You may have already wondered if incredibly tiny atoms were themselves made of even more incredibly tiny particles? The answer is “Yes”. Atoms are made of smaller particles called protons, electrons and neutrons. Protons are positively charged, electrons are negatively charged and neutrons are neutral. The protons and neutrons are about the same mass. However, the electrons are only 1/1836 th of the mass of protons and neutrons. Because these particles are even smaller than atoms, scientists call them sub-atomic particles.
Protons are positively charged, electrons are negatively charged and neutrons are neutral.
Protons and neutrons are about equal in mass, and electrons are 1/1,836 of the mass of a proton.
The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is incredibly small and heavy- more than 99% of the mass of the atoms is found here.
The much lighter electrons are found in the electron shells, sometimes called electron clouds, outside the nucleus. An electron shell is shaped like the skin of an orange. It is a spherical region with the nucleus at its centre. However, each atom has many shells of different diameters, so electrons have plenty of choice about where to go.
The diagram to the right shows where protons, neutrons and electrons are found in atoms. However, this diagram is not to scale, as the nucleus is much smaller in relation to the overall size of the atom. If an atom was the size of a football stadium, the nucleus would be the size of a pea. This means that the nucleus would be too small to see in this diagram, so we have made it look much bigger than it really is.
The number of protons and electrons are equal. This means that the positive and negative charges cancel out, making atoms overall neutral. That’s why we don’t go around with our hair sticking on end.
Number of protons = Number of electrons
The number of protons and neutrons are about equal, though this varies. The number of neutrons becomes larger than the number of protons as the atoms becomes heavier.
Compared to each other, sub-atomic particles can be described as follows:
• Protons are positive and heavy.
• Electrons are negative and light.
• Neutrons are neutral and heavy
Properties of sub-atomic particles * Compared to the mass of a proton
Protons and neutrons are located in the nucleus of the atom, and electrons whiz in orbits a loooong way from the nucleus. Electrons are about 100,000 times further away from the nucleus than the width of the nucleus itself. The diagram above makes the size of the nucleus look much larger than it really is. This means that an atom’s mass comes mostly from its nucleus, and the most of the atom’s space comes from its electron orbits. An atom is mostly empty space!
The tiny electrons are moving fast and are ‘all over the map’, and we can never be sure exactly where the electrons are, so if you could see an atom through a microscope it might look like a fuzzy ball shown at the right.
Quiz: Forces between sub-atomic particles
1. Fill in the following:
Opposite charges ______________
Like charges _______________
Neutral charges ______________
Protons carry a __________ charge.
Electrons carry a __________charge.
Neutrons carry ________ charge at all.
3. Fill in the following blanks.
Particle 2 Force between them
proton (+) proton (+)
proton (+) electron (-)
proton (0) neutron (0)
electron proton .
proton no force
4. (a) Where would you find the proton in an atom? In the nucleus/outside the nucleus.
(b) Where would you find a neutron in an atom? In the nucleus/outside the nucleus.
(c) Where would you find an electron in an atom? In the nucleus/outside the nucleus.
An atom’s type, that is, whether it is hydrogen or aluminium or gold, depends entirely on the number of protons in the nucleus. This number of protons is called its Atomic Number. 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.
Atomic Number = Number of protons in the nucleus
Teacher background: How did scientists discover that atoms were made of protons, electrons and neutrons?
See video: https://www.youtube.com/watch?v=kBgIMRV895w
In 1887, J.J. Thompson put some electrodes in a sealed evacuated glass container, and hooked them up to high voltages: see the electrodes on the left hand side of the tube in the diagram. This high voltage was able to “pull” a beam of particles out of the negative electrode, called the cathode. Because they were emitted by the cathode, they were called cathode rays, shown as a blue line in the diagram, and the device was called a cathode ray tube. When they landed at the other end of the tube, they made a fluorescent coating glow so that they could be seen. This glowing is shown as a green spot in the diagram. Thompson showed that the cathode rays could be bent by a magnetic field, or a pair of electrically charged parallel plates, shown in the middle of the diagram. Because they bent away from the negative plate and towards the positive place, Thompson knew that the rays carried a negative charge. And due to the high degree of bending, he could deduce that they were very light particles, much lighter than atoms. Finally, because he got the same results using different metals as cathodes, the particles, the particles must be common to all the different kinds of atoms. He had discovered one of the building blocks of atoms- the electron.
Thompson proposed the “plum pudding” model of the atom, in which electrons (the plums) were scattered around a sea of dispersed positive charge (the pudding).
In 1909 one of Thompson’s earlier students, Ernest Rutherford, performed his famous “gold foil” experiment, in which he shot a beam of alpha particles at a gold foil target. Alpha particles are positively charged particles that are emitted by radioactive uranium atoms, and much heavier than cathode rays (which are electrons). He used them as tiny “atomic bullets” to shoot at the gold atoms. When the alpha particles landed on the detecting screen, they would glow in a similar manner to a cathode ray tube. He expected that they most would pass through if Thompson’s plum pudding model was correct, because the thinly spread out positive charge of the gold atoms in his model would not be able to stop them. Indeed, most alpha particle did pass straight through, which confirmed that atoms were mostly empty space.
But some were deflected off course, and a very small number (about one in 20,000) were bent more than 90o, which means that they rebounded back towards the alpha particle source. Rutherford was so surprised that he said at the time “It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.” These results meant that the atoms must contain a small, very heavy, positively charged centre, which Rutherford called the “nucleus”. Later he was able to identify the charged particles that were responsible for the positive charge of the nucleus, and he called them “protons”. Since protons were so much heavier than electrons, it was thought that they protons were the main contributor to the atom’s mass.
