THE PERIODIC TABLE THE STRUCTURE OF AN ATOM

THE PERIODIC TABLE

THE STRUCTURE OF AN ATOM

You should by now know the basic structure of an atom, i.e. that it has protons, neutrons, and electrons. The protons and neutrons make up the nucleus. The protons and neutrons do not take any part in a chemical reaction. It is the electrons which are interested in. the electrons are arranged in shells and subshells. Shells must be filled at successive levels with electrons and the sequence has been determined to be 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p You are expected to be able to write the electron configuration for the first 38 elements, and their monatomic ions. You can use the following to determine the structure

1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 7s

The s subshell can hold 2 electrons The p “ “ “ 6 “ The d “ “ “ 10 “ The f “ “ “ 14 “ Some examples of this include Na11 1s22s22p63s1

Cl17 1s22s22p63s23p5

Electron configuration of the transition elements

There are 10 transition elements in period 3 between Calcium and Gallium. Generally these follow the same pattern as above

eg Sc 1s22s22p63s23d14s2 Note that there are two exceptions, with only 1 electron in the 4s orbital. These are Cr and Cu

# Note that the 4s electrons are lost first for transition metal elements only when they form ions.

Electron configuration and the Periodic Table The Periodic Table is the basis for the study of the fundamental properties of the chemical elements and their compounds. The table and the concepts of electron configuration structure, bonding, enthalpy changes and oxidation states provide the principles used to predict and explain chemical behaviour. In block outline form, the electron configurations can be emphasized. Here the groups are blocked according to the outer orbital type

The periodic table is the unifying framework for the study of the chemical elements and their compounds. Elements within each group of the periodic table have similar chemical properties that can be explained in terms of their similar outer-shell electron configurations.

ELECTONEGATIVITY

A term that you will often read about is electronegativity. Electronegativity refers to the affinity an atom had for outer electrons. Or in other word it tells us how strongly the outer electrons are held by the nucleus. A metal is an element which readily gives up its outer electrons, therefore metals are said to have a low electronegativity. Non-metals gain electrons and are said to have a high electronegativity. Metalloids are said to have an intermediate electronegativity. Across each period in the periodic table, from left to right, there is a change from metallic to non-metallic properties of the elements. The reason for this is that the nuclear charge (number of protons) is increasing across a period. Each element has one proton and one electron more than the preceding element, with the added electrons still in the same principle energy level. The effect of this is to pull the outermost electrons closer to the nucleus. Therefore the electronegativity increases. Hence metals tend to lose electrons and non-metals tend to gain electrons. This is why metals tend to end up with positive monatomic ions. Non-metals gain electrons and hence they end up with negative monatomic ions. Below is a table of electronegativity values, notice that Flourine has the highest electronegativity and Cesium has the lowest

Likely Oxidation States

Gp 8 has elements with eight outer electrons, s2p6. this configuration gives stability to the atoms of these elements and the idea of a full s2p6 configuration is an essential part of chemical bonding theory. Therefore, in terms of predicting likely oxidation states most elements will follow a pattern of trying to get a s2p6 electron structure. Eg Na 1s22s22p63s1 will lose the 3s1 and become 1s22s22p6 Therefore its oxidation number will be +1 Cl 1s22s22p63s23p5 will gain an electron to become 1s22s22p63s23p6

Therefore its oxidation state will be –1 The above is called the octet rule Second row elements never exceed the octet rule, because they only have 2s and 2p orbitals which can only hold 8 electrons. Some third row elements do not satisfy the octet rule. Phosphorus, sulphur and chlorine exceed this rule. They can share all of their outer shell electrons They do this by using their empty d orbitals to share electrons. So molecules like SF6, PCI5, are possible. Likely oxidation states of each group are shown below

