Atoms

• Matter that consists of a single type of atom is called an element. Atoms are typically described as the smallest piece of an element that still has the properties of that element. The center, or nucleus, of an atom contains particles called protons and neutrons. Tiny particles called electrons surround the nucleus.
  
  

Protons

• The proton is a positively charged particle. At least one proton exists in the nucleus of every atom. The total number of protons in the nucleus varies by element; scientists refer to this number as an element's atomic number. Protons are about 1,840 times more massive than electrons, but have roughly the same mass as neutrons. In 1911, the scientists Ernest Rutherford, Hans Geiger and Ernest Marsden were experimenting with tiny particles called alpha particles. They noticed that when they aimed a beam of these particles at metal foil, some of them were scattered. Rutherford theorized that it was the previously unknown atomic nucleus that caused the alpha particles to scatter. In 1919, Rutherford also discovered the proton.
  
  

Neutrons

• A neutron is an uncharged particle with slightly more mass than a proton. The number of neutrons in an atom is normally equal to, or greater than, the number of protons. Hydrogen is the sole exception; it doesn't have any neutrons. The total number of neutrons and protons constitutes an atom's mass number. Inside the nucleus, neutrons are stable. Outside the nucleus, they break down in a few minutes into an electron, a proton and a particle called an antineutrino.
  
  

Electrons

• An electron is a negatively charged particle found outside the nucleus. Electrons give elements their chemical properties. Normally, the number of electrons and protons is equal, so the atom has a neutral electrical charge. When an atom gains or loses electrons, it develops a negative or positive charge. Scientists once believed that electrons orbited the nucleus like planets orbiting a star. However, scientists now know that this model is incorrect. Instead, they believe that some areas around the atom have a high probability of having electrons, while others have a low probability of having electrons. The areas of high probability roughly correspond with the areas formerly depicted as orbits. Electrons also create magnetic fields around themselves by spinning; these fields affect the arrangement and interactions of electrons in an atom.