how atoms differ

how atoms differ

1. Isotopes and Mass Number

   • dalton was incorrect about atoms being indivisible and in stating that all atoms of an element are identical All atoms of an element have the same number of protons and electrons. but the number of neutrons might differ. For example. there are three types of potassium atoms that occur naturally. All three types contain 19 protons and 19 electrons. However, one type of potassium atom contains 20 neutrons, another 21 neutrons, and still another 22 neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes.

2. Mass of Atoms

   • Recall from Table 3 that the masses of both protons and neutrons are approximately 1.67 x 10¯24 g. While this is a small mass, the mass of an electron is even smaller—only about 1/1840 that of a proton or a neutron.

3. Atomic Number

   • In the modern periodic table, the elements are listed in order of increasing atomic number. The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The number of protons determines how many electrons surround the nucleus, and it is the arrangement of these electrons that determines most of the chemical behavior of an element. In a periodic table arranged in order of increasing atomic number, elements having similar chemical properties naturally line up in the same column (group). For instance, all of the elements in Group 1A are relatively soft metals, react violently with water, and form 1+ charges; all of the elements in Group 8A are unreactive, monatomic gases at room temperature, etc. In other words, there is a periodic repetition of the properties of the chemical elements with increasing mass. In the original p