Atomic Structure: Key Concepts in Chemistry (Edexcel GCSE Chemistry)

Atomic Structure

Introduction to Atomic Theory

The concept of the atom has evolved over time as scientists have discovered subatomic particles and developed new models. Understanding atomic structure is fundamental to chemistry, as it explains the properties and behaviors of elements.

• John Dalton (1803):

  • Proposed that all substances are made of atoms, which are small particles that cannot be created, divided, or destroyed.

  • Atoms of the same element are identical; atoms of different elements are different.

  • Atoms join with other atoms to make new substances.

  • Dalton's theory was later revised as new discoveries were made.

• J.J. Thomson:

  • Used cathode-ray tubes to discover electrons.

  • Proposed the "plum-pudding" model: atoms are spheres of positive charge with electrons scattered throughout.

• Ernest Rutherford (1909):

  • Conducted the gold foil experiment, discovering that most of the atom's mass is concentrated in a small, positively charged nucleus.

  • Proposed that electrons move in random paths around the nucleus.

Structure of the Atom

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells.

• Nucleus: Contains protons and neutrons; very small compared to the overall size of the atom.

• Electrons: Move in shells (energy levels) around the nucleus.

Subatomic Particles

Atoms are made up of three main subatomic particles: protons, neutrons, and electrons. Each has specific properties.

Particle	Relative Charge	Relative Mass
Proton	+1	1
Neutron	0	1
Electron	-1	1/1836

Atomic Number and Mass Number

The atomic number and mass number are used to identify and characterize atoms.

• Atomic Number (Z): Number of protons in the nucleus; unique to each element.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

Formula:

$\text{Mass Number (A)} = \text{Number of Protons} + \text{Number of Neutrons}$

Neutral Atoms

Atoms are electrically neutral because they contain equal numbers of protons and electrons. The positive charge of protons cancels out the negative charge of electrons.

• Number of Protons = Number of Electrons

Nucleus and Atomic Mass

The nucleus is extremely small compared to the overall size of the atom, but it contains most of the atom's mass.

• Most of the mass of an atom is concentrated in the nucleus.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have different mass numbers but are chemically similar.

• Example: Chlorine-35 and Chlorine-37 are isotopes of chlorine.

Calculating Numbers of Subatomic Particles

Given the atomic number and mass number, you can calculate the numbers of protons, neutrons, and electrons in an atom.

• Number of Protons: Equal to atomic number.

• Number of Neutrons: Mass number minus atomic number.

• Number of Electrons: Equal to atomic number (in a neutral atom).

Formula:

$\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$

Relative Atomic Mass (RAM)

The relative atomic mass of an element is the weighted average mass of its isotopes, taking into account their abundances. This explains why the atomic masses of some elements are not whole numbers.

• RAM is calculated using the abundance and mass of each isotope.

• Example: Chlorine has two main isotopes: chlorine-35 (75% abundance) and chlorine-37 (25% abundance).

Formula:

$\text{RAM} = \frac{(\text{mass of isotope A} \times \% \text{of isotope A}) + (\text{mass of isotope B} \times \% \text{of isotope B})}{100}$

Example Calculation

Calculate the RAM of chlorine:

• Chlorine-35: 75%

• Chlorine-37: 25%

$\text{RAM} = \frac{(35 \times 75) + (37 \times 25)}{100}$ $= \frac{2625 + 925}{100}$ $= \frac{3550}{100}$ $= 35.5$

Summary Table: Isotopes and RAM Calculation

Isotope	Relative Mass	Abundance (%)
Chlorine-35	35	75
Chlorine-37	37	25

Additional info: The concept of RAM is essential for understanding chemical formulas and reactions, as it allows chemists to calculate the proportions of elements in compounds and mixtures.