Concise Selina Solutions for Class 7 Chemistry Chapter 4 Atoms Molecules and Radicals


Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms. Atoms themselves are composed of protons, neutrons, and electrons. Each element has its own atomic number, which is equal to the number of protons in its nucleus. An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 pico-meters across. The structure of an atom, theoretically consisting of a positively charged nucleus surrounded and neutralized by negatively charged electrons revolving in orbits at varying distances from the nucleus, the constitution of the nucleus and the arrangement of the electrons differing with various chemical elements. Atom is divided into two parts - Atomic structure and extra nucleus part. Atomic structure consists of positively charged proton (P+) and neutral neutron . Extra nucleus part contains negatively charged electrons (e-). Atoms have mass as all elements and compounds have mass.