Dalton’s Atomic Theory:
This theory attempts to logically explain why substances combine in fixed proportions and why there is no loss or gain of mass during a chemical reaction.
Dalton later explained these by proposing that during a chemical reaction, atoms are indivisible and merely rearrange, rather than being created or destroyed.
Postulates of Dalton’s Atomic Theory:
- All matter is made up 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 simple whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
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.
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.
Formation of Compounds & Bonding:
Atoms of an element can combine to form a molecule of that element. Atoms of different elements combine to form a molecule of a compound.
This generally takes place in two ways. These are:
- 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.
Bonding by sharing of electrons — Covalent Bond:
A. Molecules of elements:
Formation of a hydrogen molecule:
- The electronic configuration of hydrogen (atomic number \(1\)) is \(1\). It has only one electron in the K-shell. Since, a K-shell can have a total of two electrons, it needs one more electron to become stable. Thus, a hydrogen atom shares one electron with another hydrogen atom to form a hydrogen molecule \((H_{2})\). 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.
- In case of an oxygen molecule, the two atoms are joined by two pairs of shared electrons, and are held together by a double bond. It can be depicted by drawing two lines between two oxygen atoms as \(O=O\).
Formation of hydrogen molecule
B. Molecules of compounds:
Formation of hydrogen chloride molecule:
- The electronic configurations of hydrogen atom is \(1\) and chlorine atom is \(2,8,7\) (atomic number of hydrogen is \(1\) and chlorine is \(17\)).
- The 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 and it can be depicted as \(H - Cl\).
Formation of hydrogen chloride molecule
C. Naming covalent compounds:
Covalent compounds are named by indicating the number of atoms of each element in the compound. To name these compounds, a prefix system is used to indicate the number of atoms of each element in the molecule. The first element retains its regular name, while the second element ends in -ide.
Prefixes, such as mono- \((1)\), di- \((2)\), tri- \((3)\), tetra- \((4)\), penta- \((5)\), hexa- \((6)\), etc., indicate the number of atoms. However, mono- is usually omitted for the first element but is used for the second element. If a prefix ends with ‘o’ or ‘a’ and the element starts with a vowel, drop the last vowel (for example, monoxide, pentoxide).
Prefixes, such as mono- \((1)\), di- \((2)\), tri- \((3)\), tetra- \((4)\), penta- \((5)\), hexa- \((6)\), etc., indicate the number of atoms. However, mono- is usually omitted for the first element but is used for the second element. If a prefix ends with ‘o’ or ‘a’ and the element starts with a vowel, drop the last vowel (for example, monoxide, pentoxide).
Example:
\(CO\) is named as carbon monoxide (not monooxide).
\(CO_{2}\) is named as carbon dioxide (not monocarbon dioxide).
\(CO_{2}\) is named as carbon dioxide (not monocarbon dioxide).
Important!
A few binary compounds are known only by their common names. For example, \(H_{2}O\), which would usually be named hydrogen monoxide, is commonly known as water. Similarly, \(NH_{3}\), which is actually nitrogen trihydride, is known as ammonia.
Properties of Covalent compound:
- Most covalent compounds, such as camphor and naphthalene are insoluble in water but dissolve in kerosene and petrol.
- 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.
- Covalent compounds usually have low melting and boiling points.