Atomic Structure Mind Map (GCSE)

The smallest part of an element that can exist. Atoms have no overall charge, because they contain equal numbers of protons and electrons.

A substance made of only one type of atom. There are about 100 elements, shown in the periodic table, and every atom of an element has the same number of protons.

Two or more elements chemically combined in fixed proportions. Compounds can only be separated into their elements by chemical reactions.

Two or more substances not chemically combined. Mixtures can be separated by physical processes:

• Filtration
• Crystallisation
• Simple distillation
• Fractional distillation
• Chromatography

In the early 1800s, John Dalton described atoms as tiny solid spheres that could not be divided.

In 1897, J.J. Thomson discovered the electron. His plum pudding model pictured the atom as a ball of positive charge with negative electrons embedded in it.

In the alpha particle scattering experiment (1909), alpha particles were fired at thin gold foil. Most passed straight through, but a few were deflected. This showed that an atom's mass is concentrated in a tiny, positively charged nucleus and that most of the atom is empty space. Rutherford proposed this nuclear model in 1911.

In 1913, Niels Bohr suggested that electrons orbit the nucleus in shells at fixed distances. His calculations agreed with experimental observations.

In 1932, James Chadwick showed that the nucleus also contains neutrons, which have no charge.

Relative charge +1, relative mass 1, found in the nucleus. The number of protons is the atomic number, and it defines the element.

Relative charge 0, relative mass 1, found in the nucleus. Neutrons add to the mass but not the charge, and their number varies between isotopes.

Relative charge -1, very small mass (Edexcel gives it as 1/1836), found in shells around the nucleus. Electrons decide how an atom reacts.

• Atomic number: the number of protons
• Mass number: the number of protons plus neutrons

So the number of neutrons = mass number - atomic number.

Atoms have a radius of about 0.1 nanometres (1 x 10^-10 m). The radius of the nucleus is less than 1/10,000 of the radius of the atom.

Atoms of the same element with the same number of protons but a different number of neutrons, so a different mass number.

Both have 6 protons. Carbon-12 has 6 neutrons and carbon-14 has 8.

The average mass of an element's atoms, taking account of how common each isotope is:

relative atomic mass = (mass of isotope 1 x % abundance + mass of isotope 2 x % abundance) / 100

For example, chlorine is 75% chlorine-35 and 25% chlorine-37: (35 x 75 + 37 x 25) / 100 = 35.5.

Electrons occupy shells, also called energy levels, filling the shell closest to the nucleus first. For the first 20 elements, the first shell holds up to 2 electrons, the second up to 8, and the third up to 8.

Write the number of electrons in each shell, separated by commas:

• Sodium (11 electrons): 2,8,1
• Chlorine (17 electrons): 2,8,7

Atoms gain or lose electrons to get a full outer shell. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.

Elements are in order of atomic number. Elements in the same group have the same number of outer electrons, so they have similar chemical properties. The period number is the number of electron shells.

In 1869, Dmitri Mendeleev ordered the elements mainly by atomic weight, but swapped some to fit their properties and left gaps for elements not yet discovered, correctly predicting their properties.

Most elements are metals, found on the left and in the middle of the table. Metals form positive ions; non-metals are on the right.

Full outer shells, so they are unreactive. Boiling points increase down the group.

One outer electron. They react with water to form a metal hydroxide and hydrogen. Reactivity increases down the group, because the outer electron is further from the nucleus and is lost more easily.

Seven outer electrons, and they exist as molecules made of pairs of atoms. Reactivity decreases down the group, because it is harder to gain an electron further from the nucleus, while melting and boiling points increase. A more reactive halogen displaces a less reactive one from a solution of its salt.