Atomic Structure and Electron Configuration in Biology

Atomic Structure

Subatomic Particles

Atoms are the basic units of matter and are composed of three main types of subatomic particles:

• Protons: Positively charged particles found in the nucleus of the atom.

• Neutrons: Neutral particles (no charge) also located in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells.

Atomic Number is the number of protons in an atom, which determines the element. Mass Number is the sum of protons and neutrons.

Electron Shells and Energy Levels

Electrons are arranged in shells (energy levels) around the nucleus. Each shell can hold a specific maximum number of electrons:

• First shell: up to 2 electrons

• Second shell: up to 8 electrons

• Third shell: up to 18 electrons (for basic biology, often shown as up to 8 for main group elements)

Electrons fill the lowest energy levels first (closest to the nucleus).

Bohr Model of the Atom

The Bohr model represents atoms with a central nucleus (containing protons and neutrons) and electrons in concentric circles (shells) around the nucleus.

• Example: A carbon atom (atomic number 6) has 6 protons, 6 neutrons, and 6 electrons. Its electron configuration is 2 electrons in the first shell and 4 in the second shell.

Electron Configuration Examples

Electron configuration describes the arrangement of electrons in an atom's shells. Here are some examples:

Element	Atomic Number	Electron Configuration	Bohr Model Representation
Hydrogen	1	1	1 electron in first shell
Helium	2	2	2 electrons in first shell (full)
Carbon	6	2, 4	2 electrons in first shell, 4 in second shell
Oxygen	8	2, 6	2 electrons in first shell, 6 in second shell
Sodium	11	2, 8, 1	2 electrons in first shell, 8 in second shell, 1 in third shell

Valence Electrons and Chemical Properties

Valence electrons are the electrons in the outermost shell of an atom. They determine the atom's chemical reactivity and bonding behavior.

• Atoms with full outer shells are generally stable (e.g., noble gases).

• Atoms tend to gain, lose, or share electrons to achieve a full outer shell (octet rule).

Key Equations

• Mass Number: $\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

• Atomic Number: $\text{Atomic Number} = \text{Number of Protons}$

Summary Table: Subatomic Particles

Particle	Charge	Location	Relative Mass
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Electron shells	~0 (1/1836 of a proton)

Example: Carbon Atom

• Atomic number: 6

• Protons: 6

• Neutrons: 6 (for most common isotope)

• Electrons: 6

• Electron configuration: 2 in first shell, 4 in second shell

Application: Understanding atomic structure is fundamental for studying chemical bonding, molecular biology, and the behavior of elements in biological systems.