give me notes of this chapter Related: Various Models of an Atom - Structure of an Atom, Class 9, Science

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give me notes of this chapter Related:  Various Models of an Atom - Structure of an Atom, Class 9, Science

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Various Models of an Atom

Dalton's Atomic Model

Dalton's atomic model was based on the following assumptions:

Atoms are indivisible and indestructible.

All atoms of a given element are identical in mass and properties.

Compounds are formed by the combination of atoms of different elements in simple whole-number ratios.

Thomson's Atomic Model

Thomson proposed the plum pudding model of the atom, which had the following features:

An atom consists of a positively charged sphere with electrons embedded in it like plums in a pudding.

The positive charge is distributed uniformly throughout the atom, while the electrons are scattered randomly throughout it.

Thus, the atom is electrically neutral as the positive and negative charges balance each other out.

Rutherford's Atomic Model

Rutherford's model of the atom was based on the following observations:

Most of the alpha particles passed straight through the gold foil, which meant that the atom is mostly empty space.

A few alpha particles were deflected from their path, which meant that the atom contains a small, dense, positively charged nucleus.

The electrons revolve around the nucleus in circular orbits.

Bohr's Atomic Model

Bohr's atomic model was based on the following postulates:

Electrons can only revolve around the nucleus in certain fixed circular paths called orbits.

Each orbit corresponds to an energy level of the electron, and electrons can move from one orbit to another by absorbing or emitting energy.

The energy of an electron in an orbit is quantized, i.e., it can only have certain discrete values.

Modern Atomic Model

The modern atomic model is based on quantum mechanics and describes the behavior of electrons in atoms in terms of probabilities. Some of its features are:

The electrons are not restricted to fixed orbits but occupy orbitals, which are regions of space where the probability of finding an electron is high.

Orbitals are characterized by their energy, shape, and orientation in space.

The behavior of electrons in atoms is described by the Schrödinger equation.

Answer

Atoms are the basic units of matter and the defining structure of elements. Atoms are made up of three particles: protons, neutrons and electrons.

Protons and neutrons are heavier than electrons and reside in the center of the atom, which is called the nucleus. Electrons are extremely lightweight and exist in a cloud orbiting the nucleus. The electron cloud has a radius 10,000 times greater than the nucleus.

Protons and neutrons have approximately the same mass. However, one proton weighs more than 1,800 electrons. Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.

Nucleus

The nucleus was discovered in 1911, but its parts were not identified until 1932. Virtually all the mass of the atom resides in the nucleus. The nucleus is held together by the 'strong force,' one of the four basic forces in nature. This force between the protons and neutrons overcomes the repulsive electrical force that would, according to the rules of electricity, push the protons apart otherwise.

Protons

Protons are positively charged particles found within atomic nuclei. They were discovered by Ernest Rutherford in experiments conducted between 1911 and 1919.

The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element.

Protons are made of other particles called quarks. There are three quarks in each proton — two 'up' quarks and one 'down' quark — and they are held together by other particles called gluons.

Electrons

Electrons have a negative charge and are electrically attracted to the positively charged protons. Electrons surround the atomic nucleus in pathways called orbitals. The inner orbitals surrounding the atom are spherical but the outer orbitals are much more complicated.

An atom's electron configuration is the orbital description of the locations of the electrons in an unexcited atom. Using the electron configuration and principles of physics, chemists can predict an atom's properties, such as stability, boiling point and conductivity.

Typically, only the outermost electron shells matter in chemistry. The inner electron shell notation is often truncated by replacing the long-hand orbital description with the symbol for a noble gas in brackets. This method of notation vastly simplifies the description for large molecules.

Neutrons

Neutrons are uncharged particles found within atomic nuclei. A neutron's mass is slightly larger than that of a proton. Like protons, neutrons are also made of quarks — one 'up' quark and two 'down' quarks. Neutrons were discovered by James Chadwick in 1932.

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