1 of 43 Boardworks Ltd Chemistry 11. Chemical Bonding

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1 1 of 43 Boardworks Ltd 2009 Chemistry 11 Chemical Bonding

2 2 of 43 Boardworks Ltd 2009 Electrostatic Forces An electrostatic force is a forces existing as a result of the attraction or repulsion between 2 charged particles All bonding is based on the experimentally-derived relationships of electrostatics: Opposite charges attract (+ -) Like charges repel (+ + or - -) The greater the distance between two charged particles, the smaller the attractive force The greater the charge on two particles, the greater the force of attraction (or repulsion) between them

3 3 of 44 Boardworks Ltd 2007 Electron Shells REVISITED! The periodic table shows that patterns in the properties of elements are linked to atomic number. What links atomic number and the properties of elements? Electrons! atomic number = number of protons number of protons = number of electrons atomic number = number of electrons As atomic number increases by one, the number of electrons also increases by one. This means that the elements in the periodic table are also arranged in order of the number of electrons.

4 What is periodicity? 4 of 34 Boardworks Ltd 2009 The term periodicity describes a repeating pattern in properties of elements across periods of the periodic table. The Russian chemist Dmitry Mendeleev is credited with being the creator of the first version of the periodic table. He observed that when the elements are arranged in order of atomic mass, there are recurring patterns in certain properties. The modern periodic table can be used to analyse trends in properties such as Atomic Radius across periods and down groups.

5 Atomic Radius Trends 5 of 34 Boardworks Ltd 2009

6 What is atomic radius? 6 of 34 Boardworks Ltd 2009 The atomic radius of an element is difficult to precisely define because of the uncertainty over the size of the electron cloud. Several definitions are used. One definition is half the shortest internuclear distance found in the structure of the element. For non-metallic elements, the covalent radius is often used as the atomic radius. This is half the internuclear distance between two identical atoms in a single covalent bond. covalent radius

7 More on atomic radius 7 of 34 Boardworks Ltd 2009 For non-bonded adjacent atoms (e.g. in a covalent crystal of a non-metallic element), the van der Waals radius is used as a value for atomic radius. This is half the shortest internuclear distance between two similar non-bonded atoms. van der Waals radius For metallic elements, the metallic radius is often used as the atomic radius. This is half the shortest internuclear distance between two adjacent atoms in a metallic bond.

8 Trends in atomic radius in period 3 8 of 34 Boardworks Ltd 2009

9 Trends in atomic radius in period 3 9 of 34 Boardworks Ltd 2009 Element Na Mg Al Si P S Cl Ar Atomic radius (nm) The atomic radius of the elements across period 3 decreases. This might seem counterintuitive, because as the numbers of sub-atomic particles increase, the radius might be expected to also increase. However, more than 99% of the atom is empty space the nucleus and electrons themselves occupy a tiny volume of the atom.

10 WHY? Increase in proton number 10 of 34 Boardworks Ltd 2009 The number of protons in the nucleus of the atoms increases across period 3. proton number Element 11Na 12 Mg 13 Al 14Si 15P 16 S 17 Cl 18Ar This increase in the number of protons increases the positive nuclear charge of the atoms. The nucleus has stronger attraction for the electrons, pulling them in closer and so the atomic radius decreases across the period. increased nuclear charge pulls electrons closer

11 What is shielding? 11 of 34 Boardworks Ltd 2009

12 Explaining atomic radius in period 3 12 of 34 Boardworks Ltd 2009 Element Na Mg Al Si P S Cl Ar Proton number Atomic radius (nm) Proton number increases across period 3, but shielding remains approximately constant. This causes an increase in effective nuclear charge, leading to a greater attraction between the nucleus and the outermost electrons. This pulls these electrons closer to the nucleus and results in a smaller radius.

13 15 of 44 Boardworks Ltd 2007 How are electrons arranged? Electrons are arranged in shells around an atom s nucleus. (also called energy levels-principle quantum number n 1 ). Each shell has a maximum number of electrons that it can hold. Electrons will fill the shells nearest the nucleus first. 1 st shell holds a maximum of 2 electrons 2 nd shell holds a maximum of 8 electrons This electron arrangement is written as 2,8,18. 3 rd shell holds a maximum of 18 electrons

14 16 of 44 Boardworks Ltd 2007 Electrons in period 1 Elements in period 1 only have electrons in the first shell. Why are there only two elements in period 1? H He 1 2 The first shell can only hold a maximum of two electrons, so period 1 only includes the elements hydrogen and helium. What is special about the outer shell of helium?

15 17 of 44 Boardworks Ltd 2007 Electrons in period 2 Elements in period 2 all have a complete first shell. What happens to electrons in the second shell in period 2? Li Be B C N O F Ne 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8 The second shell is completed one electron at a time going across the period from left to right. What is special about the outer shell of neon?

16 18 of 44 Boardworks Ltd 2007 Electrons in period 3 Elements in period 3 have complete first and second shells. What happens to electrons in the third shell in period 3? Na Mg Al Si P S Cl Ar 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8 The third shell is completed one electron at a time going across the period from left to right. What is special about the outer shell of argon?

17 19 of 44 Boardworks Ltd 2007 What are the patterns of electron arrangement? Consider the electron arrangements of the first 20 elements in the periodic table ,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8 4 2,8,8,1 2,8,8,2 *Remember will fill 4s 2 before we complete the 3d subshell What is the pattern of outer shell electrons in a group? What is the pattern of outer shell electrons across a period? What is the pattern of full electron shells in a group?

18 Electron trends in the periodic table 20 of 44 Boardworks Ltd 2007 Trends down a group: the number of outer shell electrons is the same; the number of complete electron shells increases by one. The number of a group is the same as the number of electrons in the outer shell of elements in that group, except for group 0. Trends across a period: the number of outer shell electrons increases by one; the number of complete electron shells stays the same. The point at which a new period starts is the point at which electrons begin to fill a new shell.

19 21 of 44 Boardworks Ltd 2007 Valence Electrons Revisited Valence electrons are: All electrons in an atom except those in the [core] or in filled d- or f- subshells / orbitals All electrons in open shells An open shell is a shell containing less than it s maximum number of electrons Valence electrons are considered reactable electrons

20 Electrons and groups 22 of 44 Boardworks Ltd 2007

21 Glossary 24 of 44 Boardworks Ltd 2007 atomic number The number of protons in an atom. Sometimes called the proton number. electron arrangement A shorthand way of writing the number of electrons in an atom s electron shells. element A substance made up of only one type of atom. group A column in the periodic table containing elements with the same number of outer shell electrons and similar chemical properties. period A row in the periodic table containing elements with the same number of full electron shells. periodic table The table that lists all the elements in order of increasing atomic number, arranged into groups and periods. property Any characteristic of an element.

22 Complete Assignment #1 25 of 43 Boardworks Ltd 2009