### What is an  atom?

• Atoms are the building blocks of universe.

• They are very tiny particles.

• Its very hard to imagine even its size.

• The centre of atom is called nucleus.

### Atom in detail

• Atom is made of tiny particles called protons and neutrons.

• Electrons circle around the nucleus in clouds, or shells, far from the nucleus.

• When an atom is in balance, it has the same number of protons and electrons.

• Atom can have a different number of neutrons.

• Electrons stay in their shells because a special force holds them.

• Protons and electrons of atom are attracted to each other.

• We say protons have a positive charge (+) and the electrons have a negative charge (-).

The electrons near the nucleus are held tight to the atom while sometimes the ones further out are not. These electrons can be made to move out of their orbits.

### Energy Levels of Atom

• An atom consists of electrons orbiting around a nucleus. But, the electrons cannot choose any orbit they wish.

• The orbits of atom are restricted to certain energies.

• Electrons can jump from one energy level to another, but they can never have orbits with energies other than the allowed energy levels.

### Example

• Consider a neutral hydrogen atom.

• Its energy levels are given in the image shown below.

• The x-axis shows the allowed energy levels of electrons in a hydrogen atom, numbered from 1 to 5. The y-axis shows each level's energy in electron volts (eV).

• One electron volt is the energy that an electron gains when it travels through a potential difference of one volt (1 eV = 1.6 x 10-19 Joules).

### Example

• Electrons in a hydrogen atom must be in one of the allowed energy levels.

• If an electron is in the first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it must have -3.4 eV of energy.

• An electron in a hydrogen atom cannot have -9 eV, -8 eV or any other value in between.

• Let's say the electron in atom wants to jump from the first energy level, n = 1, to the second energy level n = 2.

• The second energy level has higher energy than the first, so to move from n = 1 to n = 2, the electron needs to gain energy. It needs to gain (-3.4) - (-13.6) = 10.2 eV of energy to make it up to the second energy level.

### Example

• The electron of an atom can gain the energy it needs by absorbing light.

• If the electron in a atom jumps from the second energy level down to the first energy level, it must give off some energy by emitting light.

• The atom absorbs or emits light in discrete packets called photons, and each photon has a definite energy.

• Only a photon with an energy of exactly 10.2 eV can be absorbed or emitted when the electron jumps between the n = 1 and n = 2 energy levels.

### Example

• The energy of the photon depends on its wavelength.

• The wavelength, ? can be found by the following equation,

?=hc/E, where

• E is the energy of the photon (in eV), h

• is Planck's constant (4.14 x 10-15 eV s)

• c is the speed of light (3 x 108 m/s)

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