1-3 Periodic Table

III. Periodic Table

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Topic

III.

Periodic Table

Reading Assignment

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 46 – 55

Objectives

1.3 – recognise that chemical similarities exist among certain elements as illustrated by (a) sodium and potassium (b) magnesium and calcium (c) chlorine, bromine and iodine – understand that chemical properties of elements are related to the electronic arrangements of their atoms – understand that the Periodic Table is (a) an arrangement of elements in an order of increasing atomic numbers (b) divided into vertical columns called groups and horizontal rows called periods (c) a table in which elements having similar chemical properties are put together in groups – relate the positions of the elements in the Periodic Table to their electronic arrangements – recognise the pattern for the positions of metals and non-metals in the Periodic Table – predict the chemical properties of unfamiliar elements in groups I, VII and 0 – explain the change in the reactivity going down a group.

Notes

III. Periodic Table A. Electronic configuration and chemical properties There are many different elements in the world. It would be quite difficult to study all of them one by one. Scientists try to find out whether there is any pattern of reactivity among the elements. They have observed that some elements behave similarly. For examples, the followings are some of the pairs. 1.

Sodium and potassium Both of them react with water vigorously and give out a lot of energy and hydrogen gas

2.

Magnesium and calcium Both of them react with acid moderately and give out hydrogen gas.

3.

Chlorine, bromine and iodine Chlorine solution is yellowish green in colour, bromine solution is reddish brown in colour and iodine solution is brown in colour. All of them are decolorized by sodium sulphite solution (a reducing agent).

According to the electronic configuration, it was found that, sodium and potassium, magnesium and calcium, chlorine and bromine, all had the same no. of outermost shell electrons (valence electron). Group I Group II Group VII

11Na 12Mg 17Cl

2, 8, 1 2, 8, 2 2, 8, 7

19K 20Ca 35Br

2, 8, 8, 1 2, 8, 8, 2 2, 8, 18, 7

Base on this, scientists grouped the elements with the same no. of outermost shell electrons together. A modern periodic table was then created. N.B.

Indeed, the periodic table was developed by the scientist Mendeléev in 1867. The electronic configuration of atoms were only discovered by Bohr in 1913. See how clever Mendeléev was.

III. Periodic Table

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B. Structure of periodic table

1.

Position of metals and non-metals

In general, metal elements belongs to group I to III, while non-metals elements belong to group IV to 0 in the periodic table. Some of the groups of elements have special name. Group I Group II Group VII Group 0 2.

Alkali metal Alkaline earth metal Halogen Noble gas

Position and electronic configuration

In the periodic table, elements are arranged according to the atomic number (no. of proton). Because an atom is electrically neutral, the no. of proton and the no. of electron must be the same. Therefore, the electronic configuration and reactivity of an atom can be determined indirectly through the atomic number. Each vertical column in a periodic table is called a group, all elements in the same group have the same no. of outermost shell electron, thus similar chemical reactivity Each horizontal row in a periodic table is called a period, all elements in the same period have the same no. of electron shells. e.g. both 11Na 2, 8, 1 and 17Cl 2, 8, 7 have 3 electron shells and belong to period 3. Group no. = no. of outermost shell electron Period no. = no. of occupied electron shell

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The table is called periodic table because the behaviour of elements follows a repeating pattern. The first two elements, hydrogen and helium, are the exceptional ones. Atomic no. Element Metallic properties metal Physical state

1 H

2 3 4 5 He Li Be B metal solid

6 7 C N → →

8 9 10 11 12 13 14 15 16 17 18 19 20 O F Ne Na Mg Al Si P S Cl Ar K Ca non-metal → metal → non-metal → gas → solid

→

gas → solid

C. Prediction of chemical properties Reaction means change. A reactant is said to be reactive if it changes to the product quickly and its reactivity is said to be high. Indeed, in most reactions, the reactant is less stable than the product. For example, in the reaction, sodium + water → sodium hydroixde + hydrogen, the reactants (sodium and water) is less stable than the products (sodium hydroxide and hydrogen).

