the equation (i.e. the reactants) 2) Determine the right hand side of the equation, use fractions if necessary 3) Multiply throughout by the denominator of the fraction so that all fractions are removed 4) Double check all atoms to make sure no atoms were “left over”

49. Page 8 Qn 2 ___Na + ___ H 2 O → ___NaOH + ___H 2 1 Na + 1 H 2 O → ___NaOH + ___H 2 1 Na + 1 H 2 O → 1 NaOH + ½ H 2 2 Na + 2 H 2 O → 2 NaOH + 1 H 2 Count: number of Na, O and H atoms on both sides Final ans: 2 Na + 2 H 2 O → 2 NaOH + H 2

50. Page 8 Qn 3 ___HNO 3 + ___ZnCO 3 →___ Zn(NO 3 ) 2 + ___CO 2 + ___H 2 O 1 HNO 3 + 1 ZnCO 3 →___ Zn(NO 3 ) 2 + ___CO 2 + ___H 2 O doesn’t work! Look closely at the Zn(NO 3 ) 2 molecule, does it give you a hint on how to proceed? (hint: recall the fruit salad) 2 HNO 3 + 1 ZnCO 3 → ___ Zn(NO 3 ) 2 + ___CO 2 + ___H 2 O 2 HNO 3 + 1 ZnCO 3 → 1 Zn(NO 3 ) 2 + 1 CO 2 + 1 H 2 O Double check number of atoms of H, N, C, O on both sides of the equation.

51. Chemical Equations Try Qn 4 to Qn 8 by yourself!