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Note on Molecular Properties

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Importance of atom

Atoms combine together to form a molecule. Molecule combines together to from a substance. In this way, a substance is formed by atoms. To form a new substance either by combining or breaking, atoms take part in chemical reactions.

Table 1: Number of protons, neutrons, electrons in the atoms of the first 20 elements

Element Atomic number Number of proton Number of neutron Number of electrons Atomic mass
H 1 1 0 1 1
He 2 2 2 2 4
Li 3 3 4 3 7
Be 4 4 5 4 9
B 5 5 6 5 11
C 6 6 6 6 12
N 7 7 7 7 14
O 8 8 8 8 16
F 9 9 10 9 19
Ne 10 10 10 10 20
Na 11 11 12 11 23
Mg 12 12 12 12 24
Al 13 13 14 13 27
Si 14 14 14 14 28
P 15 15 16 15 31
S 16 16 16 16 32
CL 17 17 18 17 35
A r 18 18 22 18 40
K 19 19 20 19 39
Ca 20 20 20 20 40

Nucleus

The center of the atom is called the nucleus. The nucleus is a small positively charged center of an atom. The nucleus contains all the proton and neutrons that constitute in an atom. The positive charge on the nucleus is due to the presence of protons in it. The number of protons in the nucleus determines the number of positive charges on the nucleus. The neutrons which are also present in the nucleus, have no charge i. e. they are neutral.

Atomic number

The number of protons of in one atom of an element is known as atomic number of that element. That is:
The aAtomic number of an element = number of protons in one atom of the element.
For example
, one atom of sodium element has 11 protons in it, so the atomic number of sodium is 11. All the atoms of the same element have same number of protons in their nuclei, and hence they the same atomic number.

Mass number (atomic mass or atomic weight)

The total number of protons and neutrons present in one of an element is known as its atomic mass. The mass number is known as the atomic mass. That is

Mass number or atomic mass = No. of protons + No. of neutrons.

Since the number of protons in an atom is the atomic number of the element, we can write the above relation by putting the atomic number in place of number of protons. That is

Atomic mass number = Atomic number + Neutron number

The mass number does not include the electrons and so is very slightly less than the atomic weight.

Electric charge

Proton and electron posses nuclear charge. The charge is measured in Coulomb. 1 coulomb = charge of 6.25 x 1018 electrons. There are equal number of proton and electrons in an atom, so the atom is electrically neutral.

Proton, electron and neutron

Sub-atomic particle Symbol Mass Charge Location
proton p+ 1 amu + electrons
electron e- 1/1836 amu - shell
neutron n0 1 amu 0 electrons

Molecular weight

The weight of a molecule is known as molecular weight. Molecular weight can be known by adding the atomic mass of atoms present in the molecule. For example, the molecule of hydrogen has two hydrogen atoms and so, the molecular weight of hydrogen is 2.

  • Atoms combine together to form a molecule.
  • Molecule combine together to from a substance. In this way, a substance is formed by atoms.
  • The center of the atom is called the nucleus.
  • The number of protons of in one atom of an element ,is known as atomic number of that element.
  • The total number of protons and neutrons present in one of an element, is known as its atomic mass.
  • The mass number does not include the electrons and so is very slightly less than the atomic weight.
  • Molecular weight can be known by adding the atomic mass of atoms present in the molecule.
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Very Short Questions

The number of positively charged particles or protons present in the nucleus of an atom of each element is called atomic number. It is denoted by z.

The center of the atom is called nucleus.

There is the positive charge on the nucleus due to the presence of protons in it.

The number of protons in the nucleus determines the number of positive charge in the nucleus.

The number of protons in one atom of an element is known as an atomic number.

The total number of protons and neutrons present in one atom of an element is called its atomic mass.

The weight of a molecule is called its molecular weight.

The molecular weight of hydrogen is 2.

The atomic number of helium is 2 and its symbol is He.

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  • The atomic number of ______  is 6 and its mass is 13

    lithium


    boron


    carbon


    calcium


  • An element has an atomic number 19 the electronic configuration of that element is ______.

    2,8,7,2


    2,8,8,1


    2,8,9


    2,8,8,2


  • The mass number of an element is 16 and number of neutrons is 7. The number of electrons is ______.

    10


    9


    7


    8


  • Which one is true?

    Atomic mass number = Atomic number - Neutron number


    Atomic mass number = Atomic number + Neutron number


    none of the above


    Atomic mass number = No. of protons x No. of neutrons


  • One atom of sodium element has ______.

    15 proton


    11 proton


    9 proton


    5 atoms


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