C1 Atomic structure and the periodic table

An atom is the smallest part of an element that can exist on its own. Each element has a symbol (e.g. O for oxygen) and contains only one kind of atom.

A compound has two or more elements chemically joined in a fixed ratio. Compounds have different properties from their elements and are only split by chemical reactions, not physical methods.

A formula shows the atoms in a compound. H₂O contains 2 hydrogen atoms and 1 oxygen atom. With brackets, multiply everything inside by the outside number: Ca(OH)₂ has 1 Ca, 2 O, and 2 H atoms.

Balanced equations show atoms are rearranged, never created or destroyed. Both sides must have the same number of each atom. Balance equations only by putting big numbers in front; never change subscripts.

Particle models show elements as single atoms or molecules of one kind, compounds as different atoms joined together, and mixtures as different substances not joined.

Balance the equation for magnesium and oxygen: Mg + O₂ → MgO.

With 2 oxygen atoms on the left and 1 on the right, put a 2 in front of MgO: Mg + O₂ → 2MgO.

Now there are 2 magnesium atoms on the right. Put a 2 in front of Mg: 2Mg + O₂ → 2MgO.

A molecule like O₂ is an element, not a compound, because it contains only one type of atom.

Learn the symbols: S is sulfur, not sodium (Na). Capitalisation matters: CO is carbon monoxide, but Co is cobalt.

Compounds ending in '-ide' usually contain two elements. Those ending in '-ate' contain those elements plus oxygen.

A mixture is two or more elements or compounds not chemically joined. Each substance keeps its chemical properties. Mixtures can be separated by physical methods, which make no new substances and involve no chemical reactions.

Filtration separates an insoluble solid from a liquid. The solid stays in the filter paper as residue; the liquid passes through as filtrate.

Crystallisation separates a soluble solid from its solution. Heat the solution to evaporate some solvent, then let it cool. Crystals form, which can be filtered and dried.

Simple distillation separates a solvent from a solution. The liquid boils, and the vapour is condensed back to a liquid.

Fractional distillation separates a mixture of liquids with different boiling points. The liquid with the lowest boiling point is collected first.

Chromatography separates mixtures of soluble substances, such as food dyes.

Simple distillation of salt water. The thermometer bulb is level with the side-arm to measure the pure vapour.

How can you separate a mixture of sand and salt?

Salt is soluble in water, but sand is not. First, add water and stir to dissolve the salt.

Second, filter the mixture. The sand stays as the residue.

Third, heat the filtrate to evaporate water and crystallise the salt.

Dissolving is not a chemical reaction.

Evaporating to dryness is fast but can decompose some solids. Crystallisation gives purer crystals.

In distillation, the condenser has cold water entering at the bottom to keep it completely full.

In fractional distillation, the liquid with the highest boiling point distils last.

Models change when new evidence does not fit the old model.

Before electrons: atoms were thought to be tiny solid spheres that could not be divided.

Plum pudding model: after the electron was discovered, the atom was modelled as a ball of positive charge with negative electrons embedded inside.

Nuclear model: alpha particle scattering showed that the atom's mass is concentrated in a tiny, positive nucleus at the centre, and it is mostly empty space.

Bohr model: Niels Bohr showed that electrons orbit the nucleus at specific distances (energy levels).

Protons and neutrons: later experiments showed the positive charge is made of protons. James Chadwick then provided evidence for neutrons.

The plum pudding model has no nucleus and no empty space. The nuclear model has a tiny central nucleus and mostly empty space.

Positively charged alpha particles were fired at thin gold foil. What happened to them, and what did this prove?

Straight through: most alpha particles passed straight through. This proved that atoms are mostly empty space.

Deflected: a few particles changed direction. This proved they were repelled by a concentrated positive charge.

Bounced back: a very small number bounced back. This proved the mass and positive charge are concentrated in a tiny central nucleus.

Do not say the plum pudding model has a nucleus: the positive charge was a large spread-out ball.

Remember that the nucleus is positive, not negative.

Alpha particles bounced back because they hit the dense nucleus, not because they hit electrons.

Chadwick discovered neutrons about 20 years after the nucleus was accepted, not at the same time as protons.

Atoms contain three subatomic particles: protons, neutrons and electrons.

Protons have a relative charge of +1 and relative mass of 1. Neutrons have a charge of 0 and relative mass of 1. Both are in the central nucleus.

Electrons have a charge of −1 and a very small relative mass. They orbit in shells.

Atomic number = number of protons. Every atom of an element has the same number of protons. Neutral atoms have equal protons and electrons.

