Maharishi Kanada suggested that
Matter was composed of many tiny particles called paramanus, which combined to form larger particles called anus.
| Term | Definition |
|---|---|
Maharishi Kanada suggested that | Matter was composed of many tiny particles called paramanus, which combined to form larger particles called anus. |
Who called paramanu as an atom | the Greek thinker Democritus |
dalton's atomic theory | He described the atom as the smallest particle of matter that still exhibits all the properties of an element. |
Main features of Dalton's atomic theory | 1. Matter is composed of indivisible particles called atoms.
2. Atoms can neither be created nor destroyed.
3. The atoms of an element are identical in every sense but differ from those of another element.
4. Atoms of an element can combine with atoms of another element in a fixed whole number ratio to create a molecule of a compound.
5. Many atoms of the same element can combine to form a molecule of that element
5. Atoms are the smallest particles of matter that play a role in chemical reactions. |
fundamental/ sub-atomic particles | protons, neutrons, and electrons |
who discovered electrons | They were discovered by JJ Thomson in 1897 while he was studying the properties of cathode rays. |
structure of cathode ray tubes | The CRTs were made of glass tubes sealed on both ends with metal plates. The metal plate connected to the positive terminal of the current was called the anode, while the plate connected to the negative terminal was called the cathode. It contained a gas at very low pressure. |
metal plate of CRT called cathode | connected to the negative terminal of the source of current. |
metal plate of CRT called anode | connected to the positive terminal of the source of current. |
cathode rays | when high voltage current was passed through the CRT, the CRT started glowing due to the emission of rays from the cathode. |
which rays are made of negative particles [electrons] | Cathode rays |
How did jj thomson show the negative nature of the particles in cathode rays | when an electric field was applied in the path of cathode rays in the discharge tube, the cathode rays deflected to the positive side. |
properties of electrons | 1. its properties are independent of the gas in the discharge tube
2. it carries 1 unit negative charge. |
mass of electron | 9.108 * 10^-31kg |
charge of electron | 1.602* 10^-19 coulombs |
electron is denoted by | 0
e
−1
with -1 being the charge and 0 being its mass. |
protons were discovered by | E. goldstein, when he was performing an experiment with a discharge tube having a perforated cathode. |
anode / canal rays | when an electric field was applied, rays were also emitted from the anode and passed through the perforated cathode. |
Rays having positively charged particles [protons] | anode rays |
properties of protons | 1. positively charged particle
2. mass is 1.672 ^-27 kg
3. charge is 1.602 coulomb |
pum pudding model | It was the first model of an atom suggested by J. J. Thomson. According to this model, the atom is a positively charged sphere in which electrons are embedded like plums in plum pudding. |
why is an atom considered electrically neutral | because the total positive charge is always equal to the total negative charge |
discovery of nucleus | in 1911, lord rutherford discovered the nucleus through the alpha particle scattering experiment. |
gold foil experiment | it was the bombardment of a thin gold foil in an evacuated chamber with fast moving alpha particles. |
alpha particles
| they are positively charged particles that are formed when 2 electrons are removed from a helium atom. they have 2 units of charge and 4 units of mass. |
observations | 1. most of the particles passed straight through the gold foil without any deflection.
2. a small fraction of the particles were deflected at small angles.
3. very few of them reflected back. |
conclusions | 1. An atom is mostly empty
2. It caused deflection of a few particles at small angles because it was a heavy, positively charged mass
3. It was centrally located, as only a few particles bounced back |
Rutherford's atomic model | It was made of a centrally located nucleus, which contained the whole mass of the atom and the protons, and it was very small compared to the rest of the atom.
