ATOMIC RADIUS
(a) covalent radius
(b) metallic radius
(c)van der waals radius
SCREENING EFFECT
-caused by the mutual repulsion between electrons in the inner shell with those in the outer shell
-the screening effect of the inner shell electrons causes the atomic size to increase
-screening effect is also known as shielding effect
-this makes the atomic size larger
FACTOR AFFECTING ATOMIC RADII
-atomic radius is determined by two factor, screening effect and the nuclear charge
-the greater the number of filled inner shell , the greater the screening effect and the larger the atomic size
what is nuclear charge ?
measure of attractive force between nucleus and valance electron
-nuclear charge corresponds to the number of protons in the nucleus, nuclear charge pulls all the electrons closer to the nucleus and causes the atomic size to decrease
-the higher the effective nuclear charge , the smaller the atomic size
-the smaller the effective nuclear charge , the larger the atomic size
example: berryllium atom has a smaller atomic radius than lithium atom because of greater nuclear
charge while screening effect remains almost unchanged
ATOMIC RADII ACROSS PERIOD 2 AND PERIOD 3
- On going left to right , the atomic radius decrease
-nuclear charge increase and the number of electrons increase
-SCREENING EFFECT remain unchanged within the same period because the electrons are added to same quantum shell
-in conclusion , the effective nuclear charge increase, thus the valence electron are pulled closer to the nucleus and the atomic radius decreases
ATOMIC RADII DOWN A GROUP
-On going down a group , the atomic size increase as the proton number rises
-the higher the principal quantum number of the valence electrons , the bigger the atomic radius
-the increase is due to two factors
(a)increase the number of shell filled with electron
(b)increase in the screening effect
-as we move down the group , both the nuclear charge and screening effect increase , however the increase in screening effect is larger than the increase in nuclear charge
-hence the effective nuclear charge decrease as the size is becoming larger
**on going left to right , change is caused by nuclear charge
**on going down a group , change is caused by screening effect
IONIC RADII
-ionic radius is the radius of cations or anions
-isoelectronic means having the same number of electron
-the higher the nuclear charge , the higher the force of attraction and the smaller the ionic radius
-**CATIONS IS SMALLER THAN ITS NEUTRAL ATOM
-**ANION IS BIGGER THAN ITS NEUTRAL ATOM
-cations means reducing electrons
-anion is gaining electrons
-the greater the number of electrons , the greater the repulsion between the electrons and greater the atomic or ionic size
-hence ionic radius of positive ions are smaller than negative ions
MELTING POINT AND BOILING POINT AND ENTHALPY OF VAPORISATION
EXPLANATION
MELTING AND BOILING POINT DEPENDS ON
(a) chemical bonding
(b) structure , the way the atom or molecules are packed , simple , giant , etc
-elements with strong chemical bonding such as ionic solid and giant covalent molecules, their melting and boiling points are very high
-elements with low melting and boiling points are held by weak van der waals forces of attraction
PERIOD 2 AND PERIOD 3
-In period 2, the melting point increase from lithium metal to giant molecule carbon , and then decrease abruptly to the simple non metallic molecule , nitrogen
-In period 3, the melting point increase from sodium metal to the giant molecule silicon and then decrease sharply to the simple molecule, phosphorus
-same trend is repeated for boiling points
-this is due to the bonding and structure change from giant metallic structure to giant covalent structure to simple covalent structure
1.GIANT METALLIC STRUCTURE
-in the meal lattice, each positively charged metal ion is attracted to the cloud of electrons
-the mobile electrons in the valance shell of a metal are responsible for the bonding in metal
-across from left to right, there is gradual increase of valance electron
-causes the metallic bond to be stronger
-boiling point of metals are much more higher than melting point
-this is because when metals in the liquid state changes into vapour, the atoms must break all the metallic bond which still exist in liquid state
2.GIANT COVALENT STRUCTURE
-boron , carbon and silicon have giant covalent structure
-all the bond must be broken before the solid melts
-so their boiling and melting point are very high
3.SIMPLE MOLECULAR STRUCTURE
-the covalent bond within the molecule are very strong but the van der waals attraction between the two molecules are very weak
-this molecules can be separated easily
-van deer waals forces of attraction increase as the size increase
-for example chlorine which exist as diatomic has higher higher melting point than argon due to increase of RMM
ELECTRICAL CONDUCTIVITY
-metals , non metals and metalloids
-metalloids are non metal with some metallic properties
-metals are good conductors , the electrical conductivity of metals slowly falls as the temperature rise
-metalloids are poor conductor of electricity
-the conductivity of metlloids increase as the temperature increase and is affected markedly by the presence of impurities
**In metal, electron is responsible as the charge carrier. A systematic flow of electrons in one direction will cause the metal to conduct electricity efficiently. An increase in temperature will cause the electrons to get excited and this causes them to move in a not so orderly manner. Hence, they become less efficient as the charge carrier and conductivity decreases.
