The convention of using 1/12 the mass of a carbon-12 atom as the atomic mass unit was adopted because: 1) Carbon-12 has 6 protons and 6 neutrons which have nearly equal mass and the mass of electrons is negligible. 2) Dividing the total mass of protons and neutrons in carbon-12 by 12 provides a whole number for atomic masses rather than fractions. 3) Carbon-12 is the most common isotope with an equal number of protons and neutrons, making it readily available as a standard.

1. Why dowe take 1/12 mass of a carbonatom insteadof the mass of the whole elementof the carbon?
Atoms and Molecules
Summary
In 430 B.C. Democritus postulated that matter is made up ofvery small particles called“Atomos” which means"indivisible". Later, Antoine Lavoisier, fromhis experimental
observations, established laws ofchemical combinations.
Laws of Chemical Combination:
The Law of conservation of mass: This law states that, thetotal mass ofreactants is equal to the total mass ofproducts. In any chemical reaction, the total mass ofthe substance
before and after the reaction is thesame although its matter undergoes a physical change.
Law of Definite Proportions: According to law ofdefinite proportions, in a chemical substance, the elements are always present in definite proportions by mass.
Law of Multiple Proportions: When two elements combine to formdifferent compounds, then theweight ofone is constant and the other has a simple ratio.
Atom: After a series ofexperiments Dalton concluded that all matter must becomposed oftiny particles, which cannot befurther divided. He called thematoms.
Definition ofan atom: The smallest particle ofan element is called an atom.
Dalton’s Postulates:
 Matter is made ofatoms.
 Atoms are indivisible and indestructible.
 Atoms ofa given element are identical in mass and in properties.
 Compounds are formed by a combination oftwo or more different kinds ofatoms and. A chemical reaction is a rearrangement ofatoms.
 Atoms ofdifferent elements have different masses and chemical properties.
Draw backs of Dalton's atomic theory:
 According modern atomic theory atoms are divisible and arecomposed ofparticles. The three main sub-atomic particles areproton, neutron and electron.
 The assumption, atoms ofa given element areidentical in mass and in properties not applicable for isotopes ofan element.
Examples: Chlorine has 2 isotopes having mass numbers 35 and 37.
Elements andsymbols: Dalton proposed a scheme ofnotation to represent elements
Dalton's Symbols ofElements
The symbols used today were suggested by Berzelius.
Berzelius - Symbols of Elements: Berzelius Suggested to abbreviatethe names ofthe elements using one or two letters.
The first letter ofthe symbol is always in uppercase. Some elements were represented by thestarting alphabet ofits name. For instance,
Carbon is represented as C
Boron as B
Oxygen as O
Sulphuras S

2. Why dowe take 1/12 mass of a carbonatom insteadof the mass of the whole elementof the carbon?
Nitrogen is written as N and
Hydrogen as H
Symbols ofsome elements were the first two alphabets, where thesecond alphabet is in the lower case.
Examples: Aluminiumis represented as Al
Chlorine as Cl.
An element is represented by either the first or the first and any other letter ofits Latin name. There are some exceptions while writing the symbols for some ofthe elements. These
names were derived fromLatin. In general, the first letter and the second letter oftheir Latin names are used to represent thesymbols ofthese elements.
Examples:
Tungsten W wolfam
Atomic mass:
Atomic mass is the mass ofan atom.
The relative atomic mass ofan atomofan element is the number oftimes an atomofthat element is heavier than 1/12th
ofthe mass ofa carbon-12 atom.
Molecule: A molecule is the smallest particle ofan element or compound that can exist independently.
Examples:
H2O, O2, O3 etc.
Single Hydrogen is not a molecule. When this hydrogen bonds to other hydrogen (H2) or to other elements like oxygen (H2O), a moleculeis formed.
Atomicity: The number ofatoms constituting a molecule is referred to as atomicity.
A molecule which contains only one atomis called monatomic
Example: Noble gases like Helium(He), Neon (Ne), Argon (Ar) etc.
A molecule which contains two atoms is called diatomic
Example: Hydrogen (H2), Chlorine (Cl2), Nitrogen (N2)etc.
A molecule which contains three atoms is called triatomic.
Example: O3, H2O.
A molecule which contains four atoms is called teraatomic.
Example: Phosphorus (P4).
A molecule which contains more atoms is called polyatomic.
Element Symbol Latin Name
Antimony Sb Stibium
Copper Cu Cuprum
Gold Au Aurum
Iron Fe Ferrum
Lead Pb Plumbum
Mercury Hg Hydragyrum
Potassium K Kalium
Silver Ag Argentum
Sodium Na Natrium
Tin Sn Stannum

