notes of atoms and molecules class 9
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What is an Atom?
Atoms are defined as “the basic building blocks of matter”.
It is the smallest constituent unit of matter that posses the properties of the chemical element. Atoms don’t exist independently, instead, they form ions and molecules which further combine in large numbers to form matter that we see, feel and touch.
Atoms are much too small to be seen; hence experiments to find out their structure and behavior have to be conducted with large numbers of them. From the results of these experiments we may attempt to construct a hypothetical model of an atom that behaves like the true atom.
Molecules consist of one or more atoms bound together by covalent (chemical) bonds. Atoms may be depicted by circle shapes, each of which has a nucleus at the center (containing protons and neutrons), surrounded by one or more concentric circles representing the ‘shells’ or ‘levels’ in which the electrons surrounding the nucleus of the atom are located and markings indicating the electron.at each level. A molecule is the smallest thing a substance can be divided into while remaining the same substance. It is made up of two or more atoms that are bound together by chemical bonding.

Atoms and Molecules
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Atom Definition Chemistry
The smallest particle of an element, which may or may not have an independent existence but always takes place in a chemical reaction is called an atom. An atom is defined as the smallest unit that retains the properties of an element. An atom is composed of atoms and these cannot be made or destroyed. All atoms of the same element are identical and different elements have different types of atoms. Chemical reactions occur when atoms are rearranged.
Atoms consist of three fundamental types of particles, protons, electrons and neutrons. Neutrons and protons have approximately the same mass and in contrast to this the mass of an electron is negligible. A proton carries a positive charge, a neutron has no charge and an electron is negatively charged. An atom contains equal numbers of protons and electrons and therefore overall an atom has no charge. The nucleus of an atom contains protons and neutrons only, and therefore is positively charged. The electrons occupy the region of space around the nucleus. Therefore, most of the mass is concentrated within the nucleus.
The center of the atom is called the nucleus. The nucleus contains neutrons and protons that give an atom its weight and positive charges. A neutron carries no charge and has a mass of one unit. A proton carries a single positive charge and also has a mass of one unit, The atomic number of an element is equal to the number of protons or positive charges in the nucleus. The atomic weight of an element is determined by combining the total number of protons and neutrons in the nucleus. An electron carries a single negative charge. If an atom of an element is to have zero charge, it must have the same number of electrons as protons. These electrons are arranged in orbits around the nucleus of the atom like the layers of an anion.
What is the Size of an Atom?
The size of an atom is extremely small, much smaller than our imagination. A layer of an atom as thick as a thin sheet of paper is formed when more than millions of atoms are stacked together. It is impossible to measure the size of an isolated atom because it’s difficult to locate the positions of electrons surrounding the nucleus.
However, the size of an atom can be estimated by assuming that the distance between adjacent atoms is equal to half the radius of an atom. Atomic radius is generally measured in nano metres.
1 m = 109nm
Relative Sizes
ExamplesRadii (m)Atom of Hydrogen10−10mMolecule of water10−9mMolecule of hemoglobin10−8mGrain of Sand10−4m
What are Atoms made of?
An atom is composed of three particles, namely, neutrons, protons and electrons with hydrogen as an exception without neutrons.
Every atom has a nucleus that bounds one or more electrons around it.
The nucleus has typically a similar number of protons and neutrons which are together known as nucleons.
The protons are positively charged, electrons are negatively charged and neutrons are neutral.
What is Atomic Mass?
It is the mass of an atom in a chemical element. It is roughly equivalent to total neutrons and protons present in the atom. It is expressed in atomic mass units (denoted by u). 1amu is equal to the exactly one-twelfth the mass of 1 atom of C-12 and the relative atomic masses of elements is determined with respect to-12 atom.
Atomic masses of Some Elements
ElementsAtomic Mass (u)Hydrogen1Carbon12Nitrogen14Oxygen16Sodium23Magnesium24Sulfur32Chlorine35.5Calcium40