Structure of atom Flashcards
Who discovered the electrons and how and what they are? (describe the process and the properties of electrons)
The electron was discovered by J.J. Thompson using the cathode ray experiment.
-> He did this using a cathode ray tube, which is a vaccum sealed tube with a cathode and anode that created a beam of electrons travelling toward the other end of the tube.
-> the air inside the chamber was maintained as low pressure and subjected to high voltage and electricity started flowing from the negative electrode to the positive electrode.
-> this experiment showed that the atom was not an indivisible particle and had atleast one subatomic particle- electron..
Electron has negligible mass as it is 2000 times smaller than the proton. It is equal in magnitude as proton but opposite signs. It has negative charge.
Represented as e- (- is on top)
Who discovered the protons and how? What are protons, State its properties.
Eugene Goldstein discovered the proton before the electron in 1886 using the canal ray experiment.
-> the Canal ray experiment involved the use of a perforated cathode in the discharge tube.
-> The discharge tube was filled with air at a very low pressure and high voltage was applied across the electrodes in the discharge tube. A faint red glow was observed behind the perforated cathode.
-> This glow was due to another kind of rays flowing in a direction opposite to that of the cathode rays. These rays were called as anode rays or positive rays. These positively charged rays were also called as canal rays as the rays passed through the holes or canals present in the perforated cathode. The particles of the canal rays were much heavier than electrons and carried a positive charge.
- A proton has positive charge, 2000 times heavier than electrons, have the same magnitude as electrons and is represented as p+ (+ is on top)
- The no of protons is approximately equal to the number of neutrons present in the nucleus.
Who proposed the first model of an atom? What was it and what did they propose, what were it’s limitations?
J.J. Thomson was the first to propose a model for the structure of an atom.
According to his model, atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it. This model was called as plum pudding model. The electrons represent the plums in the pudding made of negative charge. This model is similar to a water-melon in which the pulp represents the positive charge and the seeds denote the electrons.
➢ An atom is composed of a positively charged sphere in which electrons are embedded in it.
➢ Atom is electrically neutral as the positive and negative charges are equal in magnitude.
Advantage: This model could explain the overall neutrality of an atom.
Limitation: The results of the other experiments carried out by other scientists could not be explained by this model.
What did Rutheford use and expect for the gold foil experiment?
Ernest Rutherford and his team performed an experiment in 1909 were fast moving alpha (α)-particles were bomabrded on a thing gold foil.
-> He selected a gold foil as he wanted the layer as thing as possible (due to gold being the most malleable metal). The gold foil was about 1000 atoms thick.
-> Alpha (α)-particles are double charged helium ions. Since they have a mass of 4u, the fast moving (α)-particles have considerable amount of energy.
-> He expected that the (α)-particles would be deflected by the sub-atomic particles in the gold atoms, since the (α)-particles were much heavier than protons, he didn’t expect to see large deflections.
radius of the nucleus is about 10^5 times less than the radius of the atom
What did Rutherford observe and conclude from the gold foil experiment?
Observation and conclusion.
1. Most of the alpha particles passed straight through the gold foil which shows that atoms contain a lot of empty space.
2. Some particles got deflected at small angles which shows that the positive charges aren;t occupying much of its space.
3. Only one out of 12000 particles bounced back 180 which shows that the protons are concentrated over a small particular area of the atom.
What was Rutheford’s Nuclear Model of the atom? What were it’s drawbacks.
Rutheford’s Nuclear model of the atom-
1. There is a positively charged centre called the nucleus. Almost all of the mass of the atom resides in nucleus.
2. The electrons revolve around the nucleus in a circular path.
3. The size of the nuxleus is very small as compared to the atom.
Drawbacks of Rutheford’s model-
1. He explained that the electrons revolved around the nucleus in a circular path but this meant that the electrons that were in the circular path would experience acceleration, this would cause an expenditure of energy and eventually the electron would lose all its energy and fall into the nucleus.
2. This cannot take place as the atoms would be extremely unstable and wouldn’t exist in the form we know.
What was Bohr’s Model Of the atom, what did he propose?
The problem of the stability of the atom and the distribution of electrons in the atom was solved by Neils Bohr’s model of the atom.
➢ Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level.
➢An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state
.
➢An electron emits or absorbs energy when it jumps from one orbit or energy level to another
➢When it jumps from a higher energy level, it emits energy while it absorbs energy when it jumps from lower energy level to higher energy level.
What are neutrons?
J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries no charge and mass nearly equal to that of a proton is known as aNeutron. It is represented as ‘n’. Neutrons are present in the nucleus of all atoms, except hydrogen.
How are electrons distributed in different orbits, by who was this distribution suggested by?
The distribution of was suggested by Bohr and Bury.
-> The maximum no of electrons presented in a shell is given by the formula 2n^2, where ‘n’ is the orbit no or energy level index, 1, 2, 3.
-> The maximum no of electrons in differens shells are-
K shell (first) - 2 x1^2= 2
L shell (second) = 2 x 2^2 = 8
M shell (third) = 2 x 3^2 = 18
N shell (fourth) = 2 x 4^2 = 32
-> The max no of electrons that can be accomodated in the outermost orbit is 8.
-> the shells are always filled in a step-wise manner from the lower to higher energy shells. Electrons are not filled in the next shell until the previous are filled.
What are valence electrons?
The electrons present in the outermost shell of an atom is called as valence electrons.
What is valency?
The combining capacity of atoms or their tendency to react and form molecules with atoms of same or different elements to attain a state of octet (8 electrons in the outermost shell)is known as valency of the atom.
Why do some atoms react with other atoms while some don’t?
-> Some atoms such as Helium (2,8) , Argon (2,8,8) and other inert gases have a completely filled outermost shell, that is why they show little to no chemical activity. Their combining capacity or valecy is zero.
-> Whereas most other atoms have an incomplete outer shell. This causes them to lose, gain or share electrons in order to attain an octet. (Outermost shell has 8 electrons)
What is atomic number (Z)
The number of protons in an atom is called the atomic number and is denoted by the symbol ‘Z’.
Number of Protons present in an atom = Atomic number (Z)
Number of Electrons present in an atom= Atomic number (Z)
What is mass number?
The mass number is the measure of the total number of protons and neutrons (Nucleons) inthe nucleus of an atom. The notation A indicates the Mass number.
Mass number (A) = number of protons (Z) + number of neutrons (n)
Number of Neutrons (n) = Massnumber (A) - Atomic number (Z).
Atomic number and mass number are represented as
Mass Number
Symbol of element
Atomic Number
What are nucleons?
Protons and neutrons are the two subatomic particles present in the nucleus. This is why they are called as nucleons.
What are isotopes?
Isotopes of a given element have the same number of protons, while the number of neutrons varies.
* Atoms of the same element with different numbers of neutrons in the nucleus are referred to as isotopes.
* The isotopes of an element have the same atomic number but different mass numbers.
* Isotopes are identified by their mass numbers. For example, the isotopes of carbon are carbon-12, carbon-13, and carbon-14.
* Isotopes of an element have similar chemical properties but have different physical properties.
What are applications of isotopes?
Applications:
➢ An Isotope of Uranium is used as a fuel in nuclear reactors.
➢ An isotope of Cobalt isused in the treatment of cancer.
➢ An isotope of Iodine is used in the treatment of goitre.
What are isobars?
Isobars are elements with different atomic numbers but the same mass numbers. Thus, Isobars is an element with different chemical properties but similar physical properties. Also, they possess different chemical properties due to the difference in the number of electrons. An isobar contains the same atomic mass, but the atomic number is different. This is because theadditional number of neutrons compensates for the difference in the number of nuclei.