It was also made of energy levels/shells/orbits containing electrons that surrounded the nucleus |
why is an atom electrically neutral | The number of electrons always equals the number of protons in an atom; hence, their charges cancel out. |
how were neutrons discovered | The mass of an atom was believed to be completely concentrated in the nucleus, which was made up only of protons. However, they realised that protons could not account for this mass by themselves, so it was predicted that there must be a 3rd subatomic particle by Rutherford, and in 1932, James Chadwick discovered this particle and called it a neutron. |
nucleons | Protons and neutrons are called nucleons because they are the particles present in the nucleus. |
properties of neutrons | 1. It is electrically neutral
2. Mass is 1.676 * 10^-27
3. Neutrons are > = protons
4. Hydrogen does not have neutrons |
why does an atom remain stable | When electrons move around the nucleus, it is expected to move closer to the centre as they are lighter and eventually lead to the collapse of the atom. However, this does not happen because of the presence of shells/ energy levels. The electrons revolve on these orbits, and they neither lose nor gain energy unless an external force is applied. Hence, the inward force of the nucleus is countered by the outward force of the orbits. |
who explained atomic stability | Neil Bohr |
which shells of an atom have the most & the least energy | The innermost shell has the least energy, while the outermost shell has the most energy. |
isotope | It is a form of an element with the same no of protons but a different no of neutrons, resulting in a different mass. |
isotopes of hydrogen | Protium, deuterium, tritium |
protium [ordinary hydrogen] | it is an isotope of hydrogen without neutrons |
deuterium [heavy hydrogen] | it is an isotope of hydrogen with 1 neutron. |
tritium [very heavy hydrogen] | it is an isotope of hydrogen with 2 neutrons. |
isotopes of carbon | carbon 12, 13, 14 |
isotopes of chlorine | chlorine 35, 37 |
properties of isotopes | 1. Isotopes of an element have the same properties despite differences in mass numbers.
2. Isotopes differ from each other only in their physical properties.
3. All isotopes of an element have the same electronic configuration.
|
ion formation [take Mg²⁺ as example] | electronic configuration of Mg = 2, 8, 2
hence, as it is easier to lose 2 electrons than gain 6 electrons to make valence electrons 8, we lose 2 electrons. hence, the result is a positive ion, or a cation. |
cation | positively charged ions |
anion | negatively charged ions |
why do elements have different properties | because no 2 elements have the same no of protons or electrons |
atomic number | number of protons present in the nucleus of an atom [the atomic number of an element = its place in the periodic table, as the periodic table is arranged in order of atomic no., in order of 1,2,3,4... etc.]. It is denoted by Z. |
mass number | Number of protons + neutrons in the nucleus of an atom. denoted by A. [neutrons= A-Z] |
how does the no of protons distinguish one element from the other? | The atomic no for each element is always fixed and can never vary, so it acts as a clue to identify the element. |
valency of hydrogen | 1 |
valency of helium | 0 |
valency of lithium | 1 |
valency of beryllium | 2 |
valency of boron | 3 |
valency of carbon | 4 |
valency of nitrogen | 3 |
valency of oxygen | 2 |
valency of fluorine | 1 |
valency of neon | 0 |
valency of sodium | 1 |
valency of magnesium | 2 |
valency of aluminium | 3 |
valency of silicon | 4 |
valency of phosphorus | 3 |
valency of sulphur | 2 |
valency of chlorine | 1 |
valency of argon | 0 |
valency of potassium | 1 |
valency of calcium | 2 |
elements with variable valency | copper, iron, lead |
electronic configuration | the arrangement of electrons in different shells of the atom. |
formula for no of electrons per shell [the bohr-bury scheme] | 2n ^2, where n is the shell number.
|
octet rule | to attain stability, the valence shell of an atom must have 8 electrons, which it can get by either losing or gaining electrons. |
duplet rule | for elements like hydrogen & helium, that have only one shell, only 2 electrons should be present in the valence shell to attain stability. |
valence shell | outermost shell of an atom |
valence electrons | The electrons present in the valence shell that determine the valency of an element. The number of electrons can vary from 1 to 8 |
drawbacks of dalton's atomic theory | 1. It said that atoms cannot be broken down further. However, as we now know, atoms can be broken down into subatomic particles
2. It said that all atoms of an element are the same. However, the existence of isotopes completely disproves this statement. |
variable valency | elements which have more than one valency |
-ous ending metal | used to show the lower valency value |
-ic ending metal | used to show the higher valency value |
how to determine which valency to use for elements which are not metallic | depends on the no of atoms of the other element. |