[Chemistry Class Notes] on Atomic Mass of Elements Pdf for Exam

In this article, we will learn about the following things: the atomic mass of elements in detail, what is the atomic mass of all elements, and what is the atomic number and atomic mass of elements.

The atomic mass of a solitary atom is its absolute mass and is regularly expressed in atomic mass units or amu. For example, a normal carbon atom with six neutrons and six protons is denoted as carbon-12. It has an atomic mass equal to 12 amu. The atomic mass number is usually rounded off to the nearest whole number.

Since an element’s isotopes have distinctive atomic masses, researchers may likewise decide the general atomic mass—once in a while called the atomic weight—for an element. The general atomic mass is the normal of the atomic masses of the apparent multitude of various isotopes in an example. Every isotope’s contribution to the normal is controlled by how huge a fraction of the example it makes up. The overall atomic masses that are given in periodic tables like the one for hydrogen are determined for the naturally occurring isotopes of each element, weighted by the weight of those particular isotopes on earth.

The Periodic Table

In 1869, when Dimitri Mendeleev put together the first periodic table, the elements in the table were arranged in the order of increasing atomic mass. This approach is because the nucleus of the atom was yet to be discovered, and the chemists at the time had no understanding of the internal structure of atoms. Until the discovery of the structure of the atom, it was not known that the properties of the elements were governed by the atomic number.

Atomic Number

The atomic number gives a number of how many protons are inside the nucleus of the atom. Elements are identified based on the number of protons in the nucleus regardless of the number of neutrons present. Neutrons are uncharged subatomic particles which are stable when bound in an atomic nucleus. Atoms of all elements have neutrons in the nucleus except hydrogen.

The atomic number is important because the number of protons determines the number of electrons that surround the nucleus. The number of electrons in an element considerably determines the chemical behavior of the element. 

When observing the periodic table, one can see that elements are arranged on the basis of increasing atomic numbers. Elements that have similar qualities fall into the same column or group. For example, the elements in Group 1A are mostly soft metals that are highly reactive with water. Similarly, the elements in Group 8A are unreactive gaseous at room temperature. In the same group, a periodic repetition of properties can be seen in the elements with increasing mass.

Atomic Number of Elements from 1 to 30

Let us take a look at what is the atomic mass of elements from 1 to 30. Given below is the atomic mass of elements list.

This was the atomic mass of the first 30 elements. Let us now learn the difference between atomic number and atomic mass of elements.

Isotopes

Before going into atomic mass, it is essential to learn about isotopes. Atoms of the same element that have a different number of neutrons are called isotopes. The isotope of a given element is defined by adding 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 isotopes of carbon, 12C, and 13C which possess 6 and 7 neutrons, respectively. Isotopes vary in their abundance in nature depending on the source of the materials.

Atomic Mass

The atomic mass is the average mass of the atoms of an element measured in the atomic mass unit (amu) or Daltons, D. The atomic mass is determined by averaging the weight of all the isotopes of the element. The averaging procedure also involves taking into consideration the abundance of each isotope and multiplying it with the mass of each. 

Consider the gas neon, which has three isotopes: 

1. neon-20: It is made up of ten protons and ten neutrons. The atomic mass unit of the gas is 19.992, and the abundance of the gas is 90.48%. 

2. neon-21: It is made up of ten protons and eleven neutrons. The atomic mass unit of the gas is 20.994, and the abundance of the gas is 0.27%. 

3. neon-22: It is made up of ten protons and twelve neutrons. The atomic mass unit of the gas is 21.991, and the abundance of the gas is 9.25%. 

To determine the atomic mass of neon, the average of the isotopes need to 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 mass is thus: 20.18 amu

The atomic mass of elements directly relates to the concept of “mole” which is an important way of measuring the amount of substance. The relation between the atomic mass and the number of moles is that, when measured, the amu directly gives a measure in grams of the element present in one mole. 

For example, iron has an atomic mass of 55.847 amu. Therefore, one mole of iron has a weight of 55.847 grams. The same concept is also used to determine the molar quantities of ionic molecules and compounds. For example, one mole of sodium chloride (NaCl) has a molecular mass of 58.44 amu (Na: 22.989 amu Cl: 35.453 amu) which gives the molar weight of salt as 58.44 grams. Similarly, the molecular mass of water is 18.02 amu which gives a molar mass of 18.02 grams.

Difference Between the Atomic Number and the Atomic Mass of Elements

Let us now learn about the difference between the atomic number of elements and their atomic mass.