SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 2

Atomic Structure and the Periodic Table of the Elements

ATOMIC SPECTRA

The Bohr model was based on a simple postulate. Bohr applied to the hydrogen atom the concept that the electron can exist only in certain energy levels without an energy change but that, when the electron changes its state, it must absorb or emit the exact amount of energy that will bring it from the initial state to the final state. The ground state is the lowest energy state available to the electron. The excited state is any level higher than the ground state. The formula for a change in energy (ΔE) is:

ΔEelectron = Efinal – Einitial

When an electron moves from the ground state to an excited state, it must absorb energy. When it moves from an excited state to the ground state, it emits energy. This release of energy is the basis for atomic spectra. (See Figure 6.)

The energy values shown were calculated from Bohr’s equation.

Figure 6. Atomic Spectra Chart

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When electrons drop to the lowest available energy level, they release energy.

When energy is released in the “allowed” values, it is released in the form of discrete radiant energy called photons. Each of the first three levels has a particular name associated with the emissions that occur when an electron reaches its ground state on that level. The emissions, consisting of ultraviolet radiation, that occur when an electron cascades from a level higher than the first level down to n = 1 are known as the Lyman series. Note in Figure 6 that the next two higher levels have the names Balmer (for n = 2) and Paschen (n = 3) series, respectively.

Spectroscopy

When the light emitted by energized atoms is examined with an instrument called a spectroscope, the prism or diffraction grating in the spectroscope disperses the light to allow an examination of the spectra or distinct colored lines. Since only particular energy jumps are available in each type of atom, each element has its own unique emission spectra made up of only the lines of specific wavelength that correspond to its atomic structure. The relationship of wavelength to frequency is shown below.

A partial atomic spectrum for hydrogen would look like this:

The right-hand group is in the visible range and is part of the Balmer series. The left-hand group is in the ultraviolet region and belongs to the Lyman series.

Spectral lines like these can be used in the identification of unknown specimens.

Mass Spectroscopy

Another tool used to identify specific atomic structures is mass spectroscopy, which is based on the concept that differences in mass cause differences in the degree of bending that occurs in a beam of ions passing through a magnetic field. This is shown in Figure 7.

Figure 7. Mass Spectroscope

Figure 7. Mass Spectroscope

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A mass spectroscope separates isotopes of the same element based on differences in their mass.

The intensity on the photographic plate indicates the amount of each particular isotope. Other collectors may be used in place of the photographic plate to collect and interpret these data.