AQA AS Level Chem Unit 1 Defintitions

AQA AS Level Chem Unit 1 Defintitions

Inclung: Atomic structure, Bonding, Amount of substance, Periodicity, Organic chemistry, Alkanes

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Atomic number

number of protons (Z / proton number)

no units

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Mass Number

Number of protons and number of neutrons


no units

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Atoms with the same numer of protons but a different number of neutrons.

  • Different isotopes of the same element react chemically in the same way
  • Vary in mass number due to the different amount of neutrons
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Mass Spectronmter

An instrument used to measure the relative atomic mass (RAM) on a scale based on the mass of an atom of Carbon-12 which has a RAM of 12

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Ionisation (Mass Spec)

A beam of electrons, fired from an electron gun, knocks an electron from an atom to create positive ions

Norally only one electron is removed creating a 1+ charge, but occasionally two electrons are removed creating a 2+ charge

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Acceleration (Mass Spec)

The postive ions are attracted towards negitivly charged plates and accelerated to a high speed

The lighter the ion, the faster they go

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Deflection (Mass Spec)

The beam of ions moves into a magnetic field, at right angles to it's direction of travel. The magnetic field deflcts the ions.

The amount of deflection depends upon the m/z vaue of the atoms, the charge mass ratio. As the charge, in most cases is 1, the deflection depends solely upon the mass number.

  • 2+ ions are deflected twice as much
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Detection (Mass Spec)

Ions hit the detector, accept electrons, lose their charge and create a current which is proportional to the abundance of each ion.

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Mass Spectrum (Mass Spec)

A graph showing the relative abundance (y-axis) and mass/charge value (x-axis) produced by the mass spectrometer

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Working out RAM (Mass Spec)

Relative atomic mass:

(m/z value x % abundance) + (m/z value x % abundance)


watch out for 2+ ions, as they may obscure your result

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Applications of Mass Spec

Used on space probes, eg. Viking Martian, to identify the elements in rock samples on other planets/moons/comets in space.

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Electron arangement

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5d6

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Electron Spin

  • Two electrons in the same orbital have opposite spins
  • Represented by an arrow pointing up or down, depending on direction of spin
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Ionisation energy

The energy required to remove one mole of electrons from a mole of atoms in the gaseous state.

measured in Kj/mol

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Sucessive Ionisation Energies

The first IE needs the leas energy to remove

The secnds needs more energy than the first beacuase it is being removed from a 1+ ion

and so on..

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Relative Atomic Mass

The average mass of one atom of an element

1/12th mas of one atom of Carbon-12

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Relative Molecular Mass

average mass of one molecule

1/12 the mass of one atom of Carbon-12

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Avagadro Constant

The number of atoms in 12 g of Carbon-12

6.022x10^23 or mole

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Moles Equation

Moles = mass in g / RAM

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Charles' Law

V is proportional to T

or using a constant (k):

  • V= kT
  • T=kV
  • k=V/T
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Gay-Lussac's Law (constant volume law)

PV/T = constant for a fixed mass of gas.

(as PV= k and V/T = k)

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Ideal Gas Equation

PV = nRT

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Empirical Formula

The simplest ratio of the atoms of each element present in a compound

To find empirical formula:

  • Find the masses of each element
  • Find the moles using moles = mass / ram
  • convert moles into whole number ratio
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Moleular Formula

The actual number of atoms of each element present in a compound.

To find molecular formula from empirical formula:

  • units of empirical formua: relative molecular mass / relative mass of empirical formula
  • times the empirical formula by the units

eg. CHv2 has a relative molecular mass of 12 and relative mass of 24

24 / 12 = 2

2 x 2 = 4 so molecular mass = Cv2Hv4

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Concentration= number of moles / volume in dm^3

NB- if given in cm^3 divide by 1000

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Atom Economy

Atom economy = (mass of disired product / total mass of reactants ) x100

Used to determin how much of the reactants will be wasted and if the experiment is useful, cost effective and sustainable.

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Percentage yield

Percentage yield = (the number of moles of a specified product / theoretical maximum number of grams of the product) x100

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Ionic bonding

  • Between a metal and a non-metal
  • Electrons are transfered from metals to non-metals
  • Positive and negative ions are formed
  • Electrostatic forces hold the ions together
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Properies of ionicly bonded compounds

  • Soild at room temperatures
  • Giant structures
  • High meltin temperatures
  • Conduct electricity when molten or disolved in water
  • Brittle
  • Shatter easily
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Covalent Bonding

  • A bond between a pair of non-metal atoms
  • Share the outer shell electrons
  • Two atoms share one pair of electrons

Occasionaly double covelent bonds are formed, when two atoms share two pairs of electrons. Called a double bond.

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Co-ordinate bonding

A single atom provides both the electrons in the shared pair. This can be called a dative covalent bond

  • The atom that accepts the pair is an atom that doesn't have a filled outer level of electrons, this atom is deficient
  • The atom doning the electrons has a pair of electrons that is not involved in a bondm called a lone pair
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The power of an atom to attract the electron density in a covelent bond towards itself. It depends upon:

  • thenuclear charge
  • the distance between the nucleus and the outer shell electrons
  • the shielding of the nuclear charge by the electrons in the inner shells
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Metalic bonding

A lattice of positive ions in a sea of outer electrons.

  • Giant structures
  • Malleable and ductile
  • High melting points- strong bonds
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Intermolecular Forces

Forces that act between molecules.

  • van der Waals
  • Dipole-dipole
  • Hydrogen bonding
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Dipole-dipole forces

An intermolecular force that results from the attraction between molecules with permenent dipoles.

  • 2nd weakest intermolecuar force
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van der Waals forces

All atoms and molecules have positive or negative charges, even if neutral overall. These charges produce a very weak electrostatic attractions.

  • The distribution of the charge changes every second
  • In addition to other forces

The weakest intermolecular force

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Hydrogen bonding

When a hydrogen atom is bonded to a very electronegative atom:

  • Oxygen
  • Nitrgoen
  • Fluorine

The strongest intermolecular force

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A saturated hydroarbon containing no double bonds.

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An orgainic compound containing one or more double bond.

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