Atom (science)
An atom (from the Greek atomos, indivisible) is the smallest physical unit that can retain its chemical properties. While atoms were once thought to be the smallest instances of mass in the universe, we now know that they are made up of smaller subatomic-particles.
Structure of the Atom
Atoms are made of a dense nucleus formed by the two nucleons (positively charged protons and zero charge neutrons) and surrounded by a much larger "cloud" of electrons. The number of protons contained in the nucleus determines the atomic number and in turn which element it is classified as. The number of neutrons further specifies the isotope number of that element. The number of electrons surrounding the nucleus is typically assumed to be equal to the number of protons in order to keep the entire atom electrically neutral. Atoms that are not neutral are called [[[ions]], they are designated by their charge in units of elementary charge, which is equal to the negative of the number of surplus electrons present around the atom. Thus an atom with one extra electron is charged -1, and one missing is charged +1.
The nucleus of the atom contains a high concentration of positively charged particles with no counter balancing negatively charged particles to keep it stable. In order to explain the existence of the nucleus scientists introduced the two nuclear forces, the [strong force]] and the weak force. We now understand that the nucleus is held together by the residual strong force despite its significant positive charge. The electrons which surround the atom are electromagnetism attracted to the nucleus due to their negative charge.
Most combinations of nucleons are inherently unstable and undergo a number of radiation in order to form more stable nuclei.