Atomic weight of elements
We know that an atom is electrically neutral, if it contains negatively charged electrons it must also contain some positively charged particles. This was confirmed by Goldstein in his discharge tube experiment with perforated cathode. Applying high voltage across the electrodes of a discharge tube it was found that some rays were coming from the side of the anode which passed through the holes in the cathode.
These anode rays (canal rays) consisted of positively charged particles formed by ionization of gas molecules by the cathode rays. The charge to mass ratio (e/m value) of positively charge particles was found to be maximum when the discharge tube was filled with hydrogen gas as hydrogen is the lightest element. These positively charged particles are called protons.
e/m varies with the nature of gas taken in the discharge tube. The positive particles are positive residues of the gas left when the gas is ionized. The neutral charge particle, neutron was discovered by James Chadwick by bombarding boron or beryllium with α–particles.
|Table of Content|
|Symbol||E or e–||P||n|
|Approximate relative mass with respect to proton||1/1836||1||1|
|Approximate relative charge with respect to proton||–1||+1||No charge|
|Absolute mass in kg||9.109×10–31||1.673×10–27||1.675×10–27|
|Absolute mass in amu||5.485×10–4||1.007||1.008|
|Absolute charge (coulomb)||1.602×10–19||1.602×10–19||0|
|Absolute charge (e.s.u.)||4.8 × 10–10||4.8 × 10–10||0|
The atomic mass unit (amu) is 1/12 of the mass of an individual atom of 6C12, i.e. 1.660 × 10–27 kg.
The neutron and proton have approximately equal masses of 1 amu and the electron is about 1836 times lighter, its mass can sometimes be neglected as an approximation.
The electron and proton have equal, but opposite, electric charges while the neutron has no charge.
|Atomic No.||Name of Element||
Ha (Z = 105) is also called as Neisbonium (Ns)
Rf (Z = 104) is also called as Kurchutovium (Ku)
Before moving on to atomic mass, it is essential to learn about isotopes. Atoms of the same element that have different numbers of neutrons are known as isotopes. A isotope of a given element is defined by the sum of the number of protons and neutrons in its nucleus. An element (which has the same atomic number) can have multiple isotopes with different numbers of neutrons. The most common examples are the carbon isotopes 12C and 13C, which have 6 and 7 neutrons, respectively. Isotopes vary in their abundance in nature depending on the source of the materials.
A atomic mass is the average mass of atoms of an element measured in the atomic mass unit (amu) or Daltons, D. Atomic mass is determined by averaging the mass of all isotopes of an element. The averaging procedure also involves taking into account the amount of each isotope and multiplying it by the given mass.
Let's consider the gas neon, which has three isotopes:
- neon-20: Consists of 10 protons and 10 neutrons. The atomic mass unit of the gas is 19.992, and the amount of the gas is 90.48%.
- neon-21: Consists of 10 protons and 11 neutrons. The atomic mass unit of the gas is 20.994, and the amount of the gas is 0.27%.
- neon-22: Consists of 10 protons and 12 neutrons. The atomic mass unit of the gas is 21.991, and the amount of the gas is 9.25%.
To determine the atomic mass of neon then, the average of the isotopes must be extracted:
0.9048 × 19.992 = 18.09 amu
0.0027 × 20.994 = 0.057 amu
0.0925 × 21.991 = 2.03 amu
The average atomic weight is, therefore: 20.18 amu
The atomic weight of elements is directly related to the term "mole," which is an important way of measuring the amount of a substance. The relationship between atomic weight and the number of moles is such that when measured, amu directly indicates the amount in grams of an element present in 1 mole.
For example, 55.847 amu is the atomic mass of iron. Therefore, one mole of iron weights 55.847 grams. Thus, the same concept is also used to explain the molar amounts of compounds and ionic molecules. E.g., one mole of sodium chloride (NaCl) has a molecular weight of 58.44 amu (Na: 22.989 amu and Cl: 35.453 amu), giving a molar mass of salt of 58.44 grams. Generally, the molecular weight of water is 18.02 amu, giving a molar mass of 18.02 grams.
|Atomic Mass||Atomic Number|
|Atomic mass is similar to the number of neutrons and protons which are present in the nucleus of an element.||Atomic number is called as number of protons that are present in the nucleus of an element.|
|It is the average weight of a specific element.||It is the total number of nucleons that are present in the atom’s nucleus.|
|Atomic mass is denoted by the letter A||The letter Z is used for representing the atomic number.|
|Atomic mass cannot be used for defining the type of element.||Atomic numbers usually help to classify and identify an element.|
|Atomic mass is also used in the classification of different isotopes of the same element||Only isotopes of an element share the same atomic number.|
|Atomic mass is always measured using the atomic mass unit (amu).||The atomic number is simply a digit which is used for placing the elements in the periodic table.|
Q1. What is Atomic Mass?
Ans. An atomic mass is the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.992646547 × 10−23 gram, which is assigned an atomic mass of 12 units. In this scale, 1 atomic mass unit (amu) corresponds to 1.660539040 × 10−24 gram.
Q2. What is Atomic Mass of Element?
Ans. An atomic mass of an element is the average mass of the atoms of an element that is measured in the atomic mass unit (amu, also known as daltons, D). An atomic mass is a weighted average of all the isotopes of that particular element, where the mass of isotope is multiplied by the amount of that particular isotope.
Q3. How do you find the atomic mass number of an element?
Ans. The number of protons and the number of neutrons together determine the elements mass number i.e, mass number = protons + neutrons. If you want to find how many neutrons an atom has, you can simply subtract the number of protons or atomic number from the mass number.
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