CHAPTER 5 ELECTRONS IN ATOMS
CHAPTER 5 ELECTRONS IN ATOMS
How do electrons behave in atoms ?
• Rutherford proposed that electrons move around the nucleus like planets around the sun.
• Rutherford’s atomic model could not explain the chemical properties of elements.
THE BOHR MODEL:
• Niels Bohr – 1885 – 1962
• Danish physicist, student of Rutherford,
• included discoveries about how the energy of an atom changes when it absorbs or emits light.
• Bohr proposed that an electron is found only in specific circular paths or orbits around the nucleus.
▪ each electron orbit has a fixed energy.
▪ Energy levels are the fixed energies an electron can have.
• closest to nucleus – lowest energy
• electrons can jump from one energy level to another
• electrons cannot be between energy levels
• to move from one energy level to another, an electron must gain or lose just the right amount of energy. This amount of energy is not always the same.
• energy levels in an atom are not equally spaced.
o highest energy levels are closest together.
o therefore it takes less energy to move from one level when they are closer together as with the higher energy levels.
• higher in energy – farther away from the nucleus.
A QUANTUM of energy is the amount of energy required to move an electron from one energy level to another energy level.
“quantum leap”
Bohr model used the hydrogen atom but did not explain the energies absorbed and emitted by atoms that have more than one electron.
QUANTUM MECHANICAL MODEL
• 1926 Erwin Schrodinger – (1887-1961)
▪ devised and solved mathematical equation describing the behavior of an electron in an hydrogen atom.
▪ thus the quantum mechanical model
• like Bohr – energy of electrons restricted to certain values
• unlike Bohr – does not involve an exact path of an electron around the nucleus.
“The Quantum Mechanical Model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus.”
Location of an electron –
• electron cloud – more dense where the probability of finding an electron is high, less dense where it is low.
ATOMIC ORBITALS:
• Schrodinger equation gives energies to electrons
▪ energy levels
▪ leads to a mathematical expression = atomic orbital
▪ atomic orbital describes the probability of finding an electron at various locations.
▪ atomic orbital = a region of space in which there is a high probability of finding an electron.
Quantum Mechanical model:
➢ energy levels are labeled by “principal quantum numbers (n).
➢ n = 1,2,3,4,etc.
➢ Each principal energy level can have several orbitals with different shapes and different energy levels. = energy sublevels.
➢ Each energy sublevel corresponds to an orbital of different shape describing where the electron is likely to be found.
Facts to remember about electrons:
□ Bohr – electrons move in circular orbits at fixed distances from the nucleus.
□ electrons are found in orbitals
□ an orbital is a region where an electron can probably be found.quantum mechanical model.
□ each electron in an atom has its own distinct amount of energy that corresponds to the energy level that it occupies.
□ an electron can only absorb a discrete or fixed amount of energy that will allow it to move to a higher energy level.
□ an electron can gain or lose energy by changing its orbit
□ the fixed energies an electron can have are called energy levels
□ a quantum of energy is the amount of energy required to move an electron from 1 energy level to another
Quantum Mechanical model:
□ modern description of the electrons in atoms.
□ like Bohr restricts energy of electrons to certain values
□ unlike Bohr it does not involve an exact path the electron takes around the nucleus
□ it determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus.
□ fuzzy cloud – cloud more dense where the probability of finding an electron is high.
Atomic orbitals:
□ electrons have energies
□ these are energy levels
□ leads to ATOMIC ORBITAL – describes the probability of finding an electron at various locations around the nucleus
□ atomic orbital is a region of space in which there is a high probability of finding an electron
□ energy levels of electrons are labeled by principal quantum numbers
▪ n=1,n=2,n=3,n=4 …………..
▪ Each principal energy level may have several orbitals with different shapes and at different energy levels
▪ energy levels within the principal energy level = sublevels
▪ different atomic orbitals denoted by letters.
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