Understanding of Atomic

Atom is a basic unit of matter, which consists of atomic nuclei and negatively charged electron cloud surrounding it. The nucleus consists of positively charged protons and electrically neutral neutrons (except hydrogen-1 nucleus, which has no neutrons). Electron-electron in an atom bound to the nucleus by the electromagnetic force. Collection of atoms can likewise bind to each other, and form a molecule. Atom containing the number of protons and electrons the same neutral, while containing a number of different protons and electrons are positive or negative and is called ions. Atoms are grouped based on the number of protons and neutrons contained in the atomic nucleus. The number of protons in an atom determines the chemical element the atom, and the number of neutrons determines the isotope of the element.
The term comes from the Greek atom (ἄτομος / Atomos, α-τεμνω), which means it can not be cut or something that can not be subdivided. The concept of atoms as a component that can not be subdivided first proposed by the philosophers of India and Greece. In the 17th century and the 18th, the chemists laid the foundations of this idea by showing that certain substances can not be divided further using chemical methods. During the late 19th century and early 20th century, physicists had found the structure and components of subatomic particles in atoms, proving that the 'atom' is not can not be subdivided. The principles of quantum mechanics used by physicists then successfully model the atom. 

In daily observations, the atom is considered a relatively very small objects that have mass also proportionally small. Atom can only be monitored by using special equipment such as atomic force microscopy. More than 99.9% of the mass of atoms centered on the nucleus, [note 1] with protons and neutrons are nearly the same mass. Each element has at least one isotope with unstable nuclei, which can experience radioactive decay. This can result in transmutation, which change the number of protons and neutrons in the nucleus. [2] Electrons are bound to the atom contains a number of energy level, or orbital, which is stable and can undergo transitions between these level by absorbing or emitting photons corresponding to the difference between the energy level. Electrons in an atom determines the chemical properties of an element, and affect the magnetic properties of these atoms.