Bohr found out that electron orbits are fixed and contain a very precise amount of energy. Electrons can orbit around an atomic nucleus only in these fixed orbits and as long as they are within these orbits they do not radiate energy. If, however, electrons jump from one orbit to the other then they will absorb or emit energy if they jump to an outer orbit or inner one respectively. This energy is expressed as a whole number or quantum.
This single quantum is called a photon. Photons can be described by a mathematical formula, which was created thanks to German theoretical physicist Max Planck. I would, however, avoid using mathematics and try to describe photons in a different way.
I imagine a photon as a quantum of energy which comes in the shape of a package. It is built up purely, or contains only electromagnetic waves, which represent the energy. How much energy a particular photon carries depends on the frequency of electromagnetic waves which make a particular photon. The more waves are in one photon, the higher frequency of these waves is in this particular photon and therefore the more energy this photon carries (Picture 1.03).
Picture 1.03
Wavelength is inversely related to wave frequency. It means the shorter the wavelength, the higher the frequency of waves or the longer the wavelength, the lower the frequency of waves.
We can return now to the main structure of the atom. An atom has positively charged protons and neutral neutron particles, both of which are placed in the centre of an atom, and form the atomic nucleus.
Around an atomic nucleus, but at a greater distance, are arranged negatively charged electrons. They are arranged in ‘shells’. Each ‘shell’ has its own energetic level. The energy of this level increases, going from the inner shells towards the outer shells. The first shell, which is closest to the atomic nucleus, is at a lower energy level than the second shell, which is outside the first shell and further from the atomic nucleus.
Why does each shell gain a higher energy level the further it is from the atomic nucleus?
The way I make sense of it lies in the existence of the positively charged nucleus (protons) and negatively charged electrons. Positively charged protons attract electrons to themselves. So, for electrons to be further away from the positive charge of the protons requires energy, in order for them to oppose the force of the positively charged protons. That is why if they are placed in further shells from the atomic nucleus, these shells have a higher energy level. They need to have more energy to oppose the attractive force of protons.
Electrons absorb energy when they jump from a lower energy level to a higher energy level, i.e. from an inner shell to an outer shell. The reverse happens when an electron goes back from an outer level or shell to a lower energy level or inner shell. In the latter case, an electron radiates the energy that it has previously absorbed to maintain itself at an outer shell. This energy is freed as a photon, with a frequency that is within the spectrum of visible light (Picture 1.04).
Picture 1.04
An example of this happening in the world is the Aurora Lights, which happen around the Northern or Southern Pole.
What actually happens here is that oxygen and nitrogen atoms get ‘excited’ by the proximity of some positive particles, which pass near these atoms. This happens during solar winds or storms, when a lot of alpha radiation reaches our planet. Alpha radiation is identified as a helium nucleus, containing protons and neutrons. The Earth is protected from such radiation by its own magnetic field. In contact with the magnetic field, the particles are redirected towards the Earth’s magnetic poles. Around that area they come into contact with air and atoms. As alpha radiation has positive particles (protons), they cause the outmost electrons in these atoms to jump into an outer shell. The electron gets an input of energy for this jump, in the shape of a photon. The excited state of the atom is not a natural state, and therefore lasts only for a short period, after which the electron goes back to its natural shell, goes back to its so-called ground state. During this, it emits a photon with a wave frequency in the spectrum of light. If this absorbed energy is high, then the wave frequency will be high, towards the blue colour of the spectrum. In the sky, it will be seen as a blue colour. If the energy of the proton is lower, with a lower wave frequency, it will lean towards the red colour of the light spectrum. We will see a red colour in the sky.
Before going back to the structure of the atom, a question can be asked as to why electrons have to go around an atomic nucleus in fixed energetic level so-called shells.
Why cannot they orbit freely around an atomic nucleus?
The answer to this is that a particular energetic level, shell, protects an electron from falling down to an atomic nucleus being attracted to it by positive charge of protons in a nucleus. To explain it better, I will use our planet and gravitational force as an analogy.
We can imagine our world as an atomic nucleus. The gravitational force of our planet is what brings us down to the surface on Earth, if we fall off a nearby cliff, for example.
We can now imagine an eight-storey car park building, which is built around the equator, going all around it making like a ring around Earth. We can imagine cars (electrons) being driven around Earth at any of these storeys with no problem doing so. If a car is driven from one storey to an upper one, it needs to use more energy (photon input). If it is driven to the lower floor, the storey (shell) does not need this energy any more as the gravitational force of Earth is attracting the car. The car can go down to the next level with the engine switched off (talking about the old type of cars).
Now let us imagine that the storeys (shells) of the car park suddenly disappear. Then all these cars located at different storeys of the car park will fall down to Earth.
This is the same as what would happen to electrons around an atomic nucleus if there were no energetic levels or shells where electrons are orbiting around an atomic nucleus. In other words, electrons would get to an atomic nucleus so the structure of a tome will cease to exist and with it the quality and property of what makes an atom or an element.