Why does sodium react with water? Stability of atom and electronic configuration of noble gas It is observed that there is a special stability associated with the completely filled outermost shell, i.e. electronic configuration of a noble gas. Group 0 elements are called noble gases because of their extremely low reactivity. They all have completely filled outermost electron shells (valence shell). From this experimental fact, we know that there must be certain connections between the reactivity and the electronic configuration of an element. Noble gas Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr)

Electronic configuration 2 2, 8 2, 8, 8 2, 8, 18, 8

Except helium, all noble gases have 8 electrons in their outermost shell (valence shell) where there is only 2 in helium. The group of 2 electrons in the first electron shell is called duplet and the group of 8 electrons in the outermost shell is called octet (oct - eight, tet - electron).

III. Periodic Table 1.

Page 4 Group trend - reactivities of different elements in the same group.

Group 0 - noble gas The first few noble gas - helium, neon and argon do not form any compound at all. The heavier noble gas, e.g. krypton and xenon, form compound with the very reactive fluorine under controlled condition. The reactivity of noble gas is beyond the scope of certificate level.

Group trend of noble gas - Reactivity of noble gas increases down a group.

Reactivity of metal and non-metal The reactivity of a metal or non-metal atom is depending on how easily it can achieve a stable electronic configuration like a noble gas. e

-

weak attraction

-

e

strong attraction

nucleus

nucleus

Big atom

Small atom

For a metal atom which has either 1, 2 or 3 outermost shell electrons, it acquires the electronic configuration of a noble gas by losing electron to the surrounding. Therefore, a big metal atom is more reactive than a small metal atom. Conversely, for a non-metal atom which has either 5, 6 or 7 outermost shell electron, it acquires the electronic configuration of a noble gas by gaining electron from the surrounding. Therefore, a small non-metal atom is more reactive than a big non-metal atom. Group I - alkali metal Lithium, sodium and potassium show similar chemical properties but the reactivities are not identical. Potassium is more reactive than sodium and sodium is more reactive than lithium. Upon reaction, an alkali metal atom loses its electrons to the surrounding and becomes a positive ion (cation ion) which has a completely filled outermost shell.

Because the outermost shell electron in potassium is much further from the nucleus than the one in sodium, the attraction between the electron and the potassium nucleus would be weaker and potassium loses the electron more readily. This accounts for the high reactivity of potassium comparing with sodium. Group trend of metal - Reactivity of metal increases down a group.

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Group VII - halogen Chlorine, bromine and iodine also show difference in chemical properties. Chlorine is more reactive than bromine and bromine is more reactive than iodine. Unlike metal, non-metal reacts by gaining electron from the surrounding. Halogen gains an electron and become a negatively charged ion (anion ion) which has a completely filled outermost shell.

Since the electron will be added to the outermost shell and the outermost shell of chlorine is closest to the nucleus. The attraction between the incoming electron and the nucleus will be greatest in chlorine. This makes chlorine gain the electron more readily and has the highest reactivity among the three. Group trend of non-metal - Reactivity of non-metal decreases down a group. 2.

Octet rule

Octet rule - in a chemical reaction, the valence electrons of an atom are redistributed in a way that octet is attained. Octet rule is only applicable to a limited number of elements and situations. e.g. from carbon to calcium. It is also only a rule which helps us to predict what will the stable electronic configuration be. It doesn’t tell us why there must be only 8 electrons in the outermost shell, but not 9 or 10. We will look at the examples which do not obey octet rule and understand the rationale behind the octet rule in Form 6. In the meantime, octet rule serves as a very useful tools for us to predict the outcome of a reaction. For most reactions, the product is more stable than the reactant. A more stable state is always preferred in nature. e.g. an object always falls from the height. Normally, an atom without a completely filled outermost shell will try to acquire one by gaining, losing or sharing electrons (redistribution of electrons) with other atoms. Furthermore, in a chemical change, a new substance is formed. Because it is a new substance, the arrangement of atoms must be different from that of the reactant. Reactant → (With old arrangement of atoms)

Product (With new arrangement of atoms)

Therefore, all chemical reactions can be interpreted as rearrangement of atoms caused by redistribution of electrons.

