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
The forces between the protons and the neutrons in the nucleus are of special kind called _________ This strong force is more powerful than gravity.
Yukawa forces.
Atoms are so tiny their mass number cannot be expressed in grams but expressed in
amu (atomic mass unit)
The outermost shell of an atom is called ______ and the electrons present in the valence shell are known as .
valence shell
valence electrons
The chemical properties of elements are decided by these _____, since they are the ones that take part in chemical reactions.
valence electrons
The elements with same number of electrons in the valence shell show similar properties and those with different number of valence electrons show different chemical properties. Elements, which have valence electrons 1 or 2 or 3 (except) Hydrogen) are ______.
Elements with 4 to 7 electrons in their valence shell are ______.
metals
non-metals
_______ of an element is the combining capacity of the element with other elements and is equal to the number of electrons that take part in a chemical reaction.
Valency
Valency of the elements having valence electrons 1, 2, 3, 4 is 1, 2, 3, 4 respectively.
Valency of an element with 5, 6 and 7 valence electrons is 3, 2 and 1 (8-valence electrons
Because 8 is the number of electrons required by an element to attain stable electronic configuration
Elements having completely filled outermost shell show
Zero valency.
For example: The electronic configuration of Neon is 2,8 (completely filled). So valency is 0.
Find the valency of Magnesium and Sulphur
Solution:
Electronic configuration of magnesium is 2, 8, 2, So valency is 2.
Electronic configuration of sulphur is 2,8 6. So valency is 2 i.e. (8-6)
_______ are defined as the atoms of the same element, having the same atomic number but different mass numbers.
isotopes
35 cl 17, 37 Cl 17
12 C 6, 13 c 6
1 H 1, 2 H 1, 3 H 1
There are two types of isotopes:
stable
unstable
The isotopes which are unstable, as a result of the extra neutrons in their nuclei are radioactive and are called .
radioisotopes
________, which is used in radiotherapy treatment are both radioisotopes.
cobalt-60
Atoms of different elements with different atomic numbers, which have the same mass number, are known as
isobars
40 Ar 18
40 ca 20
Atoms of different elements with different atomic numbers and different mass numbers, but with same number of neutrons are called
isotones
11 B 5. 6 neutron
12 C 6. 6 neutron
Law of multiple proportions
When two elements A and B combine together to form more than one compound, then masses of A which separately combines with a fixed mass of B are in simple ratio”.
law of reciprocal proportions was proposed by
Jeremias Ritcher in 1792.
Law of reciprocal proportions
If two different elements combine separately with the same weight of a third element, the ratios of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.
Law of multiple proportions was proposed by
John Dalton in 1804.
Gay Lussac’s Law of combining volumes
Whenever gases react together, the volumes of the reacting gases as well as the products bear a simple whole number ratio, provided all the volumes are measured under similar conditions of temperature and pressure.
This law may be illustrated by the following example.
It has been experimentally observed that two volumes of hydrogen reacts with one volume of oxygen to form two volumes of water as shown in the figure 11.12.
quantum numbers
the numbers which designate and distinguish various atomic orbitals and electrons present in an atom are called quantum numbers
Types of quantum numbers
Principal quantum number
Azimuthal quantum number
Magnetic quantum numbers
Spin quantum number
Change in the number of neutrons in an atom changes it to
1
a) an ion.
b) an isotope.
c) an isobar.
d) another element
an isotope
Calcium and Argon are examples of a pair of
Isobar
Total number of electrons that can be accommodated in an orbit is given by
2n^2
isotope is used in the nuclear reactors.
U235
The valency of Argon is
0 zero
Name an element which has the same number of electrons in its first and second shell.
Beryllium Be
Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached. 1 H 1 , 0 n1 , -1e0
Hydrogen
Neutron
Electron
For an atom ‘X, K, L and M shells are completely filled. How many electrons will be present in it?
28 electrons in the element, which is Nickel
What is the same about the electron structures of:
a. Lithium, Sodium and Potassium.
b. Beryllium, Magnesium and Calcium.
Lithium, sodium & potassium have 1 electron in their outermost shell.
b. Beryllium, magnesium and calcium have 2 electrons in their outermost shell
