Atomic structureIB MYP Physics: Revision notes

Every atom has a tiny, dense nucleus at its centre, surrounded by electrons in shells (energy levels). The nucleus contains protons and neutrons. Almost all of the mass of an atom is in the nucleus, but the nucleus is only a tiny fraction of the atom's size, so most of an atom is empty space.

• Proton: relative charge +1, relative mass 1, found in the nucleus
• Neutron: relative charge 0, relative mass 1, found in the nucleus
• Electron: relative charge -1, relative mass about 1/1840, found in shells around the nucleus

An atom has no overall charge because it has equal numbers of protons and electrons.

The atomic number (Z) is the number of protons in the nucleus. It decides which element the atom is. The mass number (A) is the total number of protons and neutrons (the nucleons).

Number of neutrons = A - Z

A nuclide is written with the mass number at the top left and the atomic number at the bottom left of the symbol, for example $^{23}_{11}\mathrm{Na}$.

Worked example: for $^{23}_{11}\mathrm{Na}$, protons = 11, neutrons = 23 - 11 = 12, and a neutral atom has 11 electrons.

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

For example, carbon-12 has 6 protons and 6 neutrons, and carbon-14 has 6 protons and 8 neutrons. Chlorine-35 and chlorine-37 are also isotopes.

Isotopes of an element have the same chemical properties because they have the same number of electrons, and it is the electrons that decide how an atom reacts. Some isotopes are unstable and radioactive.

Scientists change their models when new evidence appears.

• Dalton (early 1800s): atoms are tiny solid spheres that cannot be divided.
• Thomson (1897): after discovering the electron, he proposed the plum pudding model: a ball of positive charge with electrons embedded in it.
• Rutherford (1911): the nuclear model: a tiny, dense, positive nucleus with electrons around it, mostly empty space.
• Bohr (1913): electrons move in fixed shells (energy levels) around the nucleus.

Geiger and Marsden fired positively charged alpha particles at very thin gold foil.

• Most passed straight through: most of the atom is empty space.
• A small number were deflected through large angles: the nucleus is positive and repels the positive alpha particles.
• A very few bounced back: the positive charge and most of the mass are in a tiny, dense nucleus.

The plum pudding model had predicted only tiny deflections, so it was replaced by the nuclear model. This shows how evidence can make scientists reject a model.