Atoms and Radioactivity: Structure, Properties, and Applications

Atoms and Their Components

Subatomic Particles

All atoms are composed of smaller particles known as subatomic particles. The three fundamental subatomic particles are the proton, neutron, and electron.

• Protons and electrons are charged particles.

• Protons have a positive (+) charge.

• Electrons have a negative (−) charge.

• Neutrons are neutral (no charge).

• Atoms are electrically neutral overall because the number of protons equals the number of electrons.

Structure of an Atom

Although atoms and their subatomic particles are extremely small, they are considered matter. Most of the mass of an atom is concentrated in its nucleus, where protons and neutrons are located. Electrons occupy the space outside the nucleus, forming an electron cloud.

Subatomic Particle	Symbol	Electrical Charge	Relative Mass	Location in Atom
Electron	e−	1−	0.0005 (1/2000)	Outside nucleus
Proton	p or p+	1+	1	Nucleus
Neutron	n or n0	0	1	Nucleus

Example: In a lithium atom, the nucleus contains protons and neutrons, while electrons form a cloud around the nucleus.

Atomic Number and Mass Number

Atomic Number

Atoms of the same element always have the same number of protons. The atomic number is the number of protons in an atom and is unique to each element. It is found above the element symbol in the periodic table.

• Atomic number (Z): Number of protons in the nucleus.

• Determines the identity of the element.

Example: Carbon has an atomic number of 6, meaning every carbon atom has 6 protons.

Mass Number

The mass number of an atom is the sum of its protons and neutrons.

• Mass number (A): $A = \text{number of protons} + \text{number of neutrons}$

• Atoms of the same element can have different mass numbers due to varying numbers of neutrons.

Example: A carbon atom with 6 protons and 7 neutrons has a mass number of 13.

Additional info: The atomic number is always less than or equal to the mass number for any atom.