# Building blocks of matter

In general science contexts, the term matter refers to anything (a substance or a material) from which objects are made. Generally, matter is made up of atoms, has (rest) mass, and occupies space (volume). From where the word atoms came into the contexts of science and why it become important to explain it? The following example explains the whole phenomenon behind the matter and atom.

#### Example:

An object of cube shaped of solid gold has some specified length with no empty space. If the cube is cut in half, the two pieces still retain their chemical identity as solid gold. But what if the pieces are cut again and again, indefinitely? Will the smaller and smaller pieces always be gold? Questions such as these can be traced back to early Greek philosophers. Two of them - Leucippus and his student Democritus - could not accept the idea that such cuttings could go on forever. They speculated that the process ultimately must end when it produces a particle that can no longer be cut. In Greek, they were calling atomos means "not sliceable". From this comes our English word atom.

#### Discovery of Nucleus

Let us review briefly what is known about the structure of matter. All ordinary matter consists of atoms, and each atom is made up of electrons surrounding a central nucleus. Following the discovery of the nucleus in 1911, the question arose: Does it have structure? That is, is the nucleus a single particle or a collection of particles? The exact composition of the nucleus is not known completely even today, but by the early 1930s a model evolved that helped us understand how the nucleus behaves. Specifically, scientists determined that occupying the nucleus are two basic entities, protons and neutrons. The proton carries a positive charge, and a specific element is identified by the number of protons in its nucleus. This number is called the atomic number of the element. For instance, the nucleus of a hydrogen atom contains one proton (and so the atomic number of hydrogen is 1), the nucleus of a helium atom contains two protons (atomic number 2), and the nucleus of a uranium atom contains 92 protons (atomic number 92). In addition to atomic number, there is a second number characterizing atoms mass number, defined as the number of protons plus neutrons in a nucleus. As we shall see, the atomic number of an element never varies (i.e., the number of protons does not vary) but the mass number can vary (i.e., the number of neutrons varies). Two or more atoms of the same element having different mass numbers are isotopes of one another.
Fig. No.1: The Hydrogen and Helium Atoms

#### How protons and neutrons act in the nucleus?

The existence of neutrons was verified conclusively in 1932. A neutron has no charge and a mass that is about equal to that of a proton. One of its primary purposes is to act as a "glue" that holds the nucleus together. If neutrons were not present in the nucleus, the repulsive force between the positively charged particles would cause the nucleus to come apart.
But is this where the breaking down stops? Protons, neutrons, and a host of other exotic particles are now known to be composed of six different varieties of particles called quarks, which have been given the names of up, down, strange, charm, bottom, and top. The up, charm, and top quarks have charges of $+2/3$ that of the proton, whereas the down, strange, and bottom quarks have charges of $- 1/3$ that of the proton. The proton consists of two up quarks and one down quark, which you can easily show leads to the correct charge for the proton. Likewise, the neutron consists of two down quarks and one up quark, giving a net charge of zero.

#### Example 2:

How proton and neutron can be expressed according to the different varieties of particles called quarks?
Solution: As we have read that quarks consists of six other particles knows as up, top, charm, bottom, down, and strange. Each of them has given a value out of three. We also know that 1 proton has +1 charge and 1 neutron has also zero charges. Here we are going to prove this according to quarks.
Fig.no.2: The proton and neutron quarks specification.
Following are quarks values; $$up = +2/3$$ $$top = +2/3$$ $$charm = +2/3$$ where $$down = -1/3$$ $$bottom = -1/3$$ $$strange = -1/3$$ So charge on one proton contains two up and one down quarks. $$\begin{array} {rcl} \text{1 proton}& = &up + up + down\\ \text{1 proton charge}& = &({+2 \over 3}) + ({+2 \over 3}) + ({-1 \over 3})\\ & = &{4-1 \over 3} = {3 \over 3} = 1\end{array}$$ Similarly charge on one neutron contains two down and one up quarks. $$\begin{array} {rcl} \text{1 neutron charge}& = &up + down + down\\ \text{1 neutron charge}& = &({+2 \over 3}) + ({-1 \over 3}) + ({-1 \over 3})\\ & = &{2-1-1 \over 3} = {0 \over 3} = 0\end{array}$$
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