Saturday, February 11, 2023

Main particles: UNIZOR.COM - Physics4Teens - Atoms - Nucleus and Electrons

Notes to a video lecture on

Main Particles

Three particles constitute the main building blocks of any atom:
protons and
Let's examine their properties and functions in maintaining the integrity of an atom.

The basic atom model assumes that electrons are negatively charged particles flying on some orbits around an atom's nucleus that, in turn, contains certain number of positively charged protons and electrically neutral neutrons.

We discussed in details the distribution of electrons in shells (#1, #2, #3 etc.) and subshells (s, p, d etc.) around a nucleus.
These electrons (primarily, those on higher orbits) get involved in chemical reactions among atoms, facilitating creation of different molecules by either ionic or covalent bonding (see previous lectures on this topic).

The number of protons in a nucleus should be equal to the number of electrons circulating on orbits around this nucleus to maintain atom's neutrality.
This number is called an atomic number Z of an atom.

The electrostatic attraction between positive protons and negative electrons keeps the electrons on their stationary orbits.
But electrostatic forces repel from each other particles charged the same way (positive or negative). So, what keeps the protons in the nucleus and holds the atom's integrity?

Apparently, elementary particles inside a nucleus are held together by other forces, much stronger than electrostatic. They are called (not surprisingly) strong or nuclear forces.
These strong forces act only on a very small distance between particles inside a nucleus and have no noticeable influence on electrons around a nucleus.

Strong forces exist between protons, between protons and neutrons and between neutrons. Since neutrons are electrically neutral, greater number of them in the nucleus keeps the nucleus stronger, preventing electrostatic forces or external forces (like bombarding the nucleus with other particles) to break up an atom.
The total number of protons (Z) and neutrons (N) is called atomic mass of an atom (A=Z+N).

Nucleus of an atom takes a very small amount of space relatively to an atom's size.
For example, a nucleus of an atom of hydrogen, which consists of only one proton, has a diameter of the order of 10−15 meter, while a diameter of an atom of hydrogen is more than 100,000 greater (of the order of 10−10 meter).
Heavier elements that contain hundreds of particles (protons and neutrons, commonly called nucleons) in a nucleus have larger diameter of a nucleus, about 10 times larger than hydrogen, and their atoms are thousands times greater than their nuclei.

At the same time, the mass of an atom is concentrated in its nucleus. The nucleus' share in the mass of an atom is, approximately, 99.9%. The rest of mass is the mass of electrons.
These very approximate numbers do not take into consideration the effect of mass-energy relationship, according to Einstein Theory of Relativity formula E=m·c².

Since chemical properties of an element are defined by its electrons, the atomic number (that is, the number of electrons or equal to it the number of protons) defines chemical properties of an atom.
Different number of neutrons with the same number of protons in a nucleus of an element can occur, but these different compositions of a nucleus do not change the chemical properties of an element.
Atoms with the same number of protons but different number of neutrons are called isotopes.
We will address it in the next lecture.

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