The electrons are thought to orbit the nucleus within an area, the size and shape of which are related to the electrons' energy. It is the outer electrons, the so-called valence electrons, that form chemical bonds between atoms.
The number of protons in the nucleus of an atom determines the nature of the atom. For example, any atom with six protons in its nucleus will act just like carbon. That's because any atom with 6 protons is a carbon atom. In other words, the number of protons determines the identity of the atom. The number of protons in an atom's nucleus is called the atomic number because it identifies each unique type of atom. For example, every atom with one proton in its nucleus is hydrogen, every atom with two protons is helium, and every atom with 26 protons is iron. If you like, you can talk to my friends at Berkeley about smashing protons into nuclei to make new "man-made" elements (e.g., berkelium).
The number of neutrons in the nuclei of different atoms of the same element can vary. For example, most carbon atoms have six neutrons in their nuclei and are called carbon 12 atoms (12 is the atomic mass of each of those atoms - each has 6 neutrons and 6 protons). But a few carbon atoms have 8 neutrons and those atoms are called carbon 14. An atom with an unusual number of neutrons is called an isotope. Thus, carbon 14 is called an isotope of carbon. Carbon 12 is just called carbon. Carbon 13 would also be called an isotope of carbon.
The number of electrons in an atom can also vary when the atom is involved in a chemical reaction. For example, if a neutral sodium atom (Na is the chemical symbol for sodium) loses and electron to a neutral chlorine atom (Cl) during a chemical reaction, the result would be called a sodium ion (written as Na+) and a chloride ion (written as Cl-).
Chemical Bonding.
Chemical bonds form between atoms when it is energetically favorable. A chemical bond forms when two atoms share two electrons between them.
