Atoms and Molecules

Tutormate > CBSE Syllabus-Class 9th Chemistry > Atoms and Molecules

01 Properties of Matter

  • All matter is made up of extremely small particles called atoms and molecules.
  • As different kinds of atoms and molecules have different properties, different kinds of matter also possess different properties.
  • Elements combine to form compounds to become stable.
  • The chemical combinations are not random but constitute proper ways and are governed by laws.
  • Such laws which are true for chemical reactions are known as laws of chemical combination.

What is an atom, molecules?

  • Matter has basic structural and fundamental units.
  • It is like a storybook containing many pages, each page consisting of paragraphs, and each paragraph having many sentences.
  • Each sentence will further have many words and each word will have characters.
  • Matter is similarly made up of substances which contain molecules that in turn are made up of groups of atoms. Atoms are the smallest unit of matter.
  • Scientists wondered whether matter could be further divided or not in ancient times.
  • The idea of divisibility of matter evolved in India around 500 BC when a scientist named Maharishi Kanad stated that matter can be divided into smaller and smaller units.
  • The smallest unit of matter after which further division was not possible was known as parmanu.

Properties of atoms and molecules

  • A molecule is the smallest unit of a compound that contains its chemical properties.
  • Molecules are made up of groups of atoms which are also sub-divided into smaller units, namely proton, electrons and neutrons.
  • Nucleus is a bulk mass at the centre of an atom which is otherwise are largely vacant.
  • The protons and neutrons are contained inside the nucleus of atom and electrons revolve around the nucleus.
  • Protons are positively charged particles whereas electrons are negatively charged particles.
  • Neutrons do not carry any charge.
  • Nucleus is positively charged due to the presence of protons.
  • Atomic number is unique to every element.
  • The atomic number of an element is defined as the number of protons present in its nucleus, denoted by Z.
  • Mass of atoms refers to the mass of its particles.
  • Electrons have negligible mass.
  • So the mass of an atom is the sum of the mass of protons and neutrons.
  • The mass number is denoted by A.

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