Compound is a substance of atoms of two or more elements formed by the chemical combination in a definite proportion by weight. For example, water is a liquid compound of hydrogen and oxygen in which two elements are present in fixed ratio of 1:8 by weight.
The smallest particle of an element or compound, which is capable of independent existence is called molecule. It retains the properties of its own. It is very small. It is difficult to estimate the size of a molecule or to determine its shape.
The smallest particle of an element, which takes part in chemical change is called anatom.
Structure of the atom
Atom is made up of tiny particles. These tiny particles are called sub-atomic particles or elementary particles. They are electrons, neutrons and protons. The protons and neutrons are located at the center of an atom called nucleus. Nucleus shows the positive charge due to thepresence of protons. It helps to find the atomic weight. The electrons are revolving around the nucleus in their respective orbit or shell.
The number of protons or the number of electrons in an electrically neutral atom of an element is called atomic number. It is denoted by Z.
Atomic number (Z) = No. pf protons (p+) = No. of electrons (e-)
The sum of protons and the neutrons in the nucleus of an atom is called its atomic weight. It is also called mass number. It is denoted by A.
Atomic weight (A) = No. of protons (p+) + No. of neutrons (n0)
The systematic distribution of electron in different shells of an atom is called electronic configuration.
To explain the arrangement of electrons in different shells, Bohr and Bury purposed a scheme as given below:
For the K shell, the maximum number of electrons will be 2n2 = 2.(1)2 = 2 × 1 = 2
The energy level or shell nearer to the nucleus s called lower energy level and energy level way from the nucleus is called higher energy level.
Compound is a chemical substance which is formed by the combination of two or more elements in a fixed proportion by their weight. E.g. water (H2O), Ammonia (NH3) etc.
The systematic distribution of electrons outside the nucleus in different shell is called electronic configuration.
|No. of e-||2||8||1|
|No. of e-||2||8||7|
The fundamental particles which are found inside an atom are called sub-atomic particles. Electrons, protons and neutrons are the sub-atomic particles.
The differences between atom and molecules are as follows:
|The smallest particle of an element which takes part in chemical reaction without division is called atom.||The smallest particle of an element or compound which is capable of independent existence is called molecules.|
|Atom of an element can't be divided into two or more parts.
For e.g O, H, Na, N etc.
|Molcules can be divided into two particles.
For e.g O2, H2, N2 etc.
Differences between proton and neutron are as follows:
|They are positively charged sub-atomic particles which are present inside nucleus of an atom.||They are charge less sub atomic particles which are present in the nucleus of an atom.|
|They have equal mass to the hydrogen atom.||They have charge equal to hydrogen atom.|
Differences between neutron and electron are as follows:
|They are charge less sub-atomic particles which are present in the nucleus of an atom.||They are negatively charged sub-atomic particles which are present outside the nucleus in different shells.|
|Their mass is regarded as 1 (a.m.u) i.e. 1 atomic mass unit.||They have mass equal to 1/1837parts of a hydrogen atom.|
Differences between proton and electron are as follows:
|They are positively charged sub-atomic particles which are present inside nucleus of an atom.||They are negatively charged sub-atomic particles which are present outside the nucleus in different shells.|
|They have mass equal to the hydrogen atom.||They have mass equal to 1/1837parts of a hydrogen atom.|
The molecules which are made by the combination of two atoms of a same element are called diatomic molecules. E.g.: H2, N2 etc.
Electrons or pair of electrons present at the outermost shell of an atom is called valance electrons. Valance electrons in hydrogen, nitrogen, and chlorine are 1, 5, and 7 respectively.
Potassium is a metal and chlorine is a non-metal. Potassium loses one electron and becomes positively charged and chlorine gains one electron and becomes negatively charged. So, potassium chloride is called ionic compound.
Differences between chlorine atom and chlorine ion are as follows:
|Chlorine atom||Chlorine ion|
|It is electrically natural.||It is negatively charged.|
|It has equal number of electrons and protons (i.e. 17 each).||It has more electrons than protons (i.e. 18 e- and 17 P+).|
|It has 7 electrons in the valence shell. So it is very reactive.||It has 8 electrons in the valance shell. So it is non reactive (very stable).|
Differences between atom and ion are as follows:
|Atom is electrically neutral.||Ion is (+) or (-) vely charged.|
|It has equal number of protons and electrons.||It does not have equal number of protons and electrons.|
|It has usually less than eight electrons in its valence shell.||It has usually eight electrons in its valence shell.|
Cation: The positively charged ion is called cation. For e.g.: H+, Na+, Mg++ etc. A cation is formed by losing one or more electrons by an atom.
Anion: The negatively charged ion is called anion. For E.g. F-, O--, Cl- etc.
The difference between ionic compounds and covalent compounds are as follows:
|Ionic Compounds||Covalent Compounds|
|Ionic compounds are mainly solids.||Covalent compounds may be gases, liquids or solids.|
|They consist of ions.||They consist of molecules.|
|They have high melting point and boiling point.||They have generally low melting and boiling points.|
|They conduct electricity in solution.
E.g. NaCl, CaSO4
|They do not conduct electricity in solution.
E.g. H2O, CH4
|B||2, 8, 3|
|C||2, 8, 7|
|D||2, 8, 8, 1|
Here, Atomic number (Z) = 11
Atomic weight (A) = 23
Number of protons (P+) = ?
Number of electron (e-) = ?
Number of neutrons (no) = ?
Atomic number = number of protons = number of electrons.
∴Number of protons (P+) = 11
Number of neutrons (no) = 11
Atomic weight = Number of protons + Number of neutrons.
Or, A = P+ + no
Or, 23 = 11 + no
Or, no = 12
∴Number of neutrons = 12
The weight of an atom expressed by comparing with hydrogen atom is called relative weight. A hydrogen atom includes only one proton in its nucleus. Therefore, it is considered as 1.
[Note: The weight of proton or neutron is very small. The weight of a proton or neutron or atomic weight cannot be expressed in grams. Therefore, it is expressed in relative weight considering an atom of hydrogen as standard.]
Carbon and oxygen both are non-metals. Carbon needs four electrons to fulfill octet and oxygen need two electrons to fulfill octet. So, both combine together by sharing two electrons. So, it is called co-valent compound.
Which is false regarding atoms?
An atom is different than a molecule in that