In 1913, Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation. Bohr Atomic Model: In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to.
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Scientists have had the technology to observe discrete spectra since the beginning of the 19th century. They had to wait over a hundred years, though, foran explanation of how the discrete spectra were produced. They knew that it wasproduced by atoms and that atoms had negative and positive charges in them.Some models of the atom were similar to our current one: the positive chargesare concentrated in a central nucleus with the negative charges swarming aroundit, but the atoms should be unstable. As the negative charges (calledelectrons) move around the nucleus, they should radiate light and spiralinto the nucleus in about 10-16 second. This is obviously contradicted bycommon experience!
Niels Bohr (lived 1885--1962) provided the explanation in the early 20th century. He said that the electron can be only found in energy orbits of acertain size and as long as the electron is in one of those special orbits, itwould radiate no energy. If the electron changed orbits, it would radiate orabsorb energy. This model sounds outlandish, but numerous experiments haveshown it to be true.
In Bohr's model of the atom, the massive but small positively-chargedprotons and massive but small neutral neutrons arefound in the tiny nucleus. The small, light negatively-charged electrons move around the nucleus in certain specific orbits (energies).In a neutral atom the number of electrons = the number of protons.The arrangement of an atom's energy orbits depends on the number of protonsand neutrons in the nucleus and the number of electrons orbiting the nucleus.Because every type of atom has a unique arrangement of the energy orbits, theyproduce a unique pattern of absorption or emission lines.
Select here for an enlargement of the spectra.
All atoms with the same number of protons in the nucleus are grouped togetherinto something called an element. Because the atoms of an element havethe same number of protons, they also have the same number of electrons and,therefore, the same chemical properties. For example, all atoms with oneproton in the nucleus have the same chemical properties and are calledHydrogen. All atoms with two protons in the nucleus will not chemically react with any other atoms and are known as Helium. The atoms called Carbon form the basis of life and have six protons in the nucleus. In the figure below, atom (a) is Hydrogen, atom (b) is Helium,atoms (c), (d), and (e) are Lithium.
Elements are sub-divided into sub-groups called isotopes based on thenumber of protons AND neutrons in the nucleus. All atoms of an element with thesame number of neutrons in the nucleus are of the same type of isotope. An element's isotopes will have very nearly the same chemical properties but they can behave very differently in nuclear reactions. For example,all of the isotopes of the element Hydrogen have one electron orbiting thenucleus and behave the same way in chemistry reactions.The ordinary Hydrogen isotope has 0 neutrons + 1 proton while anotherHydrogen isotope called Deuterium has 1 neutron + 1 proton and another Hydrogen isotope called Tritium has 2 neutrons + 1 proton in the nucleus. Tritium is radioactive---its nucleus spontaneouly changes into anothertype of nucleus. In the figure above, atoms (c), (d), and (e) are differentisotopes of the same element called Lithium.
Most atoms in nature are neutral, the negative charges exactly cancel thepositive charges. But sometimes an atom has a hard collision with another atomor absorbs an energetic photon so that one or more electrons are knocked outof the atom. In some rare cases, an atom may temporarily hold onto an extraelectron. In either case, the atom has an extra positive or negative charge andis called an ion. For example, the carbon ion C+ has 6 protons and 5 electrons and the iron ion Fe2+ has 26 protons and 24 electrons. Because the number of electrons are different, an ion of an element will behave differently in chemical reactions than its neutral cousins. In the figure above atom (d) is a Li+ ion [compare it with atom (c) or (e)].
In order to explain discrete spectra, Bohr found that atoms obey threebasic rules:
- Electrons have only certain energies corresponding to particular distances from nucleus. As long as the electron is in one of those energyorbits, it will not lose or absorb any energy. The energy orbits are analogous to rungs on a ladder: electrons can be only on rungs of the ladder and not in between rungs.
- The orbits closer to the nucleus have lower energy.
- Atoms want to be in the lowest possible energy state called the ground state (all electrons as close to the nucleus as possible).
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last updated: January 10, 2013