The man who discovered atoms experimentally in 1802, John Dalton, thought that they were tiny solid spheres like billiard balls, and that they were the smallest particles in the universe.
But in 1896, a French scientist, Henri Becquerel, discovered even smaller positively charged particles shooting out of uranium atoms. Then in 1897, another scientist, J.J. Thompson, was able to "pull" yet smaller negatively charged particles from atoms in a metal plate connected to a high voltage inside a glass tube. They were named "electrons". A plasma ball shakes electrons from a gas inside the globe. The electrons move through the gas and escape to the outside of the globe. If you hold a normal fluorescent bulb just outside the globe, the electrons will collide with the white fluorescent coating inside the bulb and make it glow. You are "seeing electrons".
Later again in 1932, James Chadwick discovered a third particle inside atoms that carried no charge. Scientists soon realized that atoms had even smaller particles, called sub-atomic particles, inside them.
Because atoms have no overall electric charge, scientists realized that the number of positive protons equaled the number of negative electrons.
Inside an atom
Number of protons = Number of electrons
Neutrons are protons are almost the same mass, though neutrons are slightly heavier. Electrons are very light in comparison- only 1/1836 th of the mass of a proton. Although the masses of these particles are important, we will mostly focus on their different charges at this stage.
In 1909, New Zealander Earnest Rutherford did his famous "gold-foil experiment" in which he shot alpha particles- which are kind of atomic bullets- through a thin gold foil. To his surprise, most of the alpha particles went straight through the foil without bending, so he concluded that atoms were mostly "empty space". However, one in 8 000 alpha particles bounced right back, so it must have hit something very heavy inside the foil. Putting this evidence together, Rutherford concluded that atoms were made of a small heavy positive nucleus, with negatively charged electrons wrapped around them. Heavy neutrons also live in the nucleus.
Over 99.95% of the atom’s mass comes from its nucleus, and the most of the atom’s space comes from its electron orbits.
The tiny electrons are moving fast and are ‘all over the map’ so if you could see an atom through a microscope it might look like a fuzzy ball shown at the right.
When scientists could count the number of protons in the nucleus of different elements, they realized this number corresponded exactly to the Atomic Number of the element. It seemed that what element an atom belongs to depended only on this number of protons.
Atomic Number = Number of protons in the nucleus
An atom’s identity, that is, whether it is H or Al or Pb, depends completely on the number of protons in the nucleus. Atomic Number was redefined as the number of protons in the nucleus, rather than its place in the list of atoms from lightest to heaviest (although this was still true). The smallest atom, hydrogen, has one proton in its nucleus so it has an Atomic Number of 1. The next largest atom, helium, has two protons in its nucleus so it has an Atomic Number of 2, and so on until we reach the larger atoms such as Uranium with 92 protons in its nucleus giving it an Atomic Number of 92. The number of protons determines what element it is, how it will behave and what properties it will have- its colour, odour, state (solid, liquid or gas), hardness, magnetism, electrical conductivity and flammability etc. Each element has only one Atomic Number, and each Atomic Number belongs to only one element. For example, zinc is the only element with an Atomic Number of 30, and an element with Atomic Number 30 can only be zinc.
So now when you look at the Atomic Number on the Periodic Table, you know that this is the number of protons that its atoms have in its nucleus.
Electron are wrapped around the atom’s nucleus in shells. The first shell, which is closest to the nucleus, can accommodate up to 2 electrons and then it becomes ‘full’. The second shell out from the nucleus can accommodate up to 8 electrons before it becomes ‘full’. This is like a strange hotel that has only 2 beds on its first floor so after 2 guests are booked in, extra guests have to be sent to a higher floor. The hotel’s second floor has 8 beds so after the next 8 guests are booked this floor is also full.
Hydrogen, which has just 1 electron, puts it in the first shell. Helium puts both of its 2 electrons in this shell. These elements are placed on the first horizontal row on the Periodic Table to show their electron shells fill the first shell. (You can check this on your Periodic Table.) Because the first shell is now full, the elements with more than 2 electrons start filling the second electron shell.
Li, Be, B, C, N, O, F and Ne put their first 2 electrons into the first shell just like H and He, but their next 8 electrons (3rd , 4th , 5th , 6th , 7th , 8th , 9th , and 10th electrons) go into their second shell. These elements form the second row of the Period Table to show their electrons fill the first two shells. Neon’s second shell is full, so the next elements with more than 10 electrons start filling the third shell and are shown on the third row on the Periodic Table.
The Periodic Table is like the booking sheet for our strange electron hotel and it shows how each element’s electrons are ‘booked into’ the atom’s shells. The rows on the ‘booking sheet’ correspond to the atoms’ electrons in different shells. Since the first shell can take up to 2 electrons, the first row of the Periodic Table has 2 elements (H and He). The second shell can take up to 8 electrons, so there are 8 elements in this row (Li, Be, B, C, N, O, F and Ne).
Scientists have a shorthand method to show how many electrons are in each shell. For example, a nitrogen atom has 7 electrons, which means 2 electrons are located in the first shell, and the other 5 electrons are located in the second shell. Scientists write this as N [2,5]. The first number inside the brackets before the comma shows the 2 electrons in the first shell, and the second number after the comma shows the 5 electrons in the second shell. Simple, isn’t it!
This video shows where we find the protons, electrons and neutrons inside the atoms. You also discover that the Atomic Number is also exactly equal to the number of protons in the atom's nucleus. The video then explains how the electrons are wrapped around the nucleus in shells, and how this is important to how the atoms behaves in the world.
The Periodic Table is directly connected to Atomic Structure in surprising ways. This video shows you how an element's place on the Periodic Table can tell us about how it is made up of protons and electrons.