2.1 Intro

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I The atom consists of three subatomic particles:

1. the proton
2. the electron
3. the neutron

2.1.1 The Nuclear Nodel of the Atom

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II. The electron was first discovered by J.J. Thomson while he was investigating "cathode rays"

1. Thomson discovered that the "cathode rays" from metal electrodes were streams of negatively charged electrons.

2. Other types of metals release cathode rays and he concluded that all atoms consisted of electrons.

3. Thomson also found the ratio of magnitude of the electron's charge to the mass of the electron, but not the absolute values themselves.

4. The fundamental charge (e), or the magnitude of the charge of one electron, was discovered through the Millikan oil drop experiment.

1. The experiment consisted of oil drops that had an excess number of electrons.

5. The magnitude of negative charge was determined by an electric field, and the smallest increment of charge between droplets was considered the fundamental charge.

III. The proton is what allows atoms to be electrically neutral; it balances the negative charge of an electron.

1. Before the discovery of the proton, Thomson suggested a "plum pudding" model where the positive charge was spread out

2. The Geiger-Marsden experiment using platinum foil and $\alpha$ particles proved this wrong, and concluded that the positive charge was concentrated around the atom.

3. This "positive charge concentrated at the center" is known as the nucleus, and later works proved that it consisted of not only protons, but also neutrons, neutral particles that hold protons together.

2.1.2 Electromagnetic Radiation

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IV. spectroscopy: the analysis of light emitted or absorbed by substances

1. spectrometric techniques were used by early scientists to determine the arrangement of electrons within an atom.

   1. IR spectroscopy, mass spectroscopy, UV-Vis spectroscopy, EPR spectroscopy

V. electromagnetic radiation: oscillating electric and magnetic fields traveling at the speed of light

1. visible light, microwaves, x-rays, gamma rays are all forms of electromagnetic radiation.

1. wavelength (λ) represents the distance between two closest peaks of the wave

2. amplitude (A) represents the height of the wave above the centerline

3. intensity, or the brightness of the radiation, is the square of the amplitude.

4. frequency ($\nu$) is the number of cycles of the radiation. (Unit: $Hz = s^ {-1}$)

2. The relation between wavelength and frequency is: $\lambda\cross\nu = c$ (Eq. 1)

VI. types of electromagnetic radiation are dependent on its wavelength.

1. white light is a mixture of all wavelengths of visible light.
2. wavelengths between 420 to 700 nm represent visible light.

2.1.3 Atomic Spectra

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VII. When a high current is passed through hydrogen gas, the gas emits electromagnetic radiation

1. the high current “excites” the electrons from a lower energy state to a higher energy state

1. this is known as a transition; the movement from one energy level to another.

2. as the higher energy state is more unstable, it quickly returns to its original lower energy state releasing the energy as electromagnetic radiation

3. when white light is passed through a prism, a continuous spectrum emerges

4. when electromagnetic radiation emitted from the hydrogen gas is passed through a prism, emissions lines appear

1. these emission lines reveal the different wavelengths of light that the hydrogen gas emitted

2. emission lines reveal that electrons can only exist within discrete energy levels

3. with countless other observations, the frequency of the emitted light is: $\nu = R(\frac{1}{n_1^2} - \frac{1}{n_2^2})$ (Eq. 2)

   where $n_1$ = 1, 2, ... ; and $n_2 = n_1 + 1, n_1 + 2$, ..., R is the Rydberg constant

VIII. There are some series with names where n1 is a specific number:

1. if n1 = 1, it is known as the Lyman series (Ultraviolet region)
2. if n1 = 2, it is known as the Balmer series (Visible region)
3. if n1 = 3, it is known as the Paschen series (Infrared region)

IX. If white light is passed through hydrogen gas and then a prism, you would see absorption lines

1. again, white light has all frequencies of visible radiation

1. if only white light is passed through a prism, a continuous spectrum is observed

2. absorption lines suggest that hydrogen atoms can not only emit but also absorb light of a certain set of frequencies

1. If transition from 3 to 2 emits red light (in emission lines), then a transition from 2 to 3 will absorb red light (in absorption lines)

X. Note that, other gases can absorb and emit light as well, at different wavelengths