Atomic Number

This gives the number of protons or electrons required in the atom for each element. For the hydrogen atom in Fig. 1, the atomic number is 1, which means the nucleus has 1 proton balanced by 1 orbital electrons. Similarly, the carbon atom in Fig. 2 with atomic number 6 has 6 protons in the nucleus and 6 orbital electrons. Also, the copper atom has 29 electrons because its atomic number is 29.

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Structure of the Atom

Although nobody has ever seen an atom, its hypothetical structure fits experimental evidence that has been measured very exactly. The size and electric charge of the invisible particles in the atom are indicated by how much they are deflected by known forces. Our present planetary model of the atom was proposed by Niels Bohr in 1913. His contribution was joining the new ideas of the nuclear atom developed by Lord Rutherford (1871-1937) with the quantum theory of radiation developed by Max Planck (1858-1947) and Albert Einstein (1879-1955).

The nucleus contains protons for all the positive charge in the atom. The number of protons in the nucleus is equal to the number of planetary electrons. Thus, the positive and negative charges are balanced, as the proton and electron have equal and opposite charges. The orbits for the planetary electrons are also called shells or energy levels.

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Molecules and Compounds

A group of two or more atoms forms a molecule. For instance, two atoms of hydrogen (H) form a hydrogen molecule (H2). When hydrogen unites chemically with oxygen, the result is water (H2O), which is compound. A compound, then consists of two or more elements. The molecule is the smallest unit of a compound with the same chemical characteristic. We can have molecules for either elements or compounds. However, atoms exist only for the elements.

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