Chapter 27. Nuclear Physics
What is nucleus?
Our model of the atom changes as our experimental ability improves. Today’s atomic theory tries to explain the observations made with accelerators. The current “quark model” of the atom is a hypothesis based on current atomic theory. Now the physicists discovered, so far, that atoms are made up of 12 matter and antimatter particles.
Six types of fundamental matter particles are classified as leptons. Three kinds (called flavors) of these, the electron, the muon and the tau, exhibit a property called electrical charge (which can be negative, positive or zero) in the amount of -1. The other three flavors are neutrinos, which carry zero electrical charge. There is one type of neutrino corresponding to each type of charged lepton. These three neutrinos are referred to as the electron neutrino, the muon neutrino and the tau neutrino. As fundamental particles, leptons are assumed to be indivisible, and there is no evidence that they have any size.
There is a type of antilepton for each of these six kinds of lepton. The antilepton corresponding to the electron is called a positron; the other antileptons are the antimuon, antitau, anti-electron-neutrino, anti-muon-neutrino and the anti-tau-neutrino.
The six leptons (and their corresponding antileptons) are cross-classified into three lepton generations, each one generally lighter than the next: the electron neutrino and the electron (first lepton generation), the muon neutrino and the muon (second lepton generation) and the tau neutrino and the tau (third lepton generation).
And that’s just the start, my dear students…. Historically people during the ancient civilization had tried to explain the creation of the Universe. Here are some of them who helped us understand better our Universe.
Democritus (Greek philosopher around the year 400 BC.)
Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually, the smallest piece of matter would be found. He used the word atomos to describe the smallest possible piece of matter.
John Dalton (English chemist that proposed first atomic theory in 1803. )Points of Dalton’s Theory:
1. All elements are composed of indivisible particles.
2.Atoms of the same element are exactly alike.
3.Atoms of different elements are different.
4.Compounds are formed by joining atoms of two or more elements.
J. J. Thomson (English scientist who discovered electrons in 1897. )
Sometimes called the “plum pudding” model, Thomson thought of an atom as being composed of a positively charged material with the negatively charged electrons scattered through it.
Ernest Rutherford British physicist who discovered the nucleus in 1908 (His idea, but the experiment was carried out by Geiger and Marsden, his research assistants).
Rutherford’s model proposed that an atom is mostly empty space. There is a small, positive nucleus with the negative electrons scattered around the outside edge. He found that the atom consists of nucleus (proton and neutron as its constituents) where mass is concentrated and nucleus is orbited by electrons. (We use the term ‘orbit’ loosely here).
Niels Bohr Danish scientist who proposed the Planetary Model in 1913.
Electrons move in definite orbits around the nucleus, like planets moving around the Sun. Bohr proposed that each electron moves in a specific energy level.
Sir James Chadwick discovered the neutron in 1935. His wave model, based on wave mechanics, proposes that electrons have NO definite path in an atom. The probable location of an electron is based on how much energy it has. The more energy an electron has, the farther from the nucleus. The small, positively charged nucleus is surrounded by a large space in which there are enough electrons to make the atom neutral.
Posted by nazarin 
Posted by nazarin 
Posted by nazarin 
