Atoms and Nuclei Short Notes Formula Sheets Class 12 Notes

Atoms and Nuclei: Take something apart, and you’ll find something smaller inside. Engines inside cars, pipes inside apples, hearts and lungs inside humans and teddy bears. But what happens if you continue? If you take things apart, you’ll find that all matter (all the “stuff” around us) is made up of different atoms.

For example, living things are mostly carbon, hydrogen, and oxygen atoms. These are only 3 of the 100 chemical elements discovered by scientists. Other elements include copper, tin, iron, gold, and gases such as hydrogen and helium. By combining atoms of different elements like little blocks, you can make almost anything you can think of.

An atom is the smallest unit of a chemical element—so a gold atom is the smallest piece of gold you can own. I mean small, microscopically short: an atom is hundreds of thousands of times thinner than a human hair, so you can’t see it unless you have a mighty electron microscope.

What is a Nuclei?

The nuclei are the part of an atom that contains most of its mass and carries a positive electric charge. The importance of the nucleus (plural: nuclei) is the mechanism by which it controls and determines the properties of the elements around it.

Atoms And Nuclei

Everything that occupies mass and space around us is matter. Atom is the basic unit of matter. As it is the basic building block of an element, it cannot be broken down further using any chemical means. Each state of matter is solid, liquid, gas, and plasma composed of neutral (un-ionized) or ionized molecules.

Atoms and nuclei are the basic building blocks of matter. Atoms are defined as matter composed of subatomic particles such as protons, neutrons, and electrons. However, the nucleus is the central region of the atom, consisting of positively charged protons and neutral neutrons, which consist of most of the atom’s volume, except for a small space between the nucleus and the electron cloud called the orbital zone or nucleus.

Structure of an Atom

The primary origin or cause of atomic structure can be traced back to Democritus, who argued that every matter in this universe is made up of atoms. In the 1800s, English chemist John Dalton was the first to propose the atomic structure scientifically. This article will study all atomic models and their advantages and limitations. But first, let us know about the composition of the atom.

atoms and nuclei class 12 notes

Every atom is made up of three fundamental particles called subatomic particles. They are as follows:

1. Protons: A proton is a sub-atomic particle that has a positive charge.

• Electrons: Electrons are particles at the subatomic level that have a negative charge.

• Neutrons: Neutrons are neutral subatomic particles. They are particles that have no charge. 

The structure of atoms depends on these three subatomic particles. In general, the structure of a molecule is given as follows:

• At the center of every atom will be a nucleus where protons and neutrons reside.
• Around the nucleus, there are electron shells that contain electrons. Electron revolves around these shells.
• The atomic structure looks similar to our solar system, where the sun is in the center and planets revolve around it in their respective orbits. 

Structure of Nuclei

The nuclei of an atom are a tightly packed structure of protons and neutrons. Since the protons and neutrons in the nucleus are the heaviest particles in an atom, the nucleus makes up 99.9% of the atom’s mass.

atoms and nuclei formula sheet

The radius of the nucleus:

‘R’ represents the radius of the nucleus.

R=R0A1/3

where,

A is the element’s mass number.

Ro is the proportionality constant.

Atomic Models: Many greek scientists in the 18th and 19th centuries tried to explain the molecule’s structure using atomic models. Each atomic model has its advantages and disadvantages, which are essential in shaping the current molecular model.

Year	Scientist(s)	New evidence	Changes to the atomic model
1897	Thomson	The discovery of electrons.	Every molecule can be broken down into smaller parts. An atom is made of tiny negatively charged electrons surrounding a positively charged sphere like plum pudding.
1909-1911	Rutherford (and Geiger and Marsden)	Some of the positively charged particles fired at the gold foil bounce back as they pass straight through.	Atoms have a central positive nucleus. Most mass of an atom lies in the nucleus.
1913	Bohr	In-depth work on Rutherford’s model has shown that it has its limitations. Electrons should spiral only towards the positive nucleus.	Electrons move around the nucleus in fixed orbits called electron shells.

Conclusion:

Atoms and nuclei are the basic building blocks of matter. Atoms are defined as matter composed of subatomic particles such as protons, neutrons, and electrons.