Group Charge Octet

Expansion Other states in

compounds

I +1

II +2

III +3 -3

IV +2 +4 -4

V -3 +5 +3

VI -2 +4 +6 +2

VII -1 +3 +5 +7 +1

Metal Oxides It is through its chemistry that an element is classified as a metal, metalloid or non-metal. One example of the contrast in chemistry of the elements of period 3 is the nature of their oxides. Metallic oxides are solids that react with acids and are termed basic oxides. Metals form ionic oxides. These are basic because the oxide ion O2-, is present. Eg, Sodium with low electronegativity loses its valence electron to the oxygen, which has high electronegativity. The result is an ionic bond. 2 Na + O Na2O Sodium oxide reacts with water and acids to give an alkali and salts respectively. Na2O(3) + H2O 2Na+ (aq) + 2OH- (aq)

Na2O + H2SO4 Na2SO4(aq) + H2O Most ionic oxides however are insoluble and do not react with water. Magnesium oxide is an example. It will react with acids. MgO(s) + 2HCL (aq) MgCl2(aq) + H2O.

Non – metal oxides

All of the atoms in a non – metallic oxide have high electronegativity and will attract electrons from water leaving H+ ions. Hence non – metal oxides are acidic. Eg SO3 + H2O H2SO4 (aq) H2SO4 (aq) H+ (aq) + HSO4

- P4O10 (s) + 6H2O 4H3PO4 (aq) Their acidic character can be displayed by their reaction with hydroxide ions.

Metalloid Oxides

Metalloids are those elements which have both metallic and non – metallic chemistry. Metalloid oxides are amphoteric. The term amphoteric is used to describe a substance which can react with acids and bases Eg Al2O3 With Water Al2O3 + H2O 2AlO2

- + 2H+

With Water Al2O3 + 3H2O 2Al3+ + 2OH- With acids Al2O3 (s) + 6HCL (aq) 2AlCl3 (aq) + 3H2O With alkalis Al2O3 (s) 2NaOH (aq) 2NaAlO2 (aq) + H2O Sodium aluminate Eg ZnO With Water ZnO + H2O ZnO2

2- + 2H+

With Water ZnO + H2O Zn2+ + 2OH- With acids ZnO + 2HCl ZnCl2 + H2O With alkalis ZnO + NaOH + H2O Na2Zn(OH)4

These reactions generally do not occur

as aluminium oxide is not soluble

SMALL MOLECULES

Why are some molecules small while others are huge! There is a chemical pattern. Small molecules form from non – metallic atoms Eg, Oxygen (O2), Nitrogen (N2), Water (H2O) Methane (CH4) Small molecules do not contain many atoms and are discrete, i.e. separate units. They can be of either non - metallic elements or non – metallic compounds. You will need to be able to predict whether or not a compound or element is likely to be molecular, given its properties, name, elemental composition of formula. The type of bonding that occurs in these small molecules is covalent bonding, where electrons are shared between two atoms. This bond is relatively strong in holding the molecule together. By sharing electrons in this manner, all atoms in a molecule of a covalent compound become surrounded by an outer shell of eight electrons.

Eg Cl2 has an S2P5 configuration. It needs 1 more electron to get the stable S2P6 configuration

So :Cl: + :Cl: :Cl: : :Cl: (Cl – Cl) Covalent bonds are referred to as being a primary bond and an intramolecular bond.

THE SHAPES OF MOLECULES

The shapes of molecules can be explained and predicted by repulsion between pairs of bonding and non – bonding electrons. In predicting the shape of molecules the following assumptions are made: the central atom is spherical in shape

the central atoms in a molecule will usually have four pairs of electrons (an octet) for its

final electron configuration

the electron pairs will arrange themselves to give maximum distance between pairs To arrange 4 electron pairs to get a maximum distance between them, is to make a tetrahedron around the central atom

Eg CH4 (tetrahedron)

Need to improve shape Better Best NH3 (pyramid)

Water (H2O – V-shaped)

CO2 – (linear)

POLARITY IN COVALENT MOLECULES

Electrons are not always shared equally in covalent substances. They are only shared equally between identical atoms, ie. atoms of the same electronegativity.

