Physics – Concept on Atom, Molecule- Important Points

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ATOM
Atoms are very small, they are smaller than anything that we can imagine or compare with.
Relative Sizes

Radii (in m) Example

10–1 Watermelon

10–2 Ant

10–10 Atom of hydrogen

10–8 Molecule of haemoglobin

10–4 Grain of sand

10–9 Molecule of water

Dalton was the first scientist to use the symbols for elements in a very specific sense.

Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.
Symbols for some elements –
Element                             Symbol

Nitrogen                              N
Aluminium                           Al
Copper                               Cu

Fluorine                              F

Argon                               Ar

Barium                             Ba
Oxygen                            O
Gold                               Au

Potassium                       K

Silver                             Ag

Hydrogen                      H

Silicon                          Si

Bromine                      Br

Boron                         B

Iodine                         I

Sodium                     Na

Calcium                    Ca

Iron                         Fe

Sulphur                     S

Carbon                     C

Lead                       Pb

Magnesium                Mg

Chlorine                    Cl

Zinc                        Zn

Uranium                    U

Cobalt                     Co

Neon                       Ne

Atomic masses of a few elements

Element        Atomic Mass (u)

Hydrogen             1

Oxygen              16

Carbon               12

Nitrogen             14

Sodium               23

Magnesium         24

Sulphur              32

Chlorine            35.5

Calcium             40
MOLECULE
A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that
substance. Atoms of the same element or of different elements can join together to form molecules.
Molecules of some compounds
Compound         Combining Elements          Ratio by Mass

Carbon dioxide      Carbon, Oxygen                  3:8
Water                Hydrogen, Oxygen                 1:8

Ammonia               Nitrogen, Hydrogen           14:3

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