An atom is the smallest particle of an element that can be reduced to and still retains the characteristics of that element. According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons.
The nucleus consists of positively charged particles called protons and uncharged particles called neutrons.The basic particles of negative charge are called electrons.
Protons and neutrons are approximately the same mass. The mass of an electron is 1/1836 of a proton. Within protons and neutrons there are even smaller particles called quarks.
All elements are arranged in the periodic table of the elements in order according to their atomic number. The atomic number equals the number of protons in the nucleus, which is the same as the number of electrons in an electrically balanced (neutral) atom.