Mass number = protons + neutrons. To find neutrons, subtract atomic number from mass number.

Ions form when atoms gain or lose electrons. The number of protons never changes.

Size: An atom's radius is about 0.1 nm (1 × 10⁻¹⁰ m). The nucleus is less than 1/10 000 of that (about 1 × 10⁻¹⁴ m). Almost all mass is in the nucleus.

A lithium atom has 3 protons and 4 neutrons in its nucleus, and 3 electrons in its shells.

Aluminium has atomic number 13 and mass number 27. How many protons, neutrons and electrons are in an Al³⁺ ion?

Protons = atomic number = 13

Neutrons = 27 − 13 = 14

A neutral atom has 13 electrons. The 3+ charge means it lost 3 negative electrons, so it has 13 − 3 = 10 electrons.

Never add atomic number and mass number.

A positive ion has lost electrons; a negative ion has gained electrons.

When converting nm to m, 1 nm is 1 × 10⁻⁹ m. The atom's radius is 1 × 10⁻¹⁰ m.

Isotopes are atoms of the same element that have the same number of protons (atomic number) but a different number of neutrons (mass number).

Isotopes of an element have the same number of electrons, so they have the same chemical reactions.

The relative atomic mass is a weighted average mass of an element's atoms. It takes into account the mass number of each isotope and its abundance (how common it is).

Relative atomic mass = sum of (mass number × percentage abundance) ÷ 100.

The three isotopes of hydrogen. Each atom has one proton and one electron, but a different number of neutrons.

Chlorine exists as two isotopes: chlorine-35 (75% abundance) and chlorine-37 (25% abundance). Calculate the relative atomic mass of chlorine.

Multiply each mass number by its percentage abundance:

(35 × 75) + (37 × 25) = 2625 + 925 = 3550

Divide the total by 100:

3550 ÷ 100 = 35.5

Relative atomic mass is a weighted average, so it is often not a whole number.

Do not just take the mean of the mass numbers: multiply each by its abundance, add, then divide by 100.

The relative atomic mass is closer to the mass number of the more abundant isotope.

Neutrons = mass number − atomic number, e.g. carbon-13 has 13 − 6 = 7 neutrons.

Electrons in an atom occupy the lowest available energy levels, which are also called shells. The innermost shell is filled first.

For the first 20 elements, the first shell can hold up to 2 electrons. The second shell can hold up to 8 electrons. The third shell also fills up to 8 electrons before the fourth shell begins to fill.

To write an electronic structure, use numbers separated by commas. For example, calcium has 20 electrons, so its electronic structure is 2,8,8,2.

You can find an element's position in the periodic table from its electronic structure. The number of occupied shells is the period. The number of electrons in the outer shell is the group number (for groups 1 to 7). An element with a full outer shell is in Group 0.

An atom of sodium has 11 electrons in total. They fill the energy levels as 2,8,1. The inner shells are full.

An atom has the electronic structure 2,8,6. What element is this, and which group and period is it in?

The total number of electrons is 2 + 8 + 6 = 16. For an atom, this equals the atomic number, so the element is sulfur.

It has 3 occupied shells, so it is in period 3. It has 6 electrons in its outer shell, so it is in Group 6.

Remember that the first shell holds a maximum of 2 electrons, not 8.

The third shell holds 8 electrons before the fourth shell starts filling. Do not put 9 electrons in the third shell (e.g. potassium is 2,8,8,1, not 2,8,9).

The number of outer-shell electrons tells you the group, not the period.

The modern periodic table places elements in order of their atomic number (number of protons). Elements with similar chemical properties line up in vertical columns called groups.

The group number equals the outer-shell electrons. The horizontal rows are called periods, and the period number equals the number of occupied electron shells.

Before subatomic particles were known, early chemists ordered elements by atomic weight, which sometimes grouped elements with very different properties together.

Dmitri Mendeleev improved the table in 1869. He left gaps for undiscovered elements and swapped the order of some pairs (like tellurium and iodine) to keep elements with similar properties in the same group.

Mendeleev predicted the properties of the missing elements. When elements like germanium were discovered and closely matched his predictions, his table became accepted.

The later discovery of isotopes (atoms of the same element with different numbers of neutrons) explained why ordering strictly by atomic weight was not always correct.

The layout of the modern periodic table. The main groups are numbered 1 to 7 and 0. The central block holds transition metals.

Calcium is in Group 2. It reacts with water to form calcium hydroxide and hydrogen. Strontium is further down Group 2. Predict what happens when strontium is added to water.

Strontium and calcium are in the same group, so their atoms have the same number of outer electrons (2).

