Atomic Nucleus

The basic particles of an atom are the proton, neutron, and electron. One model of an atom pictures a dense, positively charged nucleus. A cloud of negative electricity surrounds it. The cloud is produced by electrons orbiting around the nucleus. The nucleus contains neutrons and positively charged protons.

A nucleus is only a tiny fraction of an atom. Yet, it contains almost the total mass of an atom. The mass of an electron is almost nothing compared to the mass of a proton or a neutron. The mass of one proton or one neutron equals the,mass of 1837 electrons.

Atoms are assigned a mass number based on a scale of atomic mass units. Atomic mass unit is abbreviated as a.m.u. For example, a helium nucleus has a mass of 4 a.m.u. Sodium has a mass of

23 a.m.u. Sulfur has a mass of 32 a.m.u. The mass is about equal to the sum of the atom's protons and neutrons.

The entire universe is held together by forces. The force of gravity holds the parts of the solar system together. Atoms and molecules are held together by the ionic and magnetic forces of charged particles. Nuclear forces hold the parts of an atomic nucleus together.

Nuclear forces hold protons and neutrons together in a nucleus.

However, they are not like the force of gravity or the forces between charged electrical particles. Nuclear forcesexist only in distances shorter than the size of an atomic nucleus (10^-12 cm or less). Thus, at distances of 10^-12 cm or less, protons are held together by nuclear forces. At larger distances, the positive charges of the protons cause them to repel each other. Nuclear forces also cause protons and neutrons to bind together within a nucleus.

· Name four kinds of forces in the universe.

PROBLEM

1) Draw a diagram of a sodium atom (11 protons,

11electrons, 12 neutrons). Name three ways in

which your diagram is different from an actual atom.

· Why does the nucleus contain most of an

atom's mass?

Figure 4-5. Forces hold the entire universe together. The Trifid nebula is held together by forces.

Nuclear forces – ядерні сили a.m.u. – а.о.м.(атомна одиниця маси)

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