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D. Electronegativity and electropositivity Different atoms have different affinity to (attraction towards) electron. This affinity to electron is called electronegativity. Non-metal atom has high electronegativity e.g. F, O, N, Cl, i.e. non-metal atom reacts by gaining electron. While metal atom has low electronegativity (or high electropositivity) e.g. Cs i.e. metal atom reacts by losing electron.

H has an electronegativity similar to C.

Glossary

pattern noble gas octet rule

Past Paper Questions

91 I 1 a i 93 I 4 a i 94 I 1 a 95 I 2 a

reactivity periodic table alkali metal alkaline earth metal group period group no. period no. group trend redistribution of electron rearrangement of atoms

halogen cation anion

91 I 1 a i 1 1a The following is a part of the Periodic Table: Group I

i

C

II

III

IV a

V VI VII 0 Second Period b Third Period c d e f g Referring to the letters indicated in the above table, answer the following questions (1) What is the name for the family of elements of which b and f are members ? 3 (2) In what way are the electronic arrangements of the atoms of elements b and f (I) similar to each other ? (II) different from each other ? (1) Halogen(s) 1 mark (Correct spelling is required.) (2) (I) Each has 7 electrons in its outermost shell / they have the same no. of electrons in their outermost shells. (Do not accept outer shell.) 1 mark (II) f has 8 electrons more than b / f has one (inner) electron shell more than b / f and b have different no. of electron shells / f has 3 electron shells and b has 2 1 mark (Do not accept layer / outer shell / different no. of electrons.) [1 mark for (2) , if only correct electronic structures of b and f are given as answers. ] (2) (I) 'Outermost shell' was often written as 'outer shell' in the answers.

III. Periodic Table Page 7 93 I 4 a i 4a The following table gives some information about W, X, Y and Z which represent particles of some elements. These particles are either atoms or ions. Mass number Atomic number No. of proton No. of electron No. of neutron W 12 12 12 X 12 10 12 Y 35 17 17 Z 17 17 20 i In which group of the Periodic Table should W be placed ? Explain your answer. 2 Group II 1 mark because W has 2 electrons in its outermost shell / W is magnesium(Mg) 1 mark 94 I 1 a 1 The table below lists some information about three metals X, Y and Z . Metal X Y Atomic number 12 20 Action of cold water No apparent change A colourless gas slowly evolves Action of 0.1 M A colourless gas evolves. — hydrochloric acid 1a To which group in the Periodic Table does Y belong ? Group II / Group IIA / Group 2 / Group two (Do NOT accept alkaline-earth metal.)

Z — No apparent change No apparent change 1 1 mark

95 I 2 a 2 In each of the following groups of substances, there is ONE substance which is different from the others in terms of their properties. In each group, identify the substance which is different from the others and explain your choice. 2a argon, fluorine, helium, neon 2 fluorine 1 mark fluorine is reactive / yellow / (colourless gas) / can form diatomic molecules 1 mark or, others are inert / unreactive / stable / colourless (gases). (Do not accept F2 is halogen while others are noble gases.) C Candidates were asked to identify the one substance which is different from the others and to explain the difference in terms of their properties However, many candidates overlooked the word 'properties' in the question. Most candidates simply mentioned that fluorine is a halogen while the others are noble gases.

III. Periodic Table 91 1 Directions : Questions 1 and 2 refer to the following table. Element W X Y Z Atomic number 4 8 14 20 B 1 Which of the following elements are likely to be metals. A. W and X B. W and Z C. X and Y D. Y and Z 92 1 B

1

Rubidium (Rb) is a group I element below potassium in the Periodic Table. Which of the following statements about rubidium is correct ? A. Rubidium forms an acidic oxide. B. Rubidium is more reactive than potassium. C. Rubidium can be obtained from its oxide by reduction with carbon. D. The formula for rubidium chloride is RbCl2.

C

1

Magnesium and calcium have similar chemical properties because A. their atoms have the same atomic structure. B. their atoms have the same number of electron shells. C. their atoms have the same number of electrons in their outermost shells. D. their atoms have the same electronic arrangement.

D

1

The chemical properties of an element depend on A. its relative atomic mass. B. the number of isotopes of the element. C. the number of electron shells in its atoms. D. the number of outermost shell electrons in its atoms.

96 1

97 1

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