Eg Cl2, O2, H2, N2 have the bonding electrons shared equally between each atom.

These molecules are symmetrical with regard to the distribution of electrical charge. They have no electrical dipole (a pole is an opposite end). Hence the above are non – polar covalent bonds. In covalent bonds between different atoms, because of different electronegativities, the electrons are not evenly distributed. The bigger atom will always want more of the electrons. Eg H – Cl Cl has a higher electronegativity and the electron pair is displaced towards the Cl nucleus A polar bond is shown using δ+ and δ –

Eg δ+ δ - δ – δ+ δ –

H – Cl O = C = O These bonds are called polar bonds The overall polarity of a molecule depends on the individual bond polarities and the shape of the molecule. In molecules containing two unlike atoms, the bond polarity will give an overall polarity to the molecule. In molecules containing more than two atoms and hence more than one bond, the polarity is the resultant of the individual polarities of all polar covalent bonds in the molecule. So a molecule that is completely symmetrical with respect to its polar covalent bonds is non – polar, even though it contains polar bonds. Eg CCl4 Cl δ- – All bonds cancel C δ+ each other, therefore A non – polar molecule Cl δ- Cl δ- Cl δ-

An unsymmetrical arrangement forms a polar molecule Eg CHCl3 H A polar compound I C δ+ This molecule has A molecular dipole Cl δ- Cl δ- Cl δ- The greater the difference between the electronegativities of the bonding atoms, the greater the polarity of the bond.

SECONDARY INTERACTIONS

The quantity and types of secondary interactions are very important in determining the melting and boiling points and also the solubility of a substance. The secondary interactions below are discussed in the order of their strength from weakest to strongest. DISPERSION FORCES Dispersion forces are the only secondary interaction that occur in all molecular substances regardless of their polarity. We know that these forces exist as the noble gases and non-polar molecules can form liquids and sometimes solids under the right conditions. Polarity in non-polar molecules is only zero over a long time period, the average is zero. The instantaneous polarity of all molecules is not zero. Electrons are constantly in motion and therefore it cannot be guaranteed that at every point in time that the distribution of electrons is perfectly symmetrical. Therefore all molecules have a weak polarity the fluctuates in both direction and strength. They can also be known as induced dipole - dipole interactions. The strength of the dispersion forces is solely reliant on the number of electrons present, and therefore larger molecules containing greater numbers of atoms will have greater dispersion forces. DIPOLE-DIPOLE INTERACTIONS Dipole-dipole interactions occur between two permanent dipoles (polar molecules) When they approach each other they will orient themselves to that the oppositely charged ends are next to each other. These molecules are held together weakly by the electrostatic attraction between the dipoles. ION-DIPOLE INTERACTIONS Ion-dipole interactions is where an ion, either positive or negative is electrostatically attracted to the oppositely charged end of a dipole. This is of particular importance when ionic substances form aqueous solutions. Water is highly polar, and ionic substances contain positively charge metal ions which will attract to the partial negative charge on the oxygen and negatively charged non-metal ions which will attract to the partial positive charge on the hydrogen.

HYDROGEN BOND

Are bonds that form between hydrogen attached to nitrogen, oxygen or fluorine, and nitrogen, oxygen or fluorine in another molecule. These bonds form due to the high electronegativities and small atomic radius of N, O and F. The hydrogen bond determines such physical properties as solubility, melting points, and boiling points The higher melting and boiling points is due to the stronger secondary bonding holding the molecules together. They are basically a very strong dipole-dipole interaction examples Eg δ+ δ-

O – H -------------- O – C H H δ+ δ- N – H -------------- O H δ+ δ- O – H ---------- N H The electrostatic attraction between the molecules is the hydrogen bond Differences in physical properties, such as melting and boiling points can generally be explained in terms of the relative strength of the Secondary Bonding between molecules. This in turn is related to both the molecular mass and the polarity of the molecules The greater the molecular mass, the stronger the secondary bonding between molecules. This is explained by the increasing melting and boiling points of Gp Vll elements. At room temperature the change from F2 gases Cl2 Br2 liquid Iodine solid Note these are non polar

