Bohr and Introduction To Electron Structure Part I

Big Picture Let’s look at three consecutive elements Chlorine – atomic # 17 Argon – atomic # 18 Potassium - atomic # 19 How are they similar and different?

Chlorine – yellow-green gas at room temperature – Highly reactive Argon – Gas at room temperature – Gas used in light bulbs – Extremely unreactive Potassium – Solid metal at room temperature – Highly reactive

Essential questions What accounts for the element’s differences? What role do electrons play in all of this?

Let’s start by looking back at our atomic theory of the atom

Rutherford’s Model Described the nucleus, but… It did not explain similarity and differences in element’s chemical properties It did not explain why the how the electrons are arranged around the nucleus. Nor why the negatively charged electrons did not fall into the positively charged nucleus and collapse the atom. 6

Emission Spectra In the early 1900’s, scientists observed that different elements emitted visible light when heated. Analysis of this emitted light reveals that chemical behavior is related to electron arrangement. (flame test demo)

Emission Spectra Atoms can absorb energy, but they must eventually release it When atoms emit energy, it is released in the form of light Atoms don’t absorb or emit all colors The spectrum of colors emitted can identify the element 8

Emission Spectra Lithium chloride Sodium chloride Potassium chloride Red Yellow Violet Calcium chloride Red-orange Strontium chloride Bright red

Line Spectra = specific wavelengths are emitted; characteristic of atoms 10

The Bohr Model of the Atom Bohr postulated that electrons do not fall into the nucleus because they can only travel in certain allowable orbits or energy levels. Proposed a model of the atom that explained the light given off by the heated or excited atoms. When atoms are excited, electrons jump up energy levels and then emit light as they fall down to their ground states. 11

Bohr Model of Atom: Electron Orbits In the Bohr Model, electrons travel in orbits or energy levels around the nucleus The farther the electron is from the nucleus the more energy it has. 12

The Bohr Model of the Atom: Orbits and Energy Each orbit (energy level) has a specific amount of energy Energy of each orbit is symbolized by n, with values of 1, 2, 3 etc; the higher the value the farther it is from the nucleus and the more energy an electron in that orbit has Tro's Introductory Chemistry, Chapter 9 13

The Bohr Model of the Atom: Energy Transitions Electrons can move from a lower to a higher (farther from nucleus) energy level by absorbing energy When the electron moves from a higher to a lower (closer to nucleus) energy level, energy is emitted from the atom as a photon of light Tro's Introductory Chemistry, Chapter 9 14

The Bohr Model of the Atom Ground and Excited States Ground state – atoms with their electrons in the lowest energy level possible; this lowest energy state is the most stable. Excited state – a higher energy state; electrons jump to higher energy levels by absorbing energy Atom is less stable in an excited state; it will release the extra energy to return to the ground state 15

The limits of Bohr’s model The Bohr Model very accurately predicts the spectrum of hydrogen with its one electron It is inadequate when applied to atoms with many electrons It has did not explain the chemical behavior of atoms. Even though the model is incorrect, it laid the groundwork for future atomic models.

Ever wonder how glow sticks work? Similar to how we use fire to excite our atoms earlier, glow sticks use a chemical reaction to excite electrons The chemical reaction causes the electrons to rise to a higher energy level When the electrons return to their ground state, they emit light in the form of chemiluminescence.

Check for Understanding Now work with a partner to complete: Electron Energy and Light Bohr Atomic Models Questions