Atomic Foundations of Matter

Class 09 Science

Many properties of the elements are not retained when they form a compound. For example, hydrogen and oxygen are gases, whereas the substance formed when they combine is water, which is a liquid at ordinary temperatures.

Law of Conservation of Mass

The total mass remains the same before and after a chemical reaction. So, matter can neither be created nor destroyed in a chemical reaction. This is known as the Law of Conservation of Mass, proposed by Antoine Lavoisier in 1789.

Law of Constant Proportions

Soon after Lavoisier, Joseph Proust proposed that in any compound formed by two or more elements, the elements combine in a fixed ratio by mass. In other words, elements in a compound have definite proportions irrespective of its source.

For example, if the water collected from various sources, such as rivers, borewell or the ocean, is purified and analysed. It is always found to contain hydrogen and oxygen in a mass ratio of 1:8.

This is known as the Law of Constant Proportions, or the Law of Definite Proportions, or sometimes as Proust’s Law.

Dalton's Atomic Theory

John Dalton proposed his atomic theory in terms of certain postulates. These postulates combined earlier experimental observations and served as the basis for modern scientific theory.

  • All matter is made of very tiny particles called atoms, which participate in chemical reactions.
  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass and chemical properties.
  • Atoms of different elements have different masses and chemical properties.
  • Atoms combine in the ratio of small whole numbers to form compounds.
  • The relative number and kinds of atoms are constant in a given compound.

For example, hydrogen and oxygen atoms combine to form water, but the atoms themselves are not destroyed or changed into something else. Similarly, when magnesium burns in air, a white powder of magnesium oxide forms. This shows that the atoms of magnesium have combined with those of oxygen to form magnesium oxide.

How Atoms Combine?

Atoms of an element can combine to form a molecule of that element. For example, a hydrogen molecule consists of two hydrogen atoms. Atoms of different elements combine to form a molecule of a compound. For example, one hydrogen atom and one chlorine atom combine to form a molecule of hydrogen chloride.

A molecule can be defined as an electrically neutral entity consisting of more than one atom that is capable of independent existence and shows all the properties of that substance.

Atoms with 8 electrons in the outermost (valence) shell (2 electrons if the K-shell is the outermost shell) are stable. If the number of electrons in a valence shell is less than eight, then they may share, gain or lose electrons to complete their valence shell and become stable. In this process, atoms of elements combine to form compounds. This generally takes place in two ways:

  • Sharing of electrons: Share a few or all of their valence electrons with another atom.
  • Transfer of electrons: Transfer one or more of the valence electrons to another atom, or accept one or more electrons from some other atom.

When atoms combine, the total energy of the system becomes lower than the sum of the energies of the individual atoms, making the resulting arrangement more stable. The force that holds atoms together is called a chemical bond.

Covalent Bond

A hydrogen atom shares one electron with another hydrogen atom to form a hydrogen molecule (H2). The shared pair of electrons attracts both the nuclei and makes the molecule stable. This type of interaction between atoms through a shared pair of electrons is called a covalent bond. If two atoms are joined by sharing one electron each, they are said to be joined by a single bond.

$$ \ce{H-H} $$

The chlorine atom has seven electrons in its valence shell. Each chlorine atom requires one electron to attain a stable electronic configuration. A chlorine molecule is formed by the sharing of one electron each by two chlorine atoms.

$$ \ce{Cl-Cl} $$

Oxygen: The number of electrons present in its valence shell is six, and it requires two more electrons to complete its octet. So, two oxygen atoms share two electrons each, forming an oxygen molecule (O2). In this case, the two atoms are joined by two pairs of shared electrons, and are held together by a double bond.

$$ \ce{O=O} $$

In hydrogen chloride, atoms of both hydrogen and chlorine need one electron each to attain stable electronic configurations. Hence, both the atoms share one electron each to form a molecule of hydrogen chloride.

Since the hydrogen and chlorine atoms in a hydrogen chloride (HCl) molecule bond by sharing electrons, it is a covalent compound. Also, since hydrogen and chlorine atoms share one pair of electrons, they are bonded by a single bond.

$$ \ce{H-Cl} $$

Hydrogen and oxygen combine to form water. Hydrogen needs only one electron, while oxygen needs two electrons to acquire stable electronic configurations. Two hydrogen atoms share an electron each with an oxygen atom. Thus the water molecule formed is represented as H2O, which indicates the presence of two hydrogen atoms and one oxygen atom.

Ionic Bond

If the valence shell of an atom has less than four electrons, it would generally donate its valence electrons to achieve a stable electronic configuration.

Example: NaCl

The atomic number of sodium is 11. Its valence shell contains only one electron, which can attain a stable electronic configuration after losing this valence electron. When sodium atom loses its valence electron, it becomes a positively charged species, called a sodium cation, represented as Na+.

On the other hand, the electronic configuration of a chlorine atom shows that it has seven valence electrons. It can attain a stable electronic configuration by gaining one electron from another atom. After gaining one extra electron, it acquires a negative charge and is called a chloride anion, represented as Cl-.

Once the sodium and chloride ions are formed, they are held together by the electrostatic force of attraction due to their opposite charges. The electrostatic force of attraction between oppositely charged ions that holds them together is called an ionic bond.

Properties of Ionic and Covalent Compounds

Ionic compounds like sodium chloride and copper sulfate are generally soluble in water but insoluble in solvents, such as kerosene and petrol. In contrast, most covalent compounds, such as camphor and naphthalene are insoluble in water but dissolve in kerosene and petrol.

Ionic compounds do not conduct electricity in the solid state because their ions are held in fixed positions by strong forces. To conduct electricity, ions must be free to move, which occurs only when the ionic compounds, such as sodium chloride and copper sulfate, are dissolved in water. On the other hand, some covalent compounds, such as sugar, are soluble in water but do not provide ions in solution; therefore, they do not conduct electricity. Other covalent compounds, such as camphor and naphthalene, also do not conduct electricity.

Ionic compounds generally have high melting and boiling points due to strong inter-ionic attractions, whereas covalent compounds usually have low melting and boiling points.