Atoms are the most basic building blocks of all materials . Elements are basic substances consisting of only one kind of atoms . In the atom negatively charged electrons form a "cloud" . At the center of the atom is a tiny area known as a nucleus , consisting of positively charged protons and neutrons that have no electrical charge . Electrical forces of attraction are active between the positive nucleus and the negative electrons . The number of protons in the atom’s nucleus distinguishes one element from another , and this is known as the element’s atomic number . In a neutral atom , the number of protons equals the number of electrons . In an electrically charged atom , an ion , the number of electrons does not equal the number of protons . The periodic table is a table in which all the known elements are organized in ascending order of their atomic numbers . The table columns are also called chemical families , or groups , since they include elements with similar properties . For e...  אל הספר
מטח : המרכז לטכנולוגיה חינוכית