GCE Chemistry 1.3 Bonding PDF

FACTFILE: GCE CHEMISTRY 1.3 BONDING Bonding Learning Outcomes 1.3.9 explain that bond polarity arises when covalently bonded atoms have different Students should be able to:...

FACTFILE: GCE CHEMISTRY 1.3 BONDING Bonding Learning Outcomes 1.3.9 explain that bond polarity arises when covalently bonded atoms have different Students should be able to: electronegativities and use partial charges to 1.3.1 demonstrate understanding that ionic show that a bond is polar; bonding is the electrostatic attraction between oppositely charged ions formed by 1.3.10 demonstrate understanding that metallic electron transfer; bonding is the attraction between positive ions and delocalised electrons in a lattice; 1.3.2 construct dot and cross diagrams for ionically bonded compounds, for example elements in Groups I, II, VI and VII, the ions of which have Bonding a noble gas structure; The physical properties of a substance depend on 1.3.3 know that a covalent bond is the electrostatic its structure and type of bonding present. Bonding attraction between a shared pair of electrons determines the type of structure. There are three and the nuclei of the bonded atoms; main types of bond that can occur between atoms: 1.3.4 define the term lone pair; An ionic bond occurs between a metal and a non- metal atom (e.g. NaCl) 1.3.5 describe the coordinate bond as a shared pair A covalent bond occurs between two non-metal of electrons with both electrons supplied by atoms (e.g. I2, CH4 ) one atom, for example the ammonium ion, A metallic bond occurs between atoms in a NH4+; metal (e.g. Cu) 1.3.6 construct dot and cross diagrams for molecules and ions with co-ordinate, single, double and triple covalent bonds; 1.3.7 define the octet rule and state its limitations, for example in BeCl2 and BF3; 1.3.8 define the term electronegativity and explain the trend in the electronegativity of elements across Periods and down Groups; 1 FACTFILE: GCE CHEMISTRY / 1.3 BONDING Ionic Bonding Metals lose electrons to form positive ions (cations) while non-metals gain electrons to form negative ions (anions). The number of electrons gained or lost by an atom is related to the group in which the element is found. Group 1 2 3 4 5 6 7 8 Charge 1+ 2+ 3+ N/A 3- 2- 1- N/A Example Na + Mg 2+ Al 3+ N/A N3- O2- F- N/A Ionic bonding is the electrostatic attraction between oppositely charged ions formed by electron transfer An ionic bond is formed when electrons are transferred from a metal atom to a non-metal atom forming oppositely charged ions. For example, in the formation of magnesium oxide, two electrons are transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom. This is represented in a dot and cross diagram. xx xx Mg O xx → Mg x x 2+ O xx x 2- x Covalent Bonding A covalent bond consists of a shared pair of electrons with one electron being supplied by each atom involved in the bond. The atoms are held together by electrostatic attraction because their nuclei, which have an overall positive charge, are attracted to the shared pair of electrons. A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of bonded atoms. Examples of covalently bonded molecules include methane, CH4 , and water, H2O. H Lone pair H C H H O H H A bonded pair of electrons is a pair of electrons shared between two atoms. A lone pair of electrons is a pair of unshared electron in the outer shell of an atom. Some covalently bonded substances have double bonds , for example CO2 and others have triple bonds for example N2. A co-ordinate bond (dative bond) is a shared pair of electrons between two atoms. One atom provides both electrons. Ammonium ion (NH4+) has a coordinate bond, shown as two crosses in the dot and cross diagram or in the structure diagram as an arrow. + + H H → H N H N H H H H 2 FACTFILE: GCE CHEMISTRY / 1.3 BONDING In some cases there is an unequal attraction for the bonding electrons between two covalently bonded atoms. This is due to the different electronegativities of different atoms. Electronegativity is the extent to which an atom attracts the bonding electrons in a covalent bond. Electronegativity decreases down groups due to the increased distance between the bonding electrons and the nucleus of the bonding atom and the corresponding increase in shielding between the nucleus and outer shell electrons; it increases across periods due to the increasing effective nuclear charge and hence attraction between the nucleus of the bonding atom and the bonding electrons. The most electronegative atom is fluorine. A covalent bond between two atoms of different electronegativities is polar; partial charges develop on the two atoms. A δ+ charge develops on the atom with a lower electronegativity value and a δ- charge develops on the atom with a higher electronegativity value. A polar bond is a covalent bond in which there is unequal sharing of the bonding electrons. For example, the covalent bonds in water are considered to be polar as the oxygen atom is significantly more electronegative than the hydrogen atom. The bonding electrons are attracted more strongly to the oxygen atom and it develops a δ- charge. δ- O δ+ δ+ H H Rather than saying that ionic and covalent are two distinct types of bonding, it is more accurate to say that they are at the two extremes of a scale. no electronegativity large electronegativity difference difference δ- δ+ X–Y X–Y X- Y+ X- Y+ polarisation of covalent bonds polarisation of ionic bonds pure covalent polar covalent polarised ionic pure ionic (favoured by + ions with high charge density – i.e. small/high charge) The octet rule states that when reacting, an atom tends to gain, lose or share electrons to achieve eight electrons in its outer shell. However there are some covalent compounds, which do not follow the octet rule. For example, in beryllium chloride the central beryllium atom does not follow the octet rule as there are only 4 electrons in its outer shell. Cl Be Cl In BF3 the fluorine atom has 8 electrons in its outer shell and follows the octet rule, but boron has 6 electrons in the outer shell and does not follow the octet rule. In SF6 sulfur has 12 electrons in the outer shell and does not follow the octet rule. 3 FACTFILE: GCE CHEMISTRY / 1.3 BONDING Metallic Bonding Metallic bonding is the attraction between positive ions and delocalised electrons. + + + + + layers of positive ions e– e– e e e– – – e– + + + + + e– e– e– e– e– delocalised electrons + + + + + Delocalised electrons are outer electrons which do not have fixed positions but move freely. 4 FACTFILE: GCE CHEMISTRY / 1.3 BONDING Revision Questions 1 Which one of the following contains a co-ordinate bond? A Ammonium, NH4+ B Boron trifluoride, BF3 C Sulfur hexafluoride, SF6 D Water, H2O 2 Fluorine is the most reactive non-metallic element. It combine with both metals and non-metals. Using dot and cross diagrams, explain how stontium atoms combine with fluorine atoms to form strontium fluoride. Show the outer electrons only. 3 Sulfur and fluorine combine to form sulfur hexafluoride, SF6. Sulfur and fluorine atoms have different electronegativity values and form a polar bond but sulfur hexafluoride is non-polar. i) Define the term electronegativity. ii) Label the diagram below to show the polarity of the S–F bond. S–F 5 FACTFILE: GCE CHEMISTRY / 1.3 BONDING Revision Questions 3 iii) Draw a dot and cross diagram to show the bonding in SF6 using outer shell electrons only. iv) Explain whether the SF6 molecule obeys the octet rule. 4 Which one of the following is involved in metallic bonding? A electron delocalisation B electron transitions C gaining electrons to form ions D sharing electron pairs © CCEA 2016 6