chem topic one

Dalton

atoms

Thompson

the atom is a ball of positive charge containing electrons (plum pudding model)

Rutherford

Atom is made up of mostly empty space and contains a positively charged nucleus (gold foil experiment)

Bohr

electrons orbit the nucleus in energy levels

Chadwick

Discovered that neutrons reduce the repulsion between protons and stabilize the nucleus

Forces within an atom

Electrostatic attraction between oppositely charged subatomic particles

Electrostatic repulsion between like charged subatomic particles

Strong nuclear force between protons and neutrons works over short distances holds the nucleus together

Atomic radius down a group

• Number of electron shells increases.

• Valence electrons are further from the positive nucleus.

• Increased shielding from inner electrons reduces the nucleus's attraction to the valence electrons.

• The weaker attraction means the outer electrons are held less tightly.

• Therefore, the atomic radius increases down a group.

Atomic radius across a period

• The number of protons increases.

• Electrons are added to the same energy level (shell), so shielding remains almost unchanged.

• Therefore, the effective nuclear charge increases.

• The stronger attraction between the nucleus and the valence electrons pulls the electrons closer to the nucleus.

• Atomic radius decreases

Successive Ionization Energy

Successive ionisation energies are the energies required to remove a mole of valence electrons from a mole of gaseous atom or ion,

Ionisation energy down a group

• Number of electron shells increases.

• Valence electrons are further from the positive nucleus.

• Increased shielding from inner electrons reduces the attraction between the nucleus and the valence electrons.

• The outermost electron is held less strongly.

• Therefore, less energy is required to remove the outermost electron, so ionisation energy decreases.

Ionisation energy across a period

• Number of protons increases.

• Number of electron shells remains the same.

• Attraction between the positive nucleus and the valence electrons increases.

• Valence electrons are pulled closer to the nucleus.

• More energy is required to remove the outermost electron.

• Therefore, ionisation energy increases across a period.

Electronegativity def

The tendency of an atom to attract bonding (shared) electrons towards itself.

Eneg down a group

• Number of electron shells increases.

• Valence electrons are further from the positive nucleus.

• Increased shielding from inner electrons reduces the nucleus's attraction.

• The nucleus has a weaker attraction for bonding (shared) electrons.

• Therefore, electronegativity decreases down a group.

Eneg across a period

• Number of protons increases.

• Number of electron shells remains the same.

• Attraction between the positive nucleus and electrons increases.

• The nucleus attracts bonding (shared) electrons more strongly.

• Therefore, electronegativity increases across a period.

5 steps involved in MS

• Vaporisation – The sample is converted into a gas.

• Ionisation – Electrons are removed from the gaseous atoms to form positive ions.

• Acceleration – The ions are accelerated by an electric field so they all have the same kinetic energy.

• Deflection – The ions pass through a magnetic field and are deflected according to their mass-to-charge ratio (m/z). Lighter ions are deflected more than heavier ions.

• Detection – The ions reach the detector, producing an electrical signal. The signal is used to determine the relative abundance of each ion.

Mass Spec def

Mass spectrometry is a technique used to identify atoms or molecules by measuring the mass-to-charge ratio of their ions

Flame Tests

• A metal salt is heated in a flame.

• Electrons absorb energy from the flame and become excited.

• The electrons move to higher energy levels.

• As the electrons return to their ground state, they release energy.

• This energy is emitted as light of a specific wavelength (colour).

• The colour produced is characteristic of that metal ion.

AAS

quantitative analysis technique used to identify the concentration of sample

AAS steps

• Vaporisation/Atomisation – The sample is sprayed into a flame, producing gaseous ground-state atoms.

• Absorption – Light of a specific wavelength is passed through the flame.

• Excitation – Atoms of the element being tested absorb the light, causing electrons to move to higher energy levels.

• Measurement – A detector measures how much light is absorbed.

• Analysis – The amount of light absorbed is proportional to the concentration of the element in the sample.