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Basics of Atom Lecture Note Matter and an Atom We can say that matter is anything that has both mass and volume or anything that occupies space. Matter is actually made up of building blocks called atoms. Atoms bonds to form matter. M ATTER HAS VOLUM E M ASS Atom is the smallest piece of matter that we can divide without losing the chemical properties of the matter. Elements and Compounds When a material is formed from only one type of atom then it is called an element. If a material is made from a combination of more than one type of atom then it is called a compound. For example oxygen is an element and carbon dioxide is a compound. Structure of an Atom and the Subatomic Particles But structurally, we can think of an atom like a tiny solar system. In the center of the atom is the nucleus which is a cluster of protons and neutrons Thus an atom consists of three main parts: protons, neutrons, and electrons. These parts are known as subatomic particles or fundamental particles. An atom has the same number of electrons (- ve charge) and protons (+ ve charge) to make the atom electrically neutral. Let us here see proper definitions of these subatomic particles one by one. Proton - “A subatomic particle which has one unit mass and one unit positive charge”.’ Electron - “A particle bearing one unit negative charge and mass 1/1835th of a hydrogen atom”. Neutron - “A subatomic particle which has a mass almost equal to that of a proton and has no charge”. Atomic number, Mass number and Atomic mass Atomic number of an element is equal to the number of protons in the nucleus of the atom of that element. It is represented by the symbol Z. The total number of protons and neutrons in the nucleus of an atom is called the Mass Number. It is represented by the symbol A. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom. Molecule Atoms bond together to form larger building blocks of matter called molecules. Two or more atoms share electrons to form a bond and when atoms form bonds they form molecules Water (H2O) is a molecule which is made up of two hydrogen atoms(H) and one oxygen atom (O). Nucleus and the Electromagnetic force between Nucleus and Electrons Nucleus can be defined as the core of the atom, which is made up of protons and neutrons. The mass of an atom is due primarily to the mass of the nucleus but it occupies only a tiny fraction of its volume Since the nucleus is made of protons and neutrons it is positively charged while the surrounding electrons are negatively charged The electromagnetic force between these two opposite charges binds the atoms together.