Elements in the same group have similar reactions, so strontium also reacts with water.

The products are strontium hydroxide and hydrogen.

Elements in the same group react in similar ways because they have the same number of outer-shell electrons.

Remember that the modern table is ordered by atomic number, not mass number or atomic weight.

Mendeleev left gaps deliberately to make predictions; they were not mistakes.

Most elements are metals. Metals are found on the left and towards the bottom of the periodic table. They react to form positive ions by losing their outer electrons.

Non-metals are found on the right and top of the periodic table. They do not form positive ions; instead they gain or share electrons in their reactions.

Elements can be classified using their properties.

Metals are typically solids with high melting points (except mercury). They are shiny, malleable (can be hammered into shape) and are good conductors of heat and electricity. Metal oxides are basic.

Non-metals often have low melting points (many are gases). Solid non-metals are dull and brittle, and they are poor conductors of electricity (except graphite). Non-metal oxides are acidic.

Metals (shaded) and non-metals (white). The thick stepped line separates them.

An unknown element melts at 650 °C, conducts electricity when solid, and forms an oxide that dissolves in water to give a solution of pH 10. Is it a metal or a non-metal?

It is a metal. The high melting point and good conductivity match the physical properties of a metal, and the basic oxide (pH > 7) confirms it.

The number of outer electrons tells you the ion formed: Group 1 atoms lose 1 electron to form 1+ ions; Group 2 atoms form 2+ ions; Group 6 atoms gain 2 electrons to form 2− ions; Group 7 atoms gain 1 electron to form 1− ions.

Do not assume all solids that conduct are metals; graphite is a non-metal that conducts electricity.

Remember that metal oxides are basic, and non-metal oxides (like sulfur dioxide) are acidic.

Group 0 elements are the noble gases (e.g. helium, neon, argon). They are very unreactive and exist as single atoms because their outer electron shells are full. This is a stable arrangement: they all have 8 outer electrons, except helium which has 2.

Group 1 elements are the alkali metals (e.g. lithium, sodium, potassium). They are soft, low-density metals. They all have one electron in their outer shell, which they lose to form 1+ ions.

Alkali metals react with water to form a metal hydroxide and hydrogen. The solution is alkaline (universal indicator turns blue or purple).

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Lithium fizzes steadily; sodium melts into a ball and fizzes rapidly; potassium also melts and the hydrogen burns with a lilac flame.

With oxygen they form metal oxides (a cut surface tarnishes). With chlorine they form white solid metal chlorides.

Boiling points of the noble gases in °C. They increase down the group, as relative atomic mass increases.

Xenon boils at −108 °C. Radon is the next element down Group 0. Predict whether radon is a solid, liquid or gas at room temperature (20 °C).

Boiling points increase down Group 0, so radon boils above −108 °C.

Each step down the group raises the boiling point by roughly 30–60 °C, so radon should boil at about −60 °C (the real value is −62 °C).

20 °C is above radon's boiling point, so radon is a gas.

Reactivity increases as you go down Group 1. The outer electron gets further from the nucleus, so it is attracted less strongly and is lost more easily.

Noble gases have a full outer shell, not an empty one.

With water an alkali metal forms a hydroxide (e.g. NaOH), not an oxide.

The elements in Group 7 are known as the halogens. They are non-metals that consist of molecules made of pairs of atoms (diatomic molecules), such as Cl₂, Br₂ and I₂.

Because they have seven electrons in their outer shell, they have similar chemical properties. They all react to gain one electron, forming ions with a 1− charge (halide ions like Cl⁻, Br⁻ and I⁻).

As you go down Group 7, the relative molecular mass, melting point and boiling point all increase. This is why chlorine is a green gas at room temperature, bromine is a red-brown liquid, and iodine is a grey-black solid.

The halogens become less reactive as you go down the group. This is because the outer shell is further from the nucleus, so the attraction from the nucleus is weaker, and an extra electron is gained less easily.

A more reactive halogen displaces a less reactive one from a solution of its salt.

Bromine water is added to potassium iodide solution. Predict whether a reaction happens, name the products and describe the colour change.

Bromine is above iodine in Group 7, so it is more reactive and displaces iodine.

bromine + potassium iodide → potassium bromide + iodine

Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

The orange bromine water turns brown as iodine forms.

Halogen means the element (e.g. Cl₂), while halide means the ion in a compound (e.g. Cl⁻ in sodium chloride).

Halogens form ionic compounds with metals, and covalent compounds with non-metals (like hydrogen chloride, HCl).

A halogen cannot displace a more reactive halogen (one above it in the group). For example, iodine added to potassium bromide gives no reaction.