3. Why dowe take 1/12 mass of a carbonatom insteadof the mass of the whole elementof the carbon?
Example: Sulphur (S8).
Compound: Elements combine to formcompounds. Thus a molecule ofa compound has two or more elements.
Example: Water (H2O), Glucose (C6H12O6), Calciumoxide (CaO), Sodiumchloride (NaCl) etc.
Differences betweena molecule and a compound: A molecule is formed when two or more atoms bonded chemically. A compound is formed when different elements bonded
chemically. Thus a molecule ofa compound has two or more elements.
All compounds are molecules but all molecules arenot compounds.
Example: Molecular hydrogen (H2) is a molecule but not compound. Hydrogen molecule is made up oftwo atoms ofhydrogen. Water is a compound. Water is made up oftwo
atoms ofoxygen and one atomofoxygen these atoms combined to forma compound known as water. Smallest particle ofwateris called a molecule.
Common salt is a compound ofsodiumand chlorine, where sodiumis a positively charged particle and chlorine a negatively charged particle, and each ofthis is called an ion.
Ion:
An ion is a charged particle.
Cation:
A positively charged particlein a molecule is called cation.
Example: Na+
, Ca+2
.
Anion:
An anion is a negatively charged particle in a molecule.
Example: F-
, Cl-
.
Differences between Cation & Anion:
Cation Anion
 Cation is a positively charged particle
 Cation is formed fromMetals
 During electrolysis cation moves towards cathode
 The size ofcation is smaller than its parent atom
Example: Na+
, K+
.
 An anion is a negatively charged particle
 Anion is formed by Non-metals
 During electrolysis anion moves towards anode
 The size ofanion is larger than its parent atom.
Example: Cl-
, Br-
.
Valency: The combining capacity ofan element is known as valency.
The combining capacity ofthe atoms to formmolecules either with same or different elements is defined as valency.
Atomcontains less than four electrons in its outermost shell; the valency ofan atomis equal to the number ofelectrons present in the valence shell.
Examples:
Sodiumhas oneelectron in its outermost shell, so the valency ofsodiumis 1.
Calciumhas two electrons in its outermost shell, so the valency ofcalciumis 2.
Aluminumhas three electrons in its outermost shell, so the valency ofaluminumis 3.
If the outer shell has more than fourelectrons, the valency = 8 - thenumber ofelectrons in the outer shell.
Question: Find the valency ofchlorine?
Solution: Theatomic number ofchlorine is 17
Electronic configuration ofChlorine= 2 8 7
Valency = 8 - 7 =1 Valency ofchlorineis 1

4. Why dowe take 1/12 mass of a carbonatom insteadof the mass of the whole elementof the carbon?
As has been pointed out by the answerbefore mine, the usage of 1/12 of acarbon is a convention that had beenadopted quite a long
time back.
It is based on the factthat the carbon-12 atom has 6 protons, 6 neutrons and 6 electrons. Ofthese the 6 electronshave negligible
w eightcompared to the others(1 electron is around 1/1836 times asheavyas a proton). Protons and neutrons weighalmost the
same atleast atthe accuracy levelsduring the time whenthe convention wasadopted.
Hence 1 proton weighs equal to 1 neutron = 1/12 of a carbon-12 atom whichhas 6 protonsand 6 neutrons.
And asthe weight of all atoms isalmost completely from the contained protons and neutrons, it ismentioned in terms of 1/12
carbon-12 atom.
So for example, oxygen has 8 protons and 8 neutrons, the weight ofoxygenisapproximately (8+8)*1/12 ofcarbon-12 atom.
I hope thatthis hassolved yourproblem.
I assume you mean asthe definition ofan atomic unit. Since protonsand neutronshave slightly different weights, and average of the
tw ois used to determine an atomic unit. Carbon-12 is the most common and readily available isotope tohave exactly the same
numberof protonsand neutrons, 6 of each, and thus provides a perfect average when divided by the total number of protons and
neutrons. (Carbon-12 alsohas 6 electrons, butthe mass ofan electron is so small as to be considered negligible). Dividingby 12
allows eventhe smallest atom, hydrogen, to have a massthat is a whole number,ratherthana fraction.
There are several atoms smaller than carbon-12 withthe same number ofprotons and neutrons, (for example, dueterium, the
hydrogen isotope presentin heavywater), but they are muchless common. By settingthe most common isotope of carbon asthe
standard, it insuresthat it isreadily available to anyone who needs access to it. Hope this helps!
Refer Periodic table web page already bookmarked in chrome{ Atomic Masses of elements }