Definition of an Atom
An atom is the smallest indivisible particle of an element which can take part in a chemical change.
The figure shows the simple structure of an atom. It consists of two parts: The nucleus and the electrons. The nucleus is the heavy portion of the atom and is made up of two types of particles called protons and neutrons. The protons carry a positive charge while neutrons carry no charge. The electrons carry a negative charge and circle in orbits around the heavy nucleus. The numbers of orbits depend on the substance, for example hydrogen has only one orbit while oxygen has two. An electron is very light (about 1 /1840 of the mass of the proton). The negative charge of an electron is equal to the positive charge of a proton and the number of electrons in an atom is equal to the number of protons. The atom is therefore electrically neutral.
The below table summarizes the properties of the elementary particles in the atom of an element.
|S/N||Elementary Particles||Charge (Coulomb)||Mass (kg)||Location in the atom|
|Particle||Charge||Mass (kg)||Relative Mass (amu)|
The important points to keep in mind are as follows:
(i) Protons and neutrons have almost the same mass, while the electron is approximately 2000 times lighter.
(ii) Protons and electrons carry charges of equal magnitude, but opposite charge. Neutrons carry no charge (they are neutral).
- What are the elementary particles contained in the nucleus of an atom?
- Write down their masses and charges.
- Which particle is located in the outermost shell?
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