Atomic Structure

All chemical behaviour — bonding, reactivity, conductivity, physical properties — traces back to atomic structure. Understanding what atoms are made of and how their electrons are arranged is the foundation for bonding, periodicity, and almost every other topic in chemistry.

The Atom¶

An atom is the smallest particle of an element that retains the chemical properties of that element. Every atom has two main regions:

• a dense central nucleus containing protons and neutrons
• a surrounding region of shells (energy levels) where electrons are found

Subatomic Particles¶

The mass of an electron is negligible compared to a proton or neutron. Nearly all of an atom's mass is concentrated in the nucleus. In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges cancel exactly.

Atomic Number and Mass Number¶

The atomic number (Z) is the number of protons in an atom. It uniquely identifies the element — every chlorine atom has exactly 17 protons, and no other element does. In a neutral atom, the atomic number also equals the number of electrons.

The mass number (A) is the total count of protons and neutrons in the nucleus:

$A = \text{protons} + \text{neutrons}$

The number of neutrons is therefore:

$\text{neutrons} = A - Z$

Atomic Notation¶

The standard nuclear notation places the mass number at the top left and the atomic number at the bottom left of the element symbol:

$^{A}_{Z}\text{X}$

So sodium is $^{23}_{11}\text{Na}$ and chlorine-35 is $^{35}_{17}\text{Cl}$.

When the species is an ion, the ionic charge appears at the top right. For example, $\text{Mg}^{2+}$ with atomic number 12 has lost two electrons, leaving it with 12 protons but only 10 electrons. The ion $\text{O}^{2-}$ has gained two electrons, so it has 8 protons and 10 electrons.

Electronic Configuration¶

Electrons occupy shells at increasing distances from the nucleus. Lower shells fill first. For elements up to atomic number 20, the shell capacities are:

Electronic configuration is written as the number of electrons in each shell from innermost outward, separated by commas.

The electrons in the outermost shell are called valence electrons. They govern bonding and reactivity. The number of valence electrons corresponds to the group number in the periodic table; the number of occupied shells corresponds to the period.

Ions¶

An ion is a charged particle formed when a neutral atom gains or loses electrons. The proton count never changes — only the electron count changes.

Cations (positive ions) form when atoms lose electrons. Metals typically do this because their valence electrons are loosely held.

Anions (negative ions) form when atoms gain electrons. Non-metals typically do this because their outer shell is nearly full and the nucleus can attract additional electrons.

Each ion achieves the electron arrangement of the nearest noble gas.

Isotopes¶

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.

Because isotopes have the same number of electrons in the same arrangement, they have identical chemical properties. Physical properties such as density and diffusion rate differ because the atoms have different masses.

Relative Atomic Mass¶

The relative atomic mass ($A_r$) of an element is the weighted average mass of its naturally occurring isotopes, expressed relative to one-twelfth the mass of a carbon-12 atom.

Carbon-12 is the international standard — its mass is defined as exactly 12, and all other atomic masses are measured against it.

When an element is a mixture of isotopes, its $A_r$ is not a whole number. Chlorine is approximately 75% Cl-35 and 25% Cl-37; the weighted average is about 35.5. This is why the periodic table shows 35.5 for chlorine rather than 35 or 37.

Uses of Radioactive Isotopes¶

Some isotopes are radioactive — their nuclei are unstable and emit radiation as they decay. This property makes them useful in several practical applications: