The development of understanding of atomic structure in line with modern chemical science awalperkembangan. The first scientist to membangunmodel (structure) atom is John Dalton, and then gradually by JJ disempurnakansecara Thomson, Rutherford, and Niels Bohr. 

1. Atomic Model DaltonDalton's atomic theory is based on quantitative measurements of chemical reactions. Dalton produces some of the following postulates.

    Material composed of particles of very dense and small that can not be broken again. The particles were called atoms.
    The atoms of an element are identical in all respects, but different from atoms of other elements.
    In chemical reactions, occurring merger or separation and rearrangement of the atoms from one composition to another composition.
    Atoms can combine with other atoms to form a molecule with a simple comparison.
The conclusion of Dalton atomic model, which consists of elements of the same atoms in all things, good shape, size, and mass, but different from atoms of other elements. In other words, the atom is the smallest particle of an element that still has the properties of that element.
Dalton's Atomic Theory 

2. Thomson Atomic ModelBased on the fact that electrons are fundamental particles making up matter, prompting Thomson to build an atomic modelto refine the atomic theory of Dalton because Dalton's atomic model does not indicate the presence of electrical properties. According to Thomson, atoms contain negatively charged electrons and the electrons are spread evenly throughout the atom. Atom itself is assumed to be a solid ball is positively charged. If Thomson's atomic model is described in three dimensionswould like a cake balls, sesame ore onde declared states of electrons and form atoms. Figure 1.11 shows the atomic modelThomson. If Thomson's atomic model halved then the electrons in the atom would look like pink stone ore spread evenly in the guava.Thomson Atomic Model
3. Rutherford Atom ModelRutherford's experiment shooting a very thin plate of gold with alpha particles emitted by radioactive elements. Data from the experiments showed that the majority of alpha particles can pass through the plates of gold, but only a small fraction of alpha particles are reflected back. Figure 1.12 shows a diagram of alpha particle scattering.Rutherford Experiment
Based on that data, Rutherford concluded that the volume of the atom mostly empty space. This is shown by the number of alpha particles that can pass through the gold plate. Presence of alpha particles that are reflected due to collide with a particle that is very hard with very small size. Rutherford named it as the core of an atomic particle. Therefore, the positively charged alpha particles it must be positively charged nucleus. If the negatively charged atomic nucleus there will be a tug between atomic nuclei and alpha particles.Based on these experiments, Rutherford devised a model of the atom (see Figure 1.13) to refine the atomic models of Thomson. The model developed by Rutherford is as follows.1. Atomic nuclei are composed of positively charged and electrons negatively charged.2. Most of the volume of an atom is empty space whose mass is concentrated in the nucleus.3. Hence the number of neutral atoms positive charge must equal the number of negative charge.4. Inside the atom, the negatively charged electrons are always moving around the atomic nucleus.Rutherford Atom Model
Weaknesses Rutherford Atomic ModelLike its predecessor the atomic model, Rutherford's atomic theory has flaws. The main drawback lies in the movement of electrons around the nucleus in an atom.According to the Law of Classical Physics of Maxwell, if an electrically charged particle moving in a circle will emit energy in the form of light which resulted in the acceleration of particles decreases and finally silent. Thus, if the negatively charged electrons moving in a circle (around the positively charged nucleus) then it will lose its energy so that the movement of electrons is reduced, which will eventually fall into the nucleus. Figure 1.14 shows the Rutherford atomic model according to Maxwell's theory.So, according to the Law of Classical Physics, Rutherford atomic model is not stable because the electron will lose energy and fall into the nucleus, the atom will eventually perish. However, the fact that the atom is stable. 

4. Bohr Atom ModelIn 1913, Danish physicist, Niels Bohr states that the failure of the Rutherford atomic model can be enhanced byapply the Quantum Theory of Planck. Bohr's atomic model is expressed in the form of four postulates relating to the movement of electrons, which are as follows.

    In the surrounding atomic nuclei, electrons are on the (track) specific. This skin is a stationary motion (settling) of the electrons in the atomic nucleus surrounded by a certain distance.
    During the electron is at a certain stationary trajectory, fixed electron energy so that no energy is emitted or absorbed.
    Electrons can switch from one skin to another skin. In this transition, which involved the same amount of energy with Planck equation, ΔE = h.
    Stationary trajectory of the electrons have angular momentum. The amount of angular momentum is a multiple of nh/2π, where n is the quantum number and h is Planck's constant.
Bohr Atom Model
Trajectory of the electrons in the skin or around the nucleus is denoted by n = 1, n = 2, n = 3, and so on. This symbol is called quantum numbers. Bohr's atomic model shown in Figure 1.15. The letters K, L, M, and so on are used to express the trajectory of the electrons in the surrounding atomic nuclei. Trajectory with n = 1 is called the K shell, the trajectory with n = 2 is called the L shell, and so on.
Basic and excited state energies
An atom is said to have the lowest energy or stable if the electrons are in the ground state. Ground state for hydrogen atom if the electron is located on the skin, n = 1. Circumstances where n> 1 for otherwise unstable hydrogen atom, a condition called excited states. This situation occurs when a hydrogen atom absorbs energy of (Δn) hv. In the excited state, electrons are returned to the original skin is accompanied by emission of energy (Δn) hv. When the electrons return to lower skin will form a spectrum. Consider Figure 1.16.
Bohr's idea of ​​an electron around the nucleus in shells