Describe the Bhor's Model Flashcards

Students should be able to describe a simple model involving electrons kept in orbit around the nucleus as a result of the electrostatic attraction between the electrons and the nucleus.
· Nuclear model of atom has positively charged nucleus made up of protons and neutrons. · Negatively charged particles called electrons orbit the nucleus. · Most of the mass of the atom is concentrated in nucleus. · Electrons are much smaller than protons and neutrons. · Neutrons have no charge. · Neutral atoms are where the charges of the protons and electrons balance; same number of protons and electrons.

A qualitative description of the Geiger–Marsden experiment and an interpretation of the results are all that is required.
· Alpha particles were shot at thin sheet of gold foil. · Most particles passed through, since atoms are made up mostly of empty space · Some particles deflected at large angles, indicating a solid, positively charged structure within atoms.

Limitations include: the model does not take into consideration isotopes and different atomic weights, and the model does not show how the atom would be stable. Electrons would lose energy with radiation.

Students should be familiar with emission and absorption spectra, but the details of atomic models are not required. Students should understand that light is not a continuous wave but is emitted as “packets” or “photons” of energy, each of energy hf.
Absorption and Emission Spectra provide evidence for energy levelsSpectra of each element is uniqueElectrons can only occupy specific energy levels.Movement between energy levels requires electron to emit or absorb energy.Energy emitted or absorbed is in the form of packets of light called photons.E=hf (Energy of a photon = Planck’s constant*frequency of light in Hz).Energy is "quantized".

1. Nuclide – a particular type of nucleus with a certain number of protons and neutrons 2. *Isotope - nuclei with the same number of protons (Z) but different number of neutrons (N) 3. *Nucleon – a proton or neutron (NOTE: Do not say “a particle in the nucleus” since that would include quarks as well.)

1. Nucleon Number (Mass Number) (A) - number of nucleons (protons + neutrons) in nucleus 2. *Proton Number (Atomic Number) (Z) - number of protons in nucleus 3. *Neutron Number (N) - number of neutrons in nucleus (N = A – Z)

Students need only know about the Coulomb interaction between protons and the strong, short-range nuclear interaction between nucleons.
Coulomb Force: The positively charged protons repel one another. Nucleons are arranged in such a way that like charges repel as far as possible. Electrostatic force of repulsion between the protons in the nucleus. Strong Nuclear Force: Strong force of attraction between nucleons that overcomes Coulomb repulsion between protons. Force is independent in whether particles are neutrons or protons. At 1.3 femtometers, force is about 100 times stronger than repulsion forces; if the distance between protons is greater than 1.3 fm, force falls to 0, if the distance is smaller than 1.3 fm, strongly repulsive.

When an unstable nucleus emits a particle (alpha, beta, gamma) (NOTE: Radioactive decay is both a random and a spontaneous process.) (NOTE: The rate of radioactive decay decreases exponentially with time.)
Process is independent of pressure, temperature, and chemical combination.

See image.

· When alpha particles pass through air, they take two electrons from the air molecules, and become neutral helium atoms. · Air molecules are left with a charge i.e. the air is ionized. · Beta particles and gamma radiations may also cause ionization; the particles of the radiation remove electrons, creating electron ion-pairs.

Students should be familiar with the direct and indirect effects of radiation on structures within cells. A simple account of short-term and long-term effects of radiation on the body is required.
· Radioactive radiations can cause severe damage to living organisms. · May affect DNA and genetic coding of future generations. · Probability of genetic damage increases with increasing intensity and exposure to radiation. · Illnesses such as leukemia and cancer can result from exposure; even death.

An explanation in terms of relative numbers of protons and neutrons and the forces involved is all that is required.
· For elements with Z less than about 20, the protons and neutrons are in equal numbers. · Due to an increase in the electrostatic repulsion forces of protons as the number of protons increases, more neutrons must be found in nucleus to hold atom together. · Each time protons and neutrons are added, they must go into higher energy state, and eventually become unstable. · Unstable nuclei emit alpha particles (two protons and two neutrons) in order to reach a more stable state.

Exponential decay need not be treated analytically. It is sufficient to know that any quantity that reduces to half its initial value in a constant time decays exponentially. The nature of the decay is independent of the initial amount.

The time taken for ½ the number of radioactive nuclei in sample to decay .
OR
The time taken for the activity of a sample to decrease to ½ its initial value.

Artificial Transmutation: When a nucleus is bombarded with a nucleon, an alpha particle or another small nucleus, resulting in a nuclide with a different proton number (a different element). · Can also be used to form radioactive isotopes of elements. · Neutrons are effective in inducing artificial transmutation to form artificial radioactive isotopes.