Atomic Structure Flashcards
In 1911, _______, a scientist from New Zealand, performed his famous experiment of bombarding a thin gold foil with very small positively charged particles called alpha(a) particles. He selected a gold foil because, he wanted as thin layer as possible and gold is the most malleable metal
Lord Rutherford
Lord Rutherford observed that
- Most of the alpha particles passed straight through the foil
- Some alpha particles were slightly deflected from their straight path.
- Very few alpha particles completely bounced back.
Rutherford’s Atomic model
i. The atom contains large empty space.
ii. There is a positively charged mass at the centre of the atom, known as nucleus.
iii. The size of the nucleus of an atom is very small compared to the size of an atom.
iv. The electrons revolve around the nucleus in close circular paths called orbits.
v. An atom as a whole is electrically neutral, i.e., the number of protons and electrons in an atom are equal
Rutherford’s model of atomic structure is similar to the structure of the ______.
solar system
Limitations in Rutherford’s model
According to Electromagnetic theory, a moving electron should accelerate and continuously lose energy. Due to the loss of energy, path of electron may reduce and finally the electron should fall into the nucleus. If it happens so, the atom becomes unstable. But atoms are stable. Thus, Rutherford’s model failed to explain the stability of an atom.
In 1913, _______ explained the causes of the stability of the atom in a different manner
Neils Bohr, a Danish physicist,
Bohr’s main postulates are
In atoms, the electron revolve around the nucleus in stationary circular paths called orbits or shells or energy levels.
ii. While revolving around the nucleus in an orbit, an electron neither loses nor gains energy.
iii. An electron in a shell can move to a higher or lower energy shell by absorbing or releasing a fixed amount of energy.
iv. The orbits or shells are represented by the letters K,L,M,N or the numbers n= 1,2,3,4,
These orbits are associated with fixed amount of energy so Bohr called them as
energy level or energy shells.
One main limitation of bohr’s model
that this model was applicable only to hydrogen and hydrogen like ions (example, Het, Li, Be, and so on). It could not be extended to multi electron nucleus.
In 1932 James Chadwick observed when ______ was exposed to alpha particles… particles with about the same mass as protons were emitted..
Beryllium
Beryllium + alpharay → carbon + neutron
Atomic number(Z) = Number of protons =
Number of electrons
Mass number = Number of protons +
Number of neutrons
Symbolically represent the following atoms using atomic number and mass number. [Refer table 11.1]
a) Carbon
c) Silicon
b) Oxygen
d) Beryllium
- Carbon = 12C6
- Oxygen = 16O 8
- Silicon = 28S14
- Beryllium = 9Be4
Number of neutrons (n) = Mass number (A)- (2)
Atomic number
Calculate the number of neutrons in the following atoms:
27Al13
31p 15
190 Os 76
54 cr 24
14
16
114
30
Atomic number is designated as Z why?
Z stands for Zahl, which means
NUMBER in German.
Z can be called Atomzahl or atomic number A is the symbol recommened in the ACS style guide instead of M (massenzahl in German).
Calculate the atomic number of an element whose mass number is 39 and number of neutrons is 20. Also find the name of the element.
Atomic Number = 19
Element having atomic number 19 is Potassium (K)
The distribution of electrons in different shells is called _______. This distribution of electrons is governed by certain rules or conditions, known as ______
electronic configuration
Bohr and Bury Rules of electronic configuration.
The maximum number of electrons that can be accommodated in a shell is equal to ______ where ‘n’ is the serial number of the shell from the nucleus,
2n²
The outermost shell of an atom cannot have more than ______, even if it has capacity to accomodate more electrons
8 electrons