An amateur Dutch physicist, Antonius Johannes van den Broek, then realised that the charge on an atom’s nucleus- the number of protons it contains- was exactly the same as its Atomic Number. Each element had its own particular number of protons in its nucleus. And since matter was generally electrically neutral, it meant that the number of electrons equalled the numb of protons in an atom, with their opposite charges cancelling each other out. The picture of the atoms was becoming clearer.
Since scientists knew how many protons and electrons each element had, they could work out how heavy they should be. But many elements’ atoms were about twice that compared to what they should be if the atoms consisted of only protons and electrons. One of Rutherford’s students, James Chadwick, designed an experiment in which he bombarded a beryllium target with alpha particles, and was able to knock a beam of particles out of its nucleus that were about the same mass as protons, but would not bend when passed through a pair of charged parallel plates. This meant that these new particles were electrically neutral. He had discovered a new sub-atomic particle that explained the extra observed mass of atoms- the neutron.
Sometimes electrons are compared to planets orbiting around the sun, with each planet having its own radius. However, there are problems with this model, because all the planets are confined to a single plane, more or less, whereas electrons are free to occupy 3 dimensions around the atomic nucleus. Atoms are shaped more like marbles than coins.
A better picture would be of electrons wrapped in 3-D shells around the nucleus. This is the version that we will use here, but keep in mind that all models have their limitations. In the case of the shell model, for example, it suggests that electrons have fixed radius of orbit around the nucleus. While it is true that the electron spends most of its time at a particular radius, it is free to wander closer and further away from the nucleus from time to time. Electrons are hard to “pin down”. In this chapter, we will think of shells as spherically shaped surfaces that electrons occupy, like the skin of an orange. But whereas oranges have only one skin, atoms have many “skins”, each getting bigger as it is further away from the nucleus. These are the electrons’ “homes”, with a particular electron confined to a particular shell. Spherically-shaped shells is quite a good approximation for some electrons, but other electrons “orbit” around the nucleus in more complicated shapes, such as dumbbells and doughnuts. Weird, eh?
Each shell is associated with a definite amount of energy. The inner shell, or the first shell, is closest to the nucleus and is most strongly bound by it by the attractive force of opposite charges, so it has the lowest energy. The bigger the orbital, the more energy the electron has. This is like the floors in a building- the lowest floor is closest to the ground, so has the lowest energy, whereas the higher floors have more energy. You can imagine that you would have more energy if you jumped off the 3 floor rather than the first floor of a building. Don’t try this!
The energy of an electron is important. For example, it takes much less effort to remove an electron from an outer shell than an inner shell. Also, the energies of the outer shell of one type of atom might be different to that of a different type. This affects how much energy is needed to remove an electron from the different types of atoms, and this means that the different types of atoms behave very differently, chemically speaking.
Maximum number of electrons per shell
When an atoms is “made” from scratch, the electrons are filled into the shells in a definite sequence. The inner shell, or first shell, is filled first, and then the outer shells are progressively filled. Like the floors of a hotel which doesn’t have a lift, you would probably select the room at the lowest possible height that wasn’t already occupied. For example, if all the rooms on the first and second floors were already filled, and only some on the third floor were filled, you would choose one of the remaining rooms on this third floor. This means that you would have to climb the least number of stairs to get to your room. Electrons follow this same energy principle.
Also like a floor on a hotel, an electron shell can accommodate only a maximum number of electrons before it becomes full. And because outer shells are bigger, they can generally fit more electrons in them before they become full.
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. The hotel floors get bigger as they get higher, and can accommodate more guests per floor.
H atom: Atomic No 1 He atom: Atomic No 2
1 proton 2 protons
1 electron in first shell 2 electrons in first shell
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 must start filling the second electron shell.
Li atom: Atomic No 3 N Atom: Atomic No 7
3 protons 7 protons
2 electron in first shell 2 electrons in first shell
1 electron in the second shell 5 electrons in second 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, or guests on different floors in the case of the hotel. 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 discovered that the third electron shell can accommodate up to 8 electrons as well, just like the second shell. The fourth and fifth shells can both accommodate up to 18 electrons each. The sixth and seventh shells can accommodate up to 32 electrons each. Our weird electron hotel is getting wider as it gets further from the ground!
If you add up the electron that can be fitted into seven shells (2 + 8 + 8 + 18 + 18 + 32 + 32) it totals 118. This means that all known elements can fit into just seven electron shells. In turn this means the Periodic Table has just 7 rows. You can count them! Atoms with more than this number of electrons are too unstable and scientists have not yet found any. If they do discover these elements they would start an eighth row on the periodic table.
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!
Video 2: Atomic Structure meets the Periodic Table (8 min) https://www.youtube.com/watch?v=3_FJIpKgdV4
Shells and Rows
Check out this coincidence! How many elements on the first row of the Periodic Table? Hydrogen and helium add up to 2 elements. How many electrons can the first shell take? 2!
How many elements in the second row of the Periodic Table? Lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon add up to 8. How many electrons can the second shell take? 8!
Each row on the Periodic Table corresponds to an electron shell.
How many electrons do you think the third and fourth shells can take? Just look at the Periodic Table and count the number of elements in the third and fourth rows, and you will have your answer.