8th chemistry

 

Class 8th Science

ICSE/ CBSE Notes

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By Er. Amit Singh

3-Atoms And Molecules

Atom

•  It also is the smallest unit of matter.
• An atom consists of a central nucleus that is usually surrounded by one or more electrons.
•  Each electron is negatively charged. 
• The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons.
• A proton is positively charged. The number of protons in the nucleus often atom is the atomic number for the chemical element.
•  A proton has a rest mass, denoted mp, of approximate 1.673 x 10-27 kilogram (kg).
•  A neutron is electrically neutral and has a rest mass, denoted mn, of approximate 1.675 x 10-27 kg.
• Protons and electrons have equal and opposite charge.

Molecules

• A molecule is an electrically neutral group of two or more atoms held together by chemical bonds
• A covalent bond is formed when two atoms share electron pairs.

chemical compound

• A compound in chemistry refers to a substance which comprises of identical molecules consisting of atoms belonging to two or more chemical elements.