The atomic number is the number of protons in the nucleus of an atom. It defines the identity of an element.

The mass number is the total number of protons and neutrons in the nucleus. This determines the isotope of an element.

Atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes behave similarly in chemical reactions but have different physical properties, such as mass.

The average mass of an atom of an element, taking into account all its isotopes and their relative abundances, compared to 1/12 of the mass of a carbon-12 atom.

The number of protons is the atomic number ($Z$). For every element, the number of protons is fixed. For example, in selenium (Se), $Z=34$, meaning selenium always has $34$ protons.

In a neutral atom, the number of electrons is equal to the number of protons. So, selenium in its neutral state has 34 electrons.

If the atom loses electrons, the ion has fewer electrons than protons. For example, $\text{Se}^{2+}$ means selenium has 2 fewer electrons, so it would have $34-2=32$ electrons.

If the atom gains electrons, the ion has more electrons than protons. For example, $\text{Se}^{2+}$ would have $34+2=36$ electrons.

Example: Let's calculate the number of fundamental particles in $\text{Cl}^{-}$(chloride ion).

Chlorine (Cl) has an atomic number of 17, meaning it has 17 protons.

The mass number of $\text{Cl}^{-}$-35 is 35.

• Neutrons: 35 - 17 = 18 neutrons.

• Electrons: Since $\text{Cl}^{-}$ is a negative ion, it has gained 1 extra electron. So, 17+1=18 electrons. So, for $\text{Cl}^{-}$:

• Protons: 17

• Neutrons: 18

• Electrons: 18

Isotopes are atoms of the same element that have the same number of protons (since they are the same element) but different numbers of neutrons, leading to different mass numbers.

Summary Tips:

• Protons = Atomic number ($Z$)
• Neutrons = Mass number ($A$) - Atomic number ($Z$)
• Electrons: Same as protons unless the atom is an ion, then adjust for charge.