So for an electron to orbit around an atomic nucleus with no fear, to be eventually pulled to an atomic nucleus, an electron has to be in an energetic level or shell. An electron cannot survive in between energetic shells so its jump from one shell to another is not a purely mechanical action. What I mean is not a situation where en electron just physically jumps from one shell to another as for this journey an electron has to pass a space between two shells in the process of jumping from one shell to another. This is actually not happening and the process of jumping from one shell to another is more precisely described as the disappearance of a particular electron from one shell and its appearance at another shell. This process is called a quantum leap.
Electrons are arranged into shells around an atomic nucleus. Every shell is, however, further divided into subshells or orbitals. Each orbital can contain only two electrons.
The number of subshells within a shell depends on that shell’s number. Shell number one (the first shell around an atomic nucleus) has only one subshell or orbit. Shell number two (the second shell around an atomic nucleus) has two subshells – the one closer to the atomic nucleus contains one orbit, the one further from the atomic nucleus contains 3 orbits. I will not go into more detail regarding this.
The atomic number refers to the number of protons a particular atom has in its nucleus. It is what determines a specific characteristic of this atom or element where the smelliest part of that element is present with this particular atom. The charge of an atom is equal zero, as the number of protons equals the number of electrons in a particular atom.
Hydrogen has an atomic number 1 consisting of 1 proton in the nucleus and 1 electron orbiting around. Helium has an atomic number 2 with 2 protons in the nucleus and 2 electrons orbiting around the nucleus.
Carbon has an atomic number 6 with 6 electrons orbiting around it while oxygen has 8 protons in the c
entre with 8 electrons orbiting around the nucleus.
Elements or their atoms are arranged in the Periodic Table in order of increasing atomic numbers.
It was Dimitri Mendaleev, a Russian chemist, who noticed a repeated pattern of chemical properties among elements, which were known at that time, in the mid-nighteenth century. He initially arranged those elements in order of an increasing mass. Later on that was changed according to an increase in the atomic number.
The mass number is the sum of the number of protons and neutrons in a nucleus of a particular atom. Usually, it is easy to memorise that number of protons corresponding to the number of neutrons. For example, helium, with an atomic number of 2, has mass number 4 consisting of 2 protons and 2 neutrons.
Oxygen, with an atomic number of 8, has a mass number close to 16, consisting of 8 protons and 8 neutrons in its nucleus. In reality, an element or atom representing a particular element with a particular atomic number comes in different forms, containing the same atomic number but a different number of neutrons.
Isotops are atoms with the same atomic number but a different number of neutrons.
The atomic number determines the property of an atom and the fact that a particular atom is the smallest part of that element. However, an atom with the same atomic number can exist in many different forms where each atom has a different number of neutrons.
Hydrogen has 1 proton with no neutron. Mostly, hydrogen exists as an element with atomic number 1 and mass number 1. However, in a small proportion there is another form of hydrogen atom with atomic number 1 and mass number 2, consisting of 1 proton and 1 neutron. This is isotope of hydrogen, which is called deuterium. There is another isotope of hydrogen consisting of 1 proton and 2 electrons, called tritium.
Ions
Not only can the number of neutrons vary within the same elements when they came as isotopes of this element, but also the number of electrons can vary.
An element has zero charge or an atom is neutral as the number of protons in the nucleus is equal to the number of electrons in the shells around the atom. However, an atom can capture en electron in the outer shell from another atom or lose an electron to another atom. Such atoms with numbers of protons and electrons that are not equal to each other are called ions.
If in this process an atom gains an electron, it becomes negatively charged as it has one more electron than number of protons. Such an ion is then called anion.
If an atom loses an electron then it becomes positively charged as it has 1 proton more than the number of electrons. Such ions are called cations (Picture 1.05).
Picture 1.05
What has been described here is chemical bonding, which can be created between 2 atoms. It is called ionic bonding. Such bonding is created between metal and nonmetal.
An example of ionic chemical bonding is table salt or sodium chloride.
Sodium has an atomic number 11 with 11 protons in the nucleus and 11 electrons, which are arranged in 2 shells. The first inner shell can have only 2 electrons while the second shell can have up to 8 electrons. With 11 electrons, sodium has 2 electrons in the inner shell, 8 in the second shell and only 1 electron in the third shell. As the third shell is furthest away from the atomic nucleus, it has the less attractive force from the positively charged nucleus. The atom of sodium can therefore easily lose the outside electron and become a cation.
Chlorine has an atomic number 17 with 17 protons and 17 electrons arranged as 2 electrons in the first shell, 8 in the second shell, and 7 in the third shell missing 1, to have 8 electrons in it. With such structure of an atom, less energy is needed to gain 1 electron to get 8 electrons in this shell than to lose 7 to maintain 8 electrons in the outer, which in the case of losing 7 electrons, will be the second shell.
