Atomic Structure and the Periodic Table — AQA Combined Science: Trilogy

This unit explains what atoms are made of, how our model of the atom developed, and how the periodic table is organised to reflect atomic structure.

Atoms, elements and compounds

• An element is a substance made of only one type of atom. There are about 100 elements, shown in the periodic table.
• A compound is two or more elements chemically combined in fixed proportions. Compounds can only be separated by chemical reactions.
• A mixture contains different substances not chemically combined, so they keep their own properties and can be separated by physical means.

Chemical equations show reactions. They must be balanced — the same number of each type of atom on both sides, because atoms are never created or destroyed (conservation of mass).

Separating mixtures

Physical separation techniques (no new substances made):

• Filtration — separates an insoluble solid from a liquid.
• Crystallisation — evaporating a solution to get soluble solid crystals.
• Simple distillation — separates a liquid from a solution (e.g. water from salty water).
• Fractional distillation — separates a mixture of liquids with different boiling points.
• Chromatography — separates substances based on how well they dissolve in a solvent.

The structure of the atom

Atoms are tiny — radius about 1 × 10⁻¹⁰ m. They have a central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels). The nucleus is about 1/10 000 the size of the atom but contains nearly all its mass.

• Atomic (proton) number = number of protons (this defines the element).
• Mass number = number of protons + neutrons.
• Atoms have no overall charge because the number of protons equals the number of electrons.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Relative atomic mass (Ar) is the average mass of the isotopes of an element, taking into account their abundance.

Development of the atomic model

The model of the atom changed as new evidence was found:

1. Atoms were first thought to be tiny solid spheres that couldn't be divided (Dalton).
2. The discovery of the electron led to the plum pudding model (a ball of positive charge with electrons embedded).
3. The alpha particle scattering experiment showed most of the atom is empty space with a small, dense, positive nucleus (the nuclear model).
4. Niels Bohr proposed that electrons orbit at specific distances (energy levels).
5. Later experiments showed the nucleus contains protons, and James Chadwick provided evidence for neutrons.

This is a great example of how scientific models change with new experimental evidence.

Electronic structure

Electrons occupy shells (energy levels) around the nucleus. The shells fill from the inside out: 2, 8, 8 for the first 20 elements. For example, sodium (11 electrons) is 2,8,1.

The electronic structure links directly to an element's position in the periodic table.

The periodic table

Elements are arranged in order of increasing atomic number.

• Groups are the vertical columns; elements in a group have the same number of electrons in their outer shell, so they have similar chemical properties.
• Periods are the horizontal rows; the period number tells you the number of occupied electron shells.

History

Early tables arranged elements by atomic weight, leaving gaps. Dmitri Mendeleev left gaps for undiscovered elements and even predicted their properties, which is why his table is regarded as the breakthrough. The discovery of isotopes later explained why ordering by atomic number (not weight) works.

Metals and non-metals

• Metals are found on the left and centre; they lose electrons to form positive ions.
• Non-metals are on the right; they tend to gain or share electrons.

Group 0 — the noble gases

• Unreactive (inert) because they have a full outer shell of electrons.
• Boiling points increase down the group as the atoms get bigger.

Group 1 — the alkali metals

• Have one electron in their outer shell.
• React with water to produce hydrogen and a metal hydroxide (an alkali).
• Reactivity increases down the group — the outer electron is further from the nucleus and more easily lost.

Group 7 — the halogens

• Have seven electrons in their outer shell; exist as diatomic molecules (e.g. Cl₂).
• Reactivity decreases down the group — it gets harder to gain an electron as atoms get bigger.
• A more reactive halogen displaces a less reactive one from a solution of its salt.
• Melting and boiling points increase down the group.

Exam tips

• Learn the three subatomic particles with their charges and masses.
• Be able to work out protons, neutrons and electrons from the atomic and mass numbers.
• Explain group reactivity trends in terms of the distance of the outer electron from the nucleus.
• Remember Group 1 reactivity increases down the group, but Group 7 reactivity decreases down the group.
• Be ready to describe the alpha scattering experiment and what it proved.