Chemistry The origin of chemistry can be traced to alchemy, or the art of converting metals like copper to gold.
Bonds • What is a chemical bond? • Electrostatic forces of attractions (between 2 atoms) between the nuclei of one atom and the electrons of the other atom. • Why do atoms bond? • Three kinds of bonding: • Ionic • Covalent • Metallic
Noble Gas Also called the inert gases or rare gases: He, Ne, Ar, Kr, Xe and Rn. Noble gases are unreactive. Exist as individual atoms, monatomic. Is there a need to bond? Are there noble gas compounds?
Noble gas compounds • Examples: • Xenon tetrafluoride (XeF4) • Xenon tetroxide (XeO4) • Krypton difluoride (KrF2) • Radon difluoride (RnF2) • Xenon trioxide (XeO2) • Notice what these compounds contain?
Noble Gas Structure Duplet or octet configurations are most stable. Also known as a noble gas configuration. Common feature – fully filled valence electron shell He Ne
Why atoms combine • Atoms WANT to achieve the noble gas configuration. • How do atoms achieve the noble gas structure? • Transferring or sharing electrons.
Recap – ions • Normally, an atom is electrically neutral • number of protons = number of electrons • An ion is formed when an atom loses or gains electrons. • An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons.
Cations Positive ions (cations) are formed by removing/ losing electrons from atoms. Loss of electrons tends to occur in atoms with few valence electrons (e.g. 1, 2 & 3) Notice that these are METALS.
Cations Electronic configuration: 1s22s22p63s1 1s22s22p6 Na Na++ e- Na Na + + e-
Anions Negative ions (cations) are formed by gaining electrons from atoms. Gain of electrons tends to occur in atoms with larger number of valence electrons (e.g. 5, 6 & 7) Notice that these are NON-METALS.
Anions Electronic configuration: 1s22s22p63s23p5 1s22s22p63s23p6 Cl+ e- Cl- - Cl + e- Cl
Discuss Consider sodium atom and sodium ion. Which do you think is bigger and why? • Consider chlorine atom and chloride ion. Which do you think is bigger and why? Na Cl- Na+ Cl 186 pm 106 pm 181 pm 100 pm
Trends in radii Atoms are always larger than any of their cations. Atoms are always smaller than any of their anions. http://chewtychem.wiki.hci.edu.sg/Ionic+Bonding
Quickcheck Is Mg+ or Mg2+ bigger? Is O- or O2- bigger?
Ionic Bond An ionic bond is a chemical bond formed by the electrostatic attraction between the positive and negative ions. Formation of an ionic bond can be viewed as a transfer of electrons from a metallic atom to a non-metallic atom. Both will gain a duplet or octet configuration.
Video Link to video
Ionic bond Two processes occuring: Na Na + + e- - Cl + e- Cl
Dot and Cross Diagram - + Na Cl Na 1s22s22p63s1 Cl 1s22s22p63s23p5 Na+ 1s22s22p6 Cl- 1s22s22p63s23p6 Isoelectronic with Isoelectronic with Neon: 1s22s22p6 Argon: 1s22s22p63s23p6
Dot and Cross Diagram Magnesium Chloride - 2+ - Cl Cl Mg Aluminium Oxide 3+ 2- 2 3 O Al
Is this possible? 7- + 7e- Na Na 1s22s22p63s1 1s22s22p63s23p6 (Ar) 7+ Cl + 7e- Cl 1s22s22p63s23p5 1s22s22p6 (Ne)
Chemical formulae • Take for example the following: • An ionic compound made of Magnesium and Fluorine. F- Mg2+ F- F- Mg2+
Chemical formulae • For Magnesium fluoride, the ions present are Mg2+ and F-. • Mg2+ has 2 positive charges • F- has 1 negative charge. • To make the overall compound electricallyneutralthere must be two F- to balance one Mg2+. • The formula is MgF2.
Chemical formulae Mg2+ F- Mg1 F2 MgF2 Na+ OH- Na1 OH1 NaOH
Find the chemical formula • Magnesium oxide • Zinc chloride • Calcium hydroxide • Iron(II) fluoride • Iron(III) sulfate • Ammonium nitrate • Silver chloride • Potassium iodide • Manganese(IV) oxide
General knowledge General knowledge 正离子 = cation 负离子 = anion 离子键 = ionic bonding 共价键 = covalent bonding 金属键 = metallic bonding Now you can hao4 lian4 to your friends!