CYCLES IN NATURE

Photosynthesis

Life on Earth would be impossible without photochemical reactions. Photosynthesis traps energy from the sun and, in the leaves of green plants, converts water and carbon dioxide into carbohydrates and oxygen. 6CO2 + 6H2O sunlight C6H12O6 + 6O2 (g) chlorophyll

Respiration

Is the process where energy is recovered from the reactions of carbon compounds (eg glucose) and oxygen to form carbon dioxide and water C6H12O6 + 6O2 6CO2 + 6H2O + energy This is aerobic respiration Both Photosynthesis and Respiration form major parts of the Carbon Cycle

BREAKDOWN OF ORGANIC MOLECULES Decomposition is a chemical reaction in which a compound breaks down into simpler compounds or into elements The presence or absence of oxygen can affect what the products of this decomposition are Aerobic Conditions – The presence of oxygen Anaerobic Conditions – The absence of oxygen Aerobic respiration is a process in which foodstuffs are fully oxidised (to CO2 and H2O) with the release of chemical energy (respiration). The process only takes place in a ready supply of atmospheric oxygen. Anaerobic Respiration is a process in which foodstuffs are partially oxidised with the release of chemical energy. The energy yield for this is lower than aerobic. It occurs in yeasts and bacteria, even muscles when oxygen is absent. Alcoholic fermentation is anaerobic. The products Aerobic Organic Matter anaerobic Carbon Dioxide CO2 Methane CH4 Water H2O Water H2O Sulphates SO4

2- Hydrogen Sulphide H2S

Nitrates NO3

- Ammonia NH3 Phosphates PO4

3- Phosphine PH3

Many of the products of anaerobic decomposition are toxic to humans and/or unpleasant smelling

THE NITROGEN CYCLE

Although nitrogen makes up 79% of the atmosphere, the stable N2 molecule cannot be used directly by higher plants or animals. Nitrogen is an essential plant food, but it must be “fixed” before it can be used by plants. Fixed means that nitrogen must be in the form of a soluble compound. Because N2 is a stable compound with a triple covalent bond which is strong, a lot of energy is needed to split the molecule. Processes that do this are Internal combustion engines Burning of fossil fuels in furnaces Lightning Nitrogen fixing bacteria Decay The energy from these processes produces nitrogen atoms which are very reactive

N2 (g) N + N

As they are reactive they combine with oxygen to form nitrogen oxides

N2 (g) + ½ O2 (g) N2O (g)

N2 (g) + O2 (g) 2NO (g)

2NO (g) + O2 (g) 2NO2 (g)

The above oxides can then be washed into the soil as acid rain and provide a useful fertiliser for plants.

Oxides of Nitrogen also contribute to:

the breakdown of the ozone layer

the formation of acid rain

photochemical smog

Humans also produce nitrogen compounds. Eg Ammonia by the Haber Process where

N2 (g) + 3H2 (g) 2NH3 (g)

The flow of nitrogen can be seen in the following diagram

Heat

Pressure

Catalyst

The lack of available nitrogen is often a major factor limiting plant growth, so applying nitrogenous fertilisers produces greatly increased crop yields.

Humans are unable to synthesise nine essential amino acids. These must be supplied by the proteins eaten. These proteins come from plants and animals.

Humans have greatly increased the availability of nitrogen by the large-scale production of synthetic fertilisers (NH3, NH4 No3 etc). this has led to Eutrophication (algal blooms) in some areas.

Two major components needed for fertile soil and plant growth are nitrogen and phosphorus compounds. They can be supplied by the use of fertilisers, however for them to be able to be used by plants they need to be in a soluble form, otherwise they cannot be dissolved a taken up by the root system.

