Year 11 Chemistry - Atomic Theory, Line Spectra and Mass Spectrometry (CAP 1)

Democritus's Theory (c. 450 BC)

Matter was made up of indivisible particles called "Atomos"

Antoine-Laurent Laviosier's Discovery

By conducting chemical reactions in sealed containers, he discovered that there is no change in mass during a chemical reaction (The Law of Conservation of Mass, 1789)

Joseph Proust's Discovery

He discovered the 'Law of Definite Proportions': That different samples of the same compound always contain the same ratio of component elements.

John Dalton's Atomic Theory (1808)

Elements are composed of tiny atoms.
All atoms of the same element are identical (mass, size, and chemical properties), all atoms of different elements have different mass, size and chemical properties.
Atoms are not created, changed into a different element or destroyed during a chemical reaction.
Chemical reactions only change the arrangement and combination of atoms.
Compounds are formed from atoms combining in specific ratios.

The Law of Multiple Proportions (Dalton)

Whenever two elements can combine to form more than one compound, each compound has a unique whole number ratio of ingredients, e.g. CO (1:1) vs CO2 (1:2).

What did Dalton think atoms looked like?

Small, hard, dense spheres with no internal structure

J.J. Thomson's Discovery (1897)

By using charged plates and a magnetic field, Thompson was able to determine that cathode rays were made out of negatively charged particles much lighter than the lightest atom and found within all elements. This particle is now known as the electron, and it disproved Dalton's idea that atoms were indivisible.

How did Thomson's experiment work?

A cathode ray tube (CRT) is a device that sends a beam of electrons from a negatively charged cathode through a vacuum to a positively charged anode, in the middle there is a phosphorescent substance that glows when hit by electrons. Since electrons have a negative charge, you can shift their course either away from a negative electromagnetic field, or towards a positive one. Thompson placed a CRT in between charged plates and a magnetic field, he then increased the charge to the plates until it cancelled out the magnetic field's effect. Thus, he was able to calculate the charge to mass ratio, and by extension mass, of the electron.

What did Thomson think atoms looked like?

Thompson proposed the plum pudding model of the atom, where small electrons are embedded within a larger positively charged sphere, like plums within a plum pudding. This accounted for the existence of electrons and the fact that atoms had neutral charge.

How did Rutherford disprove Thomson's model? (1910)

Rutherford designed an experiment where he fired positively-charged alpha particles at gold foil. If Thompson were correct, and positive charge was thinly and evenly distributed throughout the whole atom, the alpha particles would simply pass straight through. While most of the alpha particles passed straight through, some were deflected at odd angles, some were even deflected straight back to where they came from.

How did Rutherford account for this in his new model?

Rutherford though that the deflected alpha particles could be explained if the positive charge of an atom was located in a small dense nucleus (he would later discover and name the proton in 1920), while most of the atom's area was empty space with tiny electrons flying around the nucleus. This way, the small number of deflected alpha particles could be explained by them hitting or passing near the nucleus, deflecting them away since they have like charges, while the rest passed through the empty space and were unaffected.

What was Sir James Chadwick's Contribution?

Discovering the neutron, which explained why atoms were heavier than they should have been if they were only made of protons. Neutrons were hard to find as they had no charge.

How did Niels Bohr improve upon Rutherford's model?

Bohr explained both the orbits of electrons and line spectra by proposing that electrons revolved around the nucleus at certain radii determined by the specific amount (quanta) of energy that the electron had. When an electron goes from a higher quantised energy level to a lower one, it emits a photon on a certain wavelength based on the energy difference between the two levels, thus explaining line spectra.

Spectroscopy

The study of the interactions between matter and electromagnetic radiation.

Emission Spectrum

The colours (wavelengths) of light produced by a heated gas (excited state) as electrons go from high energy levels to lower levels as the gas cools (returns to ground state). They appear as vertical lines of colour on a blank/black background. The Emission spectrum of each element is unique. To the human eye, these wavelengths are combined into a single colour, but they can be separated to reveal the spectrum e.g. with a prism.

Emission lines

The lines that can be seen on emission spectra, each corresponding to the difference in energy between two energy levels electrons may occupy for that particular atom.

Absorption Spectrum

The colours of light that are absorbed when light is passed through a cool gas (ground state). If the photons have the right amount of energy they are absorbed by promoting electrons (excited), and thus are absent from an otherwise full continuous spectrum/rainbow. This appears as a continuous spectrum with specific wavelengths/colours absent. It is also unique to each element, and will produce a continuous spectrum if combined with the emission spectrum of the same element.

Absorption lines

The colours of light absent from absorption spectra, each corresponding to the difference in energy between two energy levels electrons may occupy for that particular atom.

Ground State

The smallest radius/lowest quantised energy level at which an electron can orbit its nucleus in Bohr's model. The lowest energy state of an atom.

Excited State

When an atom's electrons absorb more energy, leading them to promote to higher quantised energy levels.

The Current Atomic Model

Today we know that electrons are not found in orderly rings, but in clouds of probability called orbitals. Furthermore, we now know that protons and neutrons are themselves made up of smaller particles called quarks.

Explain how developments in technology have contributed to our understanding of the model of the atom

Laviosier used sealed containers to prove the conservation of mass (1789)

The discovery of electricity led to the discovery that atoms have electrical properties (Faraday, 1833)

A Cathode Ray Tube was used by Thomson to discover electrons and disprove that atoms were indivisible. (1897)

We were, before Chadwick, unable to find neutrons as our detection methods only worked on charged particles.

The discovery of spectroscopy necessitated that Bohr update the model of the atom.

In 1981 IBM invented a powerful enough microscope to look at atoms, confirming that they are spherical.

Flame test

A metal salt is heated using a Bunsen burner, exciting its electrons. As they return to their original state, they produce photons, turning the flame into a different colour.

Weakness of the Flame test

-Only certain metals are detectable -Metals in low concentration may be difficult to observe -Impure metals will produce confusing results -Some metals have similar colours when burned, such as Lithium and Strontium (both red). This can be fixed by viewing the flame through a spectroscope, which will separate the colours into the emission spectrum.

Hazards w/ Flame test and how to mitigate risk

Hot flame - tied back hair, no lose clothing, act with caution

Toxic Chemicals - Do not ingest, wash hands or wear gloves after contact, minimise skin contact, avoid touching your eyes

Use of emission spectra

Fireworks

Use of absorption spectra

Atomic Absorption Spectroscopy (AAS) is used to identify even tiny amounts of metals, this can be used to find toxic metals in mining operations (to ensure compliance with environmental regulations) and to find toxic metals in tissue e.g. lead in blood.

How does AAS work?

A vapourised sample of cold gas (ground state atoms) is passed in front of a light that produces a continuous spectrum. This produces an absorption spectrum and the intensity of absorbed light is measured to determine the concentration of various elements in the sample.

How does mass spectrometry work?

1. The Sample is vapourised

2. The vapour is ionised to give it charge

3. The ions are accelerated using an electric field

4. The ions are deflected by a magnetic field, with the lighter ions being deflected more

5. The ion beams, now separated by weight, hit a detector which measures their intensity

Mass Spectrum

A graph showing the abundance (y-axis) of isotopes of a certain mass/charge (m/z) ratio (x-axis) in the sample.

Uses of Mass Spectrometry

• identifying the abundance of isotopes

• detecting doping in sports

• forensic toxicology (did the victim have drugs in their system?)

• identifying samples of matter on other celestial bodies (space exploration)