User:Phaello/sandbox/Chemistry/SCI 3204P-B/Bonding

Chemical Bonding

Key points

Atoms can react with one another to form new substances called compounds The resulting compound is both chemically and physically different from the parent atoms e.g Sodium (a metal that starts burning on contact with water) and Chlorine (a poisonous gas) react to form NaCl – table salt that we consume Sodium is a silver-coloured metal, Chlorine is a greenish gas – while NaCl compound is a white solid

Key points

Chemical bonds form between atoms because electrons from the atoms interact Most atoms become stable when they contain 8 electrons in the outermost shell Atoms with fewer valence electrons than 8, bond together to share electrons to complete their valence shells There are two main types of bonding – ionic bonding and covalent bonding

Ionic Bonding

Key points

In ionic bonding electrons are transferred from one atom to another The reacting atoms form irons e.g. in the reaction of sodium and chlorine The ions formed attract An ionic bond is formed

Covalent Bonding

Key points

Occurs when atoms share electrons When two or more atoms share electrons This occurs mainly with non-metals The atoms share electrons in an attempt to complete their valence shells For example hydrogen atoms – each has one electron on the valence shell - They share each other's electrons to complete valence shells – and form H2 molecule Chlorine – each has 7 valence electrons – they share each other's electron to complete valence shells – forming Cl2 molecule - Hydrogen and Chlorine – H has 1 valence electron, Cl has 7 valence electrons – they each share one electron to complete their valence shells – forming HCl - Compounds with covalent bonding exist as true molecules – unlike ionic bonded compounds For every pair of electrons shared between atoms a single covalent bond is formed e.g. H and H bond is formed by sharing one electron from the 1st H atom, and another one from the 2nd H atom – so a pair of electrons is shared (one electron from each atom) – a single covalent bond is formed. Some atoms can share many pairs For example oxygen atoms need two electrons to complete their valence shells - They share two pairs of electrons Forming a double bond

Key points

Carbon has 4 valence electrons while H has one H needs to share one of its own electron with C also sharing one of its own Each one of the H atoms forms a single covalent bond with Carbon – 4 covalent bonds

Polar and Non-polar Covalent Bonds

Key points

Electronegativity Electronegativity is the ability of atoms to attract electrons towards themselves Elements on the left hand side of the periodic table are less electronegative than those on the right As you move across the period, from left to right, electronegativity increases As you move down the group the electronegativity decreases Periodic Table Showing Electronegativity values of elements

Activity

Why does electronegativity of elements increase as you move across the period from left to right? Why does it decrease as you move down the group?

Key points

H2, O2, N2, Cl2 are good examples of molecules with non-polar covalent bonds Because in each molecule the atoms have the same electronegativity A polar covalent bond is formed when bonding electrons are not shared equally i.e. when there is a small difference in electronegativity of the bonding atoms e.g. water molecule The more electronegative atom (oxygen in the case of H2O) will pull shared electrons more towards itself, hence becoming slightly negative The less electronegative atom (hydrogen atoms in the case of H2O) will be slightly positive because electrons are being pulled more towards the more electronegative atom

Activity

How do electronegativity values of oxygen and hydrogen compare?

Refer to the following websites for more information

Reading

http://www.elmhurst.edu/~chm/vchembook/150Anpcovalent.html http://users.stlcc.edu/gkrishnan/polar.html http://hubpages.com/hub/Polar-And-non-polar-Covalent-Bonding