Atomic mass:

(1) Atomic mass:

Atomic mass represent the mass of single isotopes or single atoms.

"Atomic mass and atomic weight do not mean the same:

Atomic mass is equal to the mass of total number of protons , neutrons  and electrons ( mass of electrons are negligible hence ignore)

Atomic mass = total mass of nucleons (total number of nucleons × mass of ons nucleons.

Atomic mass is independent of the atomic masses of isotopes.

Calculated  value of the atomic mass is whole number

Unit of atomic mass: the unit of atomic mass is atomic mass unit (amu) or unified atomic mass (u) and amu also known as Dalton(Da)

It is representation of unit of atomic weight is a.m.u. and  it is the smallest unit of mass and is define as;

We know that one mole (6.022x10^23) atoms C^12 atoms has  12 gm mass.

Hence mass of one C^12 atom is = 12/N_A=  12/6.022x 10^23 gm =1.99 x 10^-23 gm.

 The real mass of one atom of C^12 has been determined as 1.9924 × 10^-23 gram.

Conclusively we can summarise as

1 amu is = mass of one nucleons (protons or  nutrons)

1 amu is = 1.67×10^27 kg 

1 amu is = mass of one proton or nutron ( 1.67×10^27 kg )

(2) Atomic weight or relative average Atomic mass:

(3) Gram-atomic weight: When atomic weights of elements are expressed in grams, they are called gram atomic weights. 

Thus, the weight of an element in grams which is equal to its atomic weight is known as the gram atomic weight (GAW) of that element. 

  For example, atomic weight of sodium is 23 and its gram atomic weight is 23 grams. Similarly, the gram atomic weights of hydrogen, sulphur, nitrogen are 1.008 gram, 32 gram and 14 gram respectively.

Thus

1 GAW P = 31.0 gram phosphorus,

1 GAW K = 39.0 gram potassium,

1 GAW Ca = 40.0 gram calcium.

 (4) Gram atom or mole atom: The atomic weight of an element taken in grams is one gram-atom or mole atom of that element. 

For instance:

one gram atom of oxygen represents 16 gram oxygen.

One gram atom of sulphur represents 32 gram sulphur. 

Number of gram atoms of an element

  "One gram atomic weight of every element contains 6.023 × 10^23 atoms of the element.”

Other Questions:

(1) Calculate the average atomic weight of an element (X) containing three isotopes X56, X57 and X59 and natural occurrence of these isotopes are 90%, 8% and 2% respectively.

(2) For the reaction Initially 2.5 moles of Fe(NO3)3 and 3.6 moles of Na2CO3 are taken. If 6.3 moles of Na2CO3 is obtained the % yield of given reaction is ?

(3) 3.9 gm Al(OH)3 is reacted with excess of HCl , 6.5 gm AlCl3 is formed determined % yield of reaction ?

(4) 200 gm of CaCO3 on heating produces 11.2 litre of CO2 (g) at STP calculate % yield of reaction?

(5) How many grams of Fe2O3 formed by heating of 18 gm FeO with O2.

(6) How many moles O2 are required to needed produced 5 moles of Fe2O3.

(7) 367.5 gm KClO3 (122.5) when heated then calculate. (1) How many gram of O2 is produced? (2) How Many litre of O2 is produced at STP?

(8) Air contain 20 % O2 by volume. How many cm³ of air will be required for oxidation of 100 cm³(ml) of acetylene?

(9) The hydrated salt, Na2SO4.nH2O undergoes 55.9% loss in weight on heating and becomes anhydrous. The value of n will be?

(10) Haemoglobin contains 0.33% of Iron by weight. The molecular weight of it is approx. 67200. The numbers of iron atoms (Atomic wt of Fe=56 u) present in one molecule of Haemoglobin are?

Atomic weight or relative average Atomic mass:

Atom: The smallest particle of an element which cannot exist in free form is called an atom.

Atoms have a tendency to exist as clusters (group of atoms or molecules) of same or different atoms. They take part in a chemical reaction without undergoing any decomposition. 

An atom is very small (atomic size of the order 10^–6 cm) and its real mass is very low.

The smallest and lightest atom is of hydrogen. The mass of the hydrogen atom is 1.008 a.m.u.

Weights of atoms of other elements are in the range of 2 to 260 a.m.u.

Atomic weight: The atomic weight of an element is a number which indicates how many times heavier is an average atom of that element as compared with 1/12  of the mass of an atom of carbon (12) (C^12).

* Atomic weights of elements are a absolute quantity and absolute quantity having an unit average relative weights.

