REVISION NOTES FOR EXAM- OCR ..unit 1 module 1-3

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Atoms and isotopes

An atom is the smallest part of an element  composed of 3 sub-atomic particles ,protons,neutrons and electrons.

The number of protons in an atom of an element is called the atomic number ,Z.

Proton- Relative mass= 1     Relative charge=1+

Neutron-Relative mass=1     Relative charge=0

Electron-Relative mass=1/1840 or negligible   Relative charge=1-

An atom is electrically neutral, An atom contain the same anount of protons as electrons.

Isotopes- atoms of the same element with different masses and number of neutrons. Isotopes of an element have the same number of protons and electrons.

Mass number, A- The combined number of protons and neutrons in an isotope of an element.

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The electronic structure of the atom.

Energy levels or "shells"

n-1,2,3,4,     Shell- 1st,2nd,3rd,4th   Electrons-2,8,18,32

Electronic energy levels are like a ladder ,they are filled from the bottom up. The gap between the successive energy levels become less ,with increasing energy levels.

Sub-shell- s,p,d,f   Electrons- 2,6,10,14

n- 1,2,3,4    Shell- 1st,2nd,3rd,4th    Sub-shell- 1s,2s 2p,3s 3p 3d,4s 4p 4d 4f

Electrons- 2=2, 2+6=8, 2+6+10=18 , 2+6+10+14=32

s-orbital = spherical shape with one orbital and 2 electrons.

p-orbital = 3 dimensional dumbell shape with 6 electrons

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Filling the sub-shells


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Positive ions form when electrons are lost



Negative ions form when electrons are gained

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Ionisation energy

Ionisation energy measures the ease with which electrons are lost in the formation of positive ions. An element has as many ionisation energies as there are electrons.

The first ionisation energy of an element is the energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

The equation representing the first ionisation energy of sodium :

Na(g)------->Na+(g)+e-     1st ionisation energy =+496 kj mol -1

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Factors affecting ionisation energy

 Electrons are held in their shells by the attraction from the nucleus . The first electron lost will be from the highest occupied energy level. This electron experiences the least attraction from the nucleus.

Atomic radius- The greater the distance between teh nucleus and the outer electrons,the less the attractive force. Attraction falls rapidly with increasing distance and so this factor is very important and has a big effect.

Nucleur charge- The greater the number of protons in the nucleus the greater the attractive force.

Electron screening- The outer shell electrons are repelled by any inner shells between the electrons and the nucleus. The repelling effect is called shielding/screening,it reduces the overall attractive force experienced by the outer electrons.


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Atomic mass

Atomic mass unit - 1 atomic mass unit is the mass of an atom of the carbon-12 isotope. This provides the bass measurement for atomic masses.

Relative Isotopic mass-  is the mass of an atom of an isotope compared with 1/12 the mass of an atom of carbon-12.

Relative atomic mass- is the weighted mean mass of an atom compared with 1/12 of an atom of carbon-12.

Relative molecular mass- is the weighted mean mass of a molecule compared with 1/12 of an atom of carbon-12.

Relative Formula mass- is the weighted mean mass of a formula compaed with 1/12 of an atom of carbon-12.


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The mole


   Avogadro constant :

   6.02 x 10 ^23 mol-1

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a mole


A mole- is the amount of substance that contains as many single parcticles as there are atoms exactly in 12g of the carbon-12 isotope.

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Molar mass



Molar mass,Mr- is the mas of one mole of a substance . The units of molar mass are g mol-1.

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Moles equation...mass...moles...molar mass


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Moles equation...moles...concerntration...volume


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Moles equation...moles...volumes...gases

  Volume equation (

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Moles calculation example

n = mass ÷ MM

Calculate the moles in 124.5g of oxygen gas.

  • n = mass ÷ MM
  • mass = 124.5g
  • MM of O2 = 2 x 16.00 =32.00g mol-1
  • n = 124.5 ÷ 32.00 = 3.89mol


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An acid is a proton donor.

Acid examples, Sulfuric acid H2S04, hydrochloric acid HCl , nitric acid HNO3

Sulfuric acid is used in the battery acid used in cars.

When an acid is added to water ,the acid releases h+ ions (protons) into the solution:

HCl(g) + aq ----->H+(aq) + Cl-(aq)  <------ hydrochloric acid, HCl

H+ ions (protons) are an active ingrediant in acids

- A H+ ion is responsible for all acid reactions

-An acid is a proton donor

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A base is a proton acceptor.

Base examples: Metal oxides- MgO, Cuo    Metal hydroxides- NaOH,Mg(OH)2

Magnesium hydroxide is used in milk of magnesia for treating acid indigestion

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