Atomic Structure and the Elements

Many elements are familiar to us in everyday life. Iron is an element used for making ships, cars, spades, etc. There are about 90 such familiar elements, including helium, oxygen, nitrogen, mercury, platinum, and gold. As an element, iron consists of atoms of iron, the smallest building blocks, each of which is indivisible by chemical means. A lump of iron comprises millions, trillions, and zillions of atoms, and the mass of each atom of iron is very small, about 10 g In a piece of iron weighing 50 g, there are about lO atoms. 

There are 26 electrons in the Iron atom but each has little mass. The electrons are spread out around the nucleus, occupying most of the space but very little of the mass. 

The nucleus of an iron atom contains 26 protons and 30 neutrons, which occupy very little space but make up most of the mass of the atom 

A representation of atomic structure. The various spheres are not drawn to scale. The lump of iron on the left would contain almost a million million million million (10 ) atoms, one of which is represented by the sphere in the top center of the page. In turn, each atom is composed of a number of electrons, protons, and neutrons. For example, an atom of the element iron contains 26 electrons, 26 protons, and 30 neutrons. The physical size of the atom is determined mainly by the number of electrons, but almost all of its mass is determined by the number of protons and neutrons in its dense core or nucleus (lower part of figure). The electrons are spread out around the nucleus, and their number determines atomic size but the protons and neutrons compose a very dense, small core, and their number determines atomic mass. 

The electron carries one negative electric charge and has a mass only 1/2000th that of a proton. 

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