Ionic lattices In ionic compounds, ions are held in fixed positions in an orderly arrangement by strong electrostatic forces (or ionic bonds) between the cations and anions.
Ionic lattices Attractions are maximised in this structure while repulsions are minimised. How many Cl- surround one Na+ and vice versa? The coordination number is the number of nearest neighbours (atoms, ions or molecules). What are the coordination numbers of Na+ and Cl-?
Ionic Lattices Different ionic lattice structures exist. http://www.avogadro.co.uk/structure/chemstruc/ionic/g-ionic.htm
Properties High Melting and Boiling Points Hard and brittle Conducts electricity when dissolved in water or when molten Many ionic compounds are soluble in water or polar solvents like alcohol, but insoluble in most non-polar solvents like hexane WHY?
Properties - NaCl High Boiling and Melting Point due to breaking of strong ionic bonds Is brittle due to repulsion between similarly charged ions Conductor of electricity in liquid state and when in solution due to presence of free moving charges.
Linus Pauling Only person to have won two unshared Nobel Prizes (Chemistry and Peace) The Nature of the Chemical Bond and the Structure of Molecules and Crystals Pauling Electronegativity Scale
Electronegativity Click this link for the electronegativity table. http://chemwiki.ucdavis.edu/@api/deki/files/4756/=electronegativity_chart.png • Every atom has an attraction for the electrons shared in a bond. Why?
Electronegativity • Degree of attraction can be related to the electronegativity of the atom. • The higher the electronegativity, the more the electrons in a chemical bond are attracted to the atom. The high affinity for electrons of fluorine leads it to direct reactions with all other elements in which the reaction has been attempted, except for helium and neon. What patterns/ trends do you see? Metals generally have low electronegativity values, while non-metals have higher electronegativity values. Electronegativity values generally decrease down the group. Electronegativity generally increases across a period.
Predicting Ionic compounds What do you notice about ionic compounds? What are they made up of?
Metallic Bonding Metallic bonds
Metallic names Ever wondered why some metals have weird symbols in the Periodic table? E.g. Au (aurum) and Hg (hydrargyrum) Aurumactually means ‘shining dawn’. Hydragyrum means watery silver.
Comparing Ionic and Metallic Bonding ? + - Li Li Li Cl
Metallic bonding (I) Metals exist as giant structures too. In metals, atoms are packed closely together in regular three-dimensional patterns to form a giant lattice.
Metallic bonding (II) Platinum atoms Image originally created by IBM Corporation.
Metallic bonding (III) Mobile/ delocalisedelectrons. ‘Sea of electrons’ surrounding the positively charged metal cations. Opposites attract i.e. every positive ion is attracted to the ‘sea of electrons’. e e e e Li+ Li+ Li+ Li+ e e e e Li+ Li+ Li+ Li+ e e e e Li+ Li+ Li+ Li+
Metallic bonding (III) • Forces of attraction between positively charged ions and negatively charged electrons – metallic bonding. • Only found in metals! Not ionic or covalent bonding!
Metallic Bonding Properties of Metals
Malleability and Ductility (I) A malleable substance is one which can be bent or hammered out of shape without breaking. A ductile substance is one which can be stretched or drawn into thin wires without breaking.
Malleability and Ductility (II) The following video shows the malleability of metals. Metals can be forged into different shapes and sizes by beating and hammering Especially useful in making tools and machines.
Malleability and Ductility (III) The following video shows the ductility of metals. Notice that an iron strip can be twisted many times before it finally breaks. Useful in making wires and cables. Why are metals malleable and ductile?
Malleability and Ductility (IV) Metal atoms are arranged in orderly layers. Application of a force causes metal atoms to slide over each other easily. Why does the metal not break easily? Look at the following animation to understand better.
Malleability and Ductility (V) What is this property useful for? Sculpting Wire Tubing Malleability Ductility Aircraft Architecture Machinery
Melting and Boiling Points (I) Metals generally have high melting and boiling points. Most metals are packed closely together and the strong forces of attraction between the positively charged metal ions and the ‘sea of electrons’ result in strong metallic bonding. A lot of energy is required to separate the metal atoms.