The atom is made out of protons, neutrons, and electrons. In the center of the atom is the nucleus. This is made out of protons and electrons. The nucleus is very small. The nucleus is surrounded by a cloud of electrons. $^{1(32)}$$^{1(30)}$
| Name | Mass | Charge |
|---|---|---|
| Proton | 1 Unit | +1 |
| Neutron | 1 Unit | 0 |
| Electron | 0 | -1 |
Electron’s mass is so small, that we take their mass as 0. $^{1(32)}$
Each element has a unique number of protons and electrons. There are as many protons as there are electrons. This is because atoms must be neutral; as a result the positively charged protons are balanced out by the negatively charged electrons. This is called as the proton number. $^{1(32)}$
The nucleus is made out of protons and neutrons. These are collectively called as the nucleons. The total number of nucleons is called the nucleon number. $^{1(33)}$
The electrons of an atom are arranged in shells. These shells orbit around the nucleus. $^{1(37)}$
The further the shell is from the Nucleus, the more number of electrons will fit on the shell and the higher the energy level will be.$^{1(37)}$
Each shell can hold a certain number of electrons. Shell 1 can hold 2, shell 2 can hold 8, shell 3 can hold 18, shell 4 can hold 32, etc. Goes according to the rule of $2n^2$. $^{1(37)}$
However the third shell only will fill up till 8 electrons. $^{1(37)}$
Democritus, a Greek philosopher, came up with the idea that all substances can be divided again and again, until something indivisible is reached. He called these things atoms. He thought that there were 4 types: black, white, red, and green. Other philosophers thought that it was based off of the 4 elements: earth, fire, water, and air.$^{1(39)}$
Robert Boyle found that gas can be compressed, he figured that this means that it was made from many small particles with space in between them. Joseph Louis Proust later showed that when a chemical reaction is done with the same proportion of chemicals, the outcome is the same. This further supported the particle idea.$^{1(40)}$
Dalton came up with the idea that atoms of different elements can only combine in specific ratios. In 1827 Robert Brown, was studying pollen grains in water, under a microscope. He saw them jiggling around. He proved that the particle theory was actually true.$^{1(40)}$