GREENHOUSE EFFECT

The greenhouse effect is a natural warming of the earth’s lower atmosphere caused primarily by water vapour, carbon dioxide and methane. These gases maintain a steady temperature in the Earth’s atmosphere. These gases, called greenhouse gases, let the sun’s short wave rays through, but they absorb the long wave heat rays (infrared) that radiate up from the Earth’s surface

absorbed longer wavelengths Since the industrial revolution, human activities have poured increasing amounts of certain gases into the air. These gases are leading to a human – induced increase in temperatures in the lower atmosphere, the enhanced greenhouse effect.

Impacts of the Greenhouse Effect

Include: A rise in average temperatures which may lengthen summer Climate changes may occur Increase in sea levels due to expansion of water Increase in weather extremes Enhanced plant growth may favour weed species rather than crops

What can be done to limit the enhanced greenhouse effect

Limit the logging of old growth forests and stop the large scale clearance of rainforests and subtropical woodlands

Plant trees and restore native vegetation Conserve energy Use renewable energy sources Limit population growth

ACID RAIN

The problem of acid rain (acid deposition) is serious in many parts of the world, particularly in North America and Europe. They can fall as rain, snow or hail, or as microscopic particles that may occur in smog. Rainwater is naturally acidic, pH 5.6, due to the many gases present in the atmosphere from natural processes which react with water vapour and create acids. Eg CO2 + H2O H2CO3 carbonic acid solution SO2 + H2O H2SO3 sulphurous acid SO3 + H2O H2SO4 sulphuric acid 3NO2 + H2O 2HNO3 + NO nitric acid solution Since sulphuric and nitric acids are both very strong (fully ionise in water), small amounts of each acid will produce significant changes in the pH. The term acid rain refers to rain which has a pH < 5.6. Acid rain results mainly from human activities that produce oxides of nitrogen and or oxides of sulphur and emit these oxides into the troposphere.

EFFECTS OF ACID RAIN

The presence of sulphuric and nitric acids has lowered the pH of rain to such an extent in the Northern hemisphere that considerable damage has resulted Plants can be affected by acid rain in several ways direct exposure of foliage to high aid concentrations causes damage that affects water

loss from leaves and photosynthesis plant leaf protective coating also gets damaged and ions are leached from the leaves.

This effects chlorophyll production plants are also damaged through changes to soils

low ph speeds up the leaching of essential metal ions, such as Mg+2, K+ and can mobilise

potentially toxic metal ions such as Al, Pb and Zn, Cu. Free Al ions can also disturb the defence mechanisms that plants use to fight disease and damage bacteria that decompose vegetation and re-cycle plant nutrients. Mobilised toxic metal ions can also be transported in solution into sources for drinking water such as dams and wells. This can affect human health

Fish populations are affected because fish eggs and fry are particularly sensitive to low pH. Limestone and Marble building stones contain calcium/magnesium carbonates. These are readily attacked by sulphuric and nitric acids in acid rain. The carbonates are converted to soluble sulphates and nitrates, and hence get eaten away. CaCo3(s)+ H2SO4(aq) CaSo4(aq) + CO2 + H2O CaCo3(s)+ 2HNO3 Ca(NO3)2(aq) CO2(g) + H2O(l)

Acid rain also has a dramatic affect on other building materials such as Iron Fe(s) + H2SO4 FeSO4 + H2

THE pH SCALE

The level of acidity of an aqueous solution depends on the concentration of hydrogen or hydronium ions (H3O+). Because the hydrogen ion concentration in aqueous solutions is typically small, the pH scale provides a useful representation of the acidity of a solution. The pH is a logarithmic relation between pH and hydrogen ion concentration, [H+]

pH = -log [H3O

+] where [H3O+] is in mol .L-1

also [H3O

+] = 10-pH

Examples

Sometimes rather than [H3O+], [OH-] is given instead. This is easy enough to deal with. If you

have to calculate the pH of the solution use the following formulae instead

pOH = -log [OH-]

pH = 14 - pOH

PHOTOCHEMICAL SMOG

The primary pollutants in modern cities such as Adelaide come mainly from cars, buses and trucks, any transport with internal combustion engines. These pollutants include unburnt hydrocarbon fuel (VOC – volatile organic compounds), nitric oxide (NO), nitrogen dioxide NO2 (collectively NOx), carbon monoxide and particulates. During the combustion of fossil fuels a small amount of nitrogen my combine with oxygen to form nitrogen monoxide, this reaction is favoured by the high temperature in the engine. This is often rapidly oxidised by the air to form nitrogen dioxide.

