The smallest particle of an element that contains all chemical properties element is called an atom. An atom is the fundamental building block of matter and is composed of a nucleus surrounded by electrons. The nucleus contains positively charged protons and neutral neutrons, while the electrons are negatively charged and orbit the nucleus in specific shells or energy levels. The number of protons in the nucleus determines the element's atomic number and its chemical properties. Atoms of the same element can have different numbers of neutrons, these are called isotopes and have the same chemical properties but different physical properties such as mass.