Chlorine, therefore, gains an electron from sodium, becoming an anion, which makes chlorine a strong ionic chemical bond with sodium. Sodium as a positively charged cation reacts with chlorine, negatively charged anion bringing net electric charge to zero. By doing this, we now have molecules of salt composed of these two elements, chemically bound by this ionic bonding.
Ionic chemical bonds compounds when dissolved in water can conduct electricity. If a substance conducts electricity when dissolved in water, then it is called electrolyte.
Atoms can interact among each other by making covalent bonds. It happens among those elements or atoms which in outer shells do not have such a small or large number of electrons that they can easily lose or gain electrons respectively. In such cases, atoms enter into chemical reactions where instead of gaining or losing electrons, the atoms share electrons among each other (Picture 1.06).
Picture 1.06
An example of covalent bonding
Two hydrogen atoms are bound together by covalent chemical bonds. Instead of gaining or losing an electron, two-hydrogen atoms are in a covalent bond where the creating molecules of two hydrogen atoms share their electrons. In doing so, the outer shells of both hydrogen atoms now have 2 electrons circling around both atoms and in that way establishing the stable state of an element; hydrogen in this case.
We mentioned earlier that the atom is the smallest part of an element which still has the physical and chemical characteristics of that element. The particular chemical characteristic depends on the number of electrons each atom of a particular element has in its own outermost shell.
Apart from the first shell around the atom which has to have 2 electrons to be full, each next shell needs to have 8 electrons in order for that element to be chemically stable, that is to say not to react with other elements.
So-called noble gases such as neon, argon, krypton, xenon, and radon have 8 electrons in their outer shells. Helium is the first noble gas and has only one shells, the first one with 2 electrons. With such atomic structure, they are chemically stable and do not need to get involved in chemical reactions. In fact, it would be extremely difficult to get these elements involved in any chemical reactions.
All the other elements have a number of electrons in their outer shells ranging from 1 to 7 depending on their atomic number and corresponding number of electrons. All those elements with a number of electrons up to 3 in the outer shell can engage in a chemical reaction where they can lose electrons, forming ionic chemical bonds. They would obviously become cautions in such a reaction.
Atoms can also gain a maximum of up to 3 electrons so all elements whose atoms have 5 electrons in their outer shells can gain 3 electrons, becoming anions and forming ionic chemical bonds.
Carbon with an atomic number of 6 and 6 corresponding electrons (2 in the inner and 4 in the outer shell) is a typical element, which gets involved in chemical reactions by covalent binding.
Many so-called organic compounds, where the carbon atom is the main skeleton or part of a structure of an organic molecule, are made by covalent bonds. Examples are sugar vinegar, carbodioxide (CO2) or a very complex organic molecule such as DNA.
Ionic compounds are mostly between metals and nonmetals. They are usually solid. The melting point is usually higher than for covalent compounds. Ionic compounds tend to be electrolytes.
Covalent compounds are between nonmetals. They can be solid, liquid or gas. They have a lower melting point and do not tend to be electrolyte.
We now have a rough idea about what is matter and what matter is made of. We have roughly elaborated on different kinds of matter called elements where the smallest part of an element makes an atom.
Atoms can react among themselves, creating compounds. When 2 or more different atoms react chemically among themselves they create a molecule, which is the smallest part of a particular compound having property, which makes this compound unique.
A molecule of water or glucose is still water or glucose until it breaks down to its elements, which initially this molecule was made of. If we go in an upwards direction trying to grasp chemical reactions, diff
erent compounds from the simple to the most complicated ones, we will go into the field of chemistry, biochemistry, molecular biology and so on. I do not want to go in this direction and I will go to the core of our matter or downwards to the tiniest particle of which the matter consists. That is within the field of physics of subatomic particles or elementary partials and within the scope of quantum physics or theories.
SUBATOMIC PARTICLES
With modern progress in physics of subatomic particles it is now well known that there are more than 200 subatomic particles. Most of them last only an incredibly short period of time.
Every subatomic particle can be characterised and defined by the three main properties they can express. These are their mass, charge and spin (rotation around their own axis).
With present technology, we are able to calculate the mass of many particles. We know, for example, that the mass of an electron is almost 2000 times smaller than the mass of a proton. We also know that the mass of a neutron is slightly higher than the mass of a proton. They are also massless particles like photons.
Charge is the second characteristic that a particle can express. We are familiar with positively charged particles such as protons and negatively charged electrons. There are also particles which carry no charge and are neutral, such as neutrons.
The mass of a particle is very important, as the quantity of that mass will determine the strength of gravitational force, which exists among particles on a large scale.
The charge of a particle has an important rule when electromagnetic force comes into effect. A neutron, as no charge or neutral particle, has no rule in fundamental reactions among particles, which are carried out by electromagnetic forces.
Journey Through Time Page 2
