physics- atomic structure revision (developing the atom (The plum pudding…
physics- atomic structure revision
developing the atom
The plum pudding model- After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding.
Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum. Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom..
Bohr's 'solar system' model of the atom is the way that most people think about atoms today.
The modern view of the atom is of a nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.
Isotopes are forms of an element that have the same number of protons but different numbers of neutrons.
an element's mass numbers can vary, which means that it can have different numbers of neutrons. So although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons.
Atoms can, however, lose or gain electrons due to collisions or other interactions. When they do, they form charged particles
if the atom loses one or more electrons, it becomes a positively-charged ion
if the atom gains one or more electrons, it becomes a negatively-charged ion
Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay by emitting radiation.
unstable nucleus can decay by emitting an alpha particle, a beta particle, a gamma ray
beta particle- The beta particle is an electron but it has come from the nucleus, not the outside of the atom.
gamma ray- High energy particles will emit energy as they drop to lower energy levels. Since energy levels in the nucleus are much higher than those in the gas, the nucleus will cool down by emitting a more energetic electromagnetic wave called a gamma ray.
alpha particle- If the nucleus has too few neutrons, it will emit a ‘package’ of two protons and two neutrons called an alpha particle.
Half-life is the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve. Count-rate is the number of decays recorded each second by a detector, such as the Geiger-Muller tube.
a fraction - a ½ of a ½ of a ½ of a ½ remains, which is ½ × ½ × ½ × ½ = 1/16 of the original sample
Alpha decay (two protons and two neutrons) changes the mass number of the element by -4 and the atomic number by -2. An alpha particle is the same as a helium-4 nucleus.
Shining visible radiation from a torch beam onto a hand, lights the hand up because the hand has been exposed to light.
Exposing objects to beams of radiation is called irradiation. The term applies to all types of radiation including radiation from the nuclei of atoms.
Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard.
Contamination occurs if an object has a radioactive material introduced into it. An apple exposed to the radiation from cobalt-60 is irradiated but an apple with cobalt-60 injected into it is contaminated.