An atom is the smallest particle of an element that retains its (elements) chemical properties. An atom of one element is different in size and mass from the atoms of the other elements. These atoms were considered "indivisible" by Indian and Greek Philosophers in the beginning and the name "atom" emerged out of this basic philosophy.
Today, we know that atoms are not indivisible. They can be broken down into still smaller particles although they lose their chemical identity in this process.
Atoms are very small, they are smaller than anything that we can imagine or compare with. Atoms of different elements not only differ in mass as proposed by Dalton but also they differ in size.
Every matter is made of atoms. It is difficult to imagine the real shape of an atom but for all practical purposes it is taken as spherical in size and that is why we talk of its radius. Since size is extremely small and invisible to the eyes, we adopt a scale of nanometer (1 nm = 10–9 m) to express its size.
Dalton gave the concept of atomic mass. According to him, atoms of the same element have same atomic masses but atoms of different elements have different atomic masses. Since Dalton could not weigh individual atoms, he measured relative masses of the elements required to form a compound. From this, he deduced relative atomic masses.
For example, you can determine by experiment that 1.0000 g of hydrogen gas reacts with 7.9367 g of oxygen gas to form water. If you know formula of water, you can easily determine the mass of an oxygen atom relative to that of hydrogen atom.
Dalton did not have a way of determining the proportions of atoms of each element forming water during those days. He assumed the simplest possibility that atoms of oxygen and hydrogen were equal in number. From this assumption, it would follow that oxygen atom would have a mass that was 7.9367 times that of hydrogen atom. This, in fact, was not correct. We now know that in water number of hydrogen atoms is twice the number of oxygen atoms (formula of water being H2O). Therefore, relative mass of oxygen atom must be 2 × 7.9367 = 15.873 times that of hydrogen atom.
After Dalton, relative atomic masses of several elements were determined by scientists based on hydrogen scale. Later on, hydrogen based scale was replaced by a scale based on oxygen as it (oxygen) was more reactive and formed a large number of compounds.
In 1961, C-12 (or 126C ) atomic mass scale was adopted. This scale depends on measurement of atomic mass by an instrument called mass spectrometer. Mass spectrometer invented early in 20th century, allows us to determine atomic masses precisely. The masses of atoms are obtained by comparison with C-12 atomic mass scale. In fact C-12 isotope is chosen as standard and arbitrarily assigned a mass of exactly 12 atomic mass units.
One atomic mass unit (amu) equals exactly one-twelfth of mass of a carbon-12 atom, Atomic mass unit (amu) is now-a-days is written as unified mass unit and is denoted by the letter "u".
The relative atomic mass of an element expressed in atomic mass unit is called its atomic weight. Now-a-days we are using atomic mass in place of atomic weight. Further, Dalton proposed that masses of all atoms in an element are equal. But later on it was found that all atoms of naturally occurring elements are not of the same mass. Atomic masses that we generally use in our reaction or in chemical calculations are average atomic masses which depend upon relative abundance of isotopes of elements.
Dalton considered an atom as an indivisible particle. Later researches proved that an atom consists of several fundamental particles such as electrons, protons and neutrons. An electron is negatively charged and a proton is positively charged particle.
Number of electrons and protons in an atom is equal. Since charge on an electron is equal and opposite to charge of a proton, therefore an atom is electrically neutral. Protons remain in the nucleus in the centre of the atom, and nucleus is surrounded by negatively charged electrons.
The number of protons in the nucleus is called atomic number denoted by Z. For example, there are 8 protons in the oxygen nucleus, 6 protons in carbon nucleus and only one proton in hydrogen nucleus. Therefore atomic numbers of oxygen, carbon and hydrogen are 8, 6 and 1 respectively.
There are also neutral particles in the nucleus and they are called neutrons. Mass of a proton and of a neutron is nearly the same.
Total mass of the nucleus = mass of protons + mass of neutrons
Total number of protons and neutrons is called mass number (A). By convention atomic number is written at the bottom of left corner of the symbol of the atom of a particular element and mass number is written at the top left corner. For example, symbol 126C indicates that there is a total of 12 particles (nucleons) in the nucleus of a carbon atom, 6 of which are protons. Thus, there must be 12-6 = 6 neutrons.
Similarly 168O indicates 8 protons and 16 nucleons (8 protons + 8 neutrons). Since atom is electrically neutral, oxygen has 8 protons and 8 electrons in it. Further, atomic number (Z) differentiates the atom of one element from the atoms of the other elements.
An element may be defined as a substance where all the atoms have the same atomic number.
But the nuclei of all the atoms of a given element do not necessarily contain the same number of neutrons. For example, atoms of oxygen, found in nature, have the same number of protons which makes it different from other elements, but their neutrons (in nucleus) are different. This is the reason that the masses of atoms of the same element are different. For example, one type of oxygen atom contains 8 protons and 8 neutrons in one atom, second type 8 protons and 9 neutrons and the third type contains 8 protons and 10 neutrons. We represent these oxygen atoms as 168O , 178O and 188O respectively.
Atoms of an element that have the same atomic number (Z) but different mass number (A) are called isotopes. In view of difference in atomic masses of the same element, we take average atomic masses of the elements. This is calculated on the basis of the abundance of the isotopes.
Example: Chlorine is obtained as a mixture of two isotopes 3517Cl and 3717Cl. These isotopes are present in the ratio of 3:1. What will be the average atomic mass of chlorine?
Out of four atoms, 3 atoms are of mass 35 and one atom of mass 37. Therefore,
Average atomic mass = (35×3 + 37×1)/4 = 142/4 = 35.5 u
Thus, average atomic mass of chlorine will be 35.5 u.