Scientific Models of the Atom

Atoms are the smallest particles of an element that retains its properties.
Early scientific models of the atom proposed the idea that atoms were solid spheres, indivisible and indestructible.
J.J. Thompson’s ‘plum pudding’ model, proposed in 1897, described atoms as a positively charged ‘pudding’, with electrons (negative charges) embedded like ‘plums’. This suggested that atoms were divisible.
Ernest Rutherford’s gold foil experiment in 1911 disproved the plum pudding model. Some alpha particles aimed at a thin gold foil were deflected, indicating they hit a concentrated central charge - the nucleus.
The Rutherford model, or nuclear model, proposed that the atom consists of a positively charged nucleus, containing protons, surrounded by negative electrons.
The Rutherford model was insufficient to explain why electrons do not crash into the nucleus. Niels Bohr expanded upon this model in 1913.
Bohr’s model proposed that electrons resided in specific energy levels or shells around the nucleus. They could absorb energy to move an outer shell or release energy to move into an inner shell, but they could not exist between shells.
The modern quantum mechanical model further refines this, suggesting that we cannot know both the exact location and velocity of an electron. Instead, we can only predict the probability of its location in a fuzzy ‘cloud’ around the nucleus.
The nucleus of an atom contains both protons (positively charged) and neutrons (neutral), while electrons (negatively charged) move in orbitals around the nucleus.
The number of protons in the nucleus is the atomic number and determines the identity of an element.
The sum of protons and neutrons gives the mass number.
Isotopes are atoms of the same element with different numbers of neutrons, hence they have different mass numbers.

Remember, each model has played an important role in developing our current understanding of atoms. All models are based on available evidence and may be refined or replaced as new evidence emerges. Always consider how our concept of the atom has changed over time and the experimental evidence that has led to these developments.