In electrolyte, it is the ions (both positive and negative) that is responsible as the charge carrier. An increase in temperature will increase the energy of the ions and they will move faster. Hence its efficiency as charge carrier increases and this will cause the conductivity to increase.
-lithium , berylium , sodium , magnesium , aluminium are metals and can conduct electricity
-this is because metals have delocalised electrons which will move freely across the metal
-the non metals use valence electron to form covalent bond, so they don't conduct electricity
-boron , carbon (graphite) and silicon are metalloids ,the electrical conductivity of these metals are higher than non metals but lower than metals
ELECTRONEGATIVITY
-ability or power of the the atom in a covalent bond to attract shared electron to itself
-the power of nucleus to attract surrounding electrons
1. ACROSS FROM LEFT TO RIGHT
-the atomic radius decrease but the nuclear charge increase
-as a result , the attraction of electrons increase hence electronegativity increase
**due to nuclear charge
2. ACROSS FROM DOWN A GROUP
-on going down , the nuclear charge increase and screening effect also increase
-as a result, the effective nuclear charge decrease and the attraction for electron decrease
-hence the electronegativity decrease
IONIATION ENERGY
-minimum energy required to remove 1 mol of electrons from 1 mol of atoms in the gaseous state
-high ionisation energy means harder to remove the electrons
1. FACTORS AFFECTING THE IONISATION ENERGY
-distance of the outer electrons from the nucleus
-size of the nuclear charge
-screening effect of the electrons in the shell
(a) ionisation energy decrease as the atomic radius increase
(b)nuclear charge increase hence ionisation energy increase
(c)increase in screening effect cause decrease in ionisation energy
IONISATION ACROSS PERIOD 2 AND PERIOD 3
-on moving across a period , the nuclear charge increase but the screening remain almost constant
-valence electrons held more tightly by the nucleus
-hence ionisation energy increase
-the increase is not uniform
-notice that , ionisation energy of berylium and nitrogen is higher than boron and oxygen
-ionisation energy of magnesium and phosphorus is higher than aluminium and sulphur
- this is due to electronic configuration
IN PERIOD 2
Be : 1s2 2s2
B : 1s2 2s1
IN PERIOD 3
Mg : 1s2 2s2 2p6 3s2
Al : 1s2 2s2 2p6 3s2 3s1
**MORE ENERGY IS NEEDED TO REMOVE ELECTRON FROM A FULLY OCCUPIED S ORBITAL THAN A SINGLY OCCUPIED P ORBITAL
**SAME GOES FOR N2 O2 P AND S
**MORE ENERGY IS REQUIRED TO REMOVE AM ELECTRON FROM HALF FILLED ELECTRON
IONISATION ENERGRY DOWN A GROUP
-first ionisation energy decrease on going down a group
-this is because increase in atomic radius due to screening effect
SUCCESSIVE IONISATION ENERGIES
Why do successive ionisation energies get larger?
Once you have removed the first electron you are left with a positive ion. Trying to remove a negative electron from a positive ion is going to be more difficult than removing it from an atom. Removing an electron from a 2+ or 3+ (etc) ion is going to be progressively more difficult.
REACTIONS OF PERIOD 3 ELEMENTS WITH OXYGEN
-the reactivity decrease from left to right
-na mg al are the metals involved
-these metals are strong reducing agent
-silicon is the weakest reducing agent hence react slowly with oxygen
-phosphorus burns readily in water
-in limited supply of air, phosphorus form p4o6
-in excess air or oxygen, form white fumes p4o10
-sulphur burn readily in air with a blue flame, forming sulphur dioxide
-so3 is produced when sulphur dioxide react with oxygen in high temperature and in the presence of catalyst such vanadium oxide
REACTIONS OF PERIOD 3 ELEMENTS WITH WATER
-sodium and magnesium reacts vigorously in water
-aluminium is not attacked by water or steam because it consists of a very thin aluminium oxide film forms on the surface
-this thin oxide film is non porous ton water and protects the metal from attack by water
-the protective oxide layer can be removed by rubbing mercury to form aluminium mercury amalgam
-silicon , phosphorus , sulphur and argon do not react with water
-chlorine dissolve in water to form two types of acids
-hydrochloric acid and chloric acid
-this reaction is called disproportionation reaction
-is a chemical substance which simultaneously oxidised and reduced
USES OF SULPHUR DIOXIDE IN FOOD PRESERVATION
-it acts as a germicide to prevent the growth of microbes and as antioxidant to prevent the oxidation
-at low concentration , so2 inhibits the growth of bacteria and moulds
-at high temperature it can kill a large number of bacteria and fungi
-so2 is a reducing agent , it can retard the oxidation of unsaturated fats in foods
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