* Both atomic weight and molecular weight are unit less quantities. These quantities are also regarded as relative atomic mass and relative molecular mass.

Average (relative) atomic mass: 

Most often the atomic weight of elements are fractional. It is due to the existence of isotopes of the elements which are atoms with different masses. 

Since an element is a mixture of all of its isotopes, the atomic weight of an element is the average of weights of all its isotopes. So, atomic weight have non –integral (fractional) values.

"Atomic weight (Average atomic weight or relative atomic mass) can be define as average weight of an elements with respect to all its isotopes and their relative abundance.

Calculation of average (relative) atomic mass: 

(A) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’ in the ratio  X: Y, Then   

                         or

(B) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’  and their abundant  percentage (%)  are  X % and Y % then 

(C) the calculated value may be or may not be a whole number.

Exercise: (1) Find the average atomic weight of mixture of containing 25% by mol Cl^35 and 75% by mol Cl^37.

Ans key: Cl^35 = 75%, Cl^37 = 35%

Exercise: (2) Boron have two isotopes B^11 and B^A and their relative abundant %  are 80% and 20% respectively and average atomic weight of Boron is 10.8 find the value of  “A”

Ans key: A = 10

Exercise: (3) Silver contains two isotopes Ag^107 and Ag^109 and average atomic weight of silver is 108.5 find percentage abundant of  isotopes of silver.

Ans key: Ag^107 = 25%, Ag^109 = 75%

Exercise: (4) Calculate the % abundance of Ag^109  it is know that silver contains two isotopes Ag^107 and Ag^109 and average atomic weight of silver is 108.5.

 Ans key: Ag^107 = 25%, Ag^109 = 75%

Exercise: (5) An element (X) have three isotopes X^20, X^21 and X^22. The percentage abundance of X^20 is 90% and it's average atomic mass is of element is 20.18. the percentage of abundance of X^21 should be.

Ans key: X^21 = 2%, X^22 = 8%

Exercise: (5) Calculate the average atomic weight of an element (X) containing three isotopes X56, X57 and X59 and natural occurrence of these isotopes are 90%, 8% and 2% respectively.

Other topic:

(1) What is the % composition of each element in (Mg(OH)2) magnesium hydroxide?

(2) Haemoglobin contains 0.33% of Iron by weight. The molecular weight of it is approx. 67200. The numbers of iron atoms (Atomic wt of Fe=56 u) present in one molecule of Haemoglobin are?

(3) The hydrated salt, Na2SO4.nH2O undergoes 55.9% loss in weight on heating and becomes anhydrous. The value of n will be?

(4) Air contain 20 % O2 by volume. How many cm³ of air will be required for oxidation of 100 cm³(ml) of acetylene?

Is Cyclocta-1,3,5,7-tetraene (COT) is aromatic, anti-aromatic OR non- aromatic?

Cyclocta-1,3,5,7-tetraene (COT) have four double bonds  and four single bond alternatively hence their is lack of  resonance. The reason behind this is bigger size of ring and finally  cyclocta-1,3,5'7-tetraene is to be non-planar.
The structure is:

However, the compound have four  double bonds  means eight (8) π electrons.

It may be aromatic, it has to have 4n+2 electrons (2, 6, 10, etc.).

Anti-aromatic systems have 4n electrons (4, 8, 12, etc.).

It may be expected that the molecule was planar and anti-aromatic.

However, actually the molecule takes a tub-shaped (non planer ) conformation in its native state. Since, it is not a planar molecule, it becomes non-aromatic instead of anti-aromatic.

Related Questions:

1. How is base strength related to the availabihty of the electron-pair?
2. Amines are more basic than ammonia why?
3. What is relative basic strength order 1° amines , 2°amines and 3° amines ? Explain:
4. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
5. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
6. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
7. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
8. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
9. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
10. Why pyridine is more basic than Pyrrole?
11. Why pyrimidine is less basic than pyridine?
12. Imidazole is more basic than pyridine? Why?
13. Biological Important of Imidazole and structure:
14. Pyridine is almost 1 million times less basic than piperidine? Why?
15. Cyclohexylamine amine is the stronger base than Aniline? Why?
16. Tetrahydroquinoline amine is the stronger base than Tetrahydroisoquinoline? Why?
17. Arrange the following in the order of increasing basicity : p-Toluidine, N, N-Dimethyl-p-toluidine, p-Nitroaniline, Aniline. (I.I.T.1986)
18. Arrange the following in the increasing order of their acid strength : Methyl amine, Dimethyl amine, Aniline, N-methyl aniline (I.I.T, 1988).