N2 + O2 2NO

2NO + O2 2NO2 In the presence of sunlight, a number of secondary pollutants are derived from the chemical cocktail including ozone, aldehydes and peroxyacetyl nitrate, PAN. The resultant photochemical smog is a yellow – brown haze containing species which irritate the eyes and respiratory tract, causing long term effects on health. Nitrogen dioxide, a yellow – brown coloured gas, absorbs radiation with wavelengths lower than 400 nm and undergoes photolysis which leads to ozone formation. NO2 + U.V. light NO + O These oxygen radicals are then able to react to form ozone. O + O2 + M O3 + M (M removes energy) NO + O3 NO2 + O2 This shows that NO2 is reformed from NO by the destruction of ozone. Hence a steady state is established and ozone levels remain low and stable However in the presence of VOC and sunlight, NO is converted to NO2 and this leads to a significant increase in ozone concentration and will peak some hours after the nitric oxide and VOC are emitted from cars. Even at low concentrations, ozone affects lung function in humans and causes damage to plants. It is possible to reduce the quantities of nitrogen oxides generated by cars. This is done by using catalytic converters in cars. They use platinum, which speeds up the following reactions 2NO(g) + 2CO(g) N2(g) + 2CO2(g) Unburnt hydrocarbons in the presence of platinum also oxidise more rapidly to produce carbon dioxide and water. CO(g) + ½ O2(g) CO2(g)

Effects of ozone

It causes rubber to crack and perish (ozone attacks the double bonds)

It is a greenhouse gas

It attacks lung function in humans

It causes leaves on plants to yellow causing a decrease in photosynthesis Effects of Nitrogen Dioxide

It is a greenhouse gas

It is a major component of acid rain

WATER TREATMENT

Water treatment occurs in a number of step one of those steps is flocculation. In water there are a number of small particles such as silt, algae and other micro-organisms, these can affect the colour, smell, taste and health of the water. Flocculation involves adding substances such as Aluminium Sulfate (Al2(SO4)3) to the water. Small particles are attracted to the Al2(SO4)3 forming clumps which can be collected by sedimentation of filtration. Chlorine is the worlds most widely used bleaching agent and disinfectant. It is added to drinking water supplies, swimming pools and to household bleaches. Chlorine kills bacteria by their oxidising action. For water purification we use Hypochlorus acid, Chlorine and hypochlorites. Hypochlorus is derived from the reactions of chlorine and hypochlorites with water.

Cl2(g) + H2O(l) H+(aq) + Cl (aq) + HClO(aq) Chlorine hypochlorus acid

ClO-(aq) + H2O OH-(aq) + HOCl(aq) Hypochlorite ion The hypochlorous acid (being non-polar) enters through cell walls of the bacteria and when inside oxidises the enzymes which control bacteria growth. The bacteria die.

THE IMPORTANCE OF pH TO SWIMMING POOL CHEMISTRY

HOCl is a very weak acid. It ionises only a little to produce ClO- and H+.

HOCl OCl- + H+

The shows there is a balance between the forward and back reaction. This balance can be changed by taking away or adding H+, ie by changing the pH of the solution. In low pH more HOCl forms, therefore less H+ + OCl-

In high pH less HOCl forms, therefore more H+ + OCl- When pH is 7.5, a 50% ClO- and 50% HClO solution forms. This is ideal. If pool water is too acidic ( pH <7), red eyes result. Algae also grows well. If too alkaline CLO- forms which decomposes in sunlight, therefore sterilization is less efficient.