GCSE Science₈₄₆₅

Explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number.

The elements in the periodic table are arranged in order of atomic (proton) number, so that elements with similar properties are in columns known as groups. The table is called a periodic table because similar properties occur at regular intervals.

Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level (in a particular shell). The electrons in an atom occupy the lowest available energy levels (innermost available shells). Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.

WS 1.2

Represent the electronic structure of the first 20 elements of the periodic table in the following forms:

sodium

2,8,1

Predict possible reactions and probable reactivity of elements from their positions in the periodic table.

This topic links with Atomic structure .

Explain in terms of isotopes how this changes the arrangement proposed by Mendeleev.

Following Mendeleev, the elements in the periodic table were arranged in order of relative atomic mass. In this order some elements appeared to be in the wrong group. These problems were solved once it was realised that most elements occur as mixtures of isotopes and that elements should be arranged in the table in order of atomic number.

WS 1.1

Show how scientific methods and theories have changed over time.

This topic links with Atomic structure .

Explain how the atomic structure of metals and non-metals relates to their position in the periodic table.

Explain how the reactions of elements are related to the arrangement of electrons in their atoms and hence to their atomic number.

The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table. Non-metals are found towards the right and top of the periodic table.

Elements that react by losing their outer electrons to form positive ions are metals.

Elements that do not form positive ions are non-metals. The more reactive non-metals, such as the halogens, react with metals by gaining electrons to form negative ions.

WS 1.2

Recall the simple properties of Group 0.

Explain how the observed simple properties of Group 0 depend on the outer shell of electrons of the atoms and predict properties from given trends down the group .

The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons. The noble gases have eight electrons in their highest energy level (outer shell), except for helium, which has only two electrons.

The boiling points of the noble gases increase with increasing relative atomic mass (going down the group ).

WS 1.2

Predict properties from given trends down Group 0.

Recall the simple properties of Group 1.

Explain how the observed simple properties of Group 1 depend on the outer shell of electrons of the atoms and predict properties from given trends down the group .

The elements in Group 1 of the periodic table are known as the alkali metals. They:

• are soft metals with low density
• react with non-metals, including chlorine and oxygen, to form colourless ionic compounds
• react with water
• form hydroxides that give alkaline solutions in water.

In Group 1, the further down the group an element is, the more reactive the element.

WS 1.2

Predict properties from given trends down Group 1.

Recall the simple properties of Group 7.

Explain how the observed simple properties of Group 7 depend on the outer shell of electrons of the atoms and predict properties from given trends down the group .

The elements in Group 7 of the periodic table are known as the halogens. They:

• are non-metals
• consist of molecules
• react with metals to form ionic compounds
• form molecular compounds with other non-metallic elements.

In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.

In Group 7, reactivity of the elements decreases going down the group.

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.

WS 1.2

Predict properties from given trends down Group 7.

Use the names and symbols of common elements and compounds and the principle of conservation of mass to write formulae and balanced chemical equations.

(HT only) and half equations.

Use chemical symbols to write the formulae of elements and simple covalent and ionic compounds.

Describe the physical states of products and reactants using state symbols (s, l, g and aq).

Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium.

There are about 100 different elements. Elements are shown in the periodic table.

Compounds are formed from elements by chemical reactions.

Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. Compounds can only be separated into elements by chemical reactions.

Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change. Chemical reactions can be represented by word equations or equations using symbols and formulae.

In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions.

WS 4.1

Use the names and symbols of the first 20 elements, groups 1, 7 and 0 and other common elements from a supplied periodic table to write formulae and balanced chemical equations where appropriate.

Name compounds of these elements from given formulae or symbol equations.

Write word equations for the reactions in this specification.

Write formulae and balanced chemical equations for the reactions in this specification.

Recall and use the law of conservation of mass.

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

This means that chemical reactions can be represented by symbol equations that are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

MS 1a

Use arithmetic computation and ratio when writing and balancing equations.

Explain any observed changes in mass in non-enclosed systems during a chemical reaction and explain them using the particle model.

Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account.

WS 1.2

Explain any observed changes in mass in non-enclosed systems during a chemical reaction given the balanced symbol equation for the reaction.

Calculate relative formula masses of species separately and in a balanced chemical equation.

Students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses.

The relative atomic mass of an element compares the mass of atoms of the element with the¹² C isotope. It is an average value for the isotopes of the element.

The relative formula mass ( M _r ) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

Students will not be expected to calculate relative atomic masses from isotopic abundances.

MS 1a, 3a

Calculate the relative formula mass ( M _r ) of a compound from its formula, given the relative atomic masses.

WS 3.3

Carry out and represent mathematical and statistical analysis.

Explain how the mass of a given substance is related to the amount of that substance in moles and vice versa.

Recall and use the definitions of the Avogadro constant (in standard form) and of the mole.

Chemical amounts are measured in moles. The symbol for the unit mole is mol.

The mass of one mole of a substance in grams is numerically equal to its relative formula mass.

One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance.

The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 × 10²³ per mole.

MS 1a, 1b, 1c, 2a

Recognise and use expressions in decimal form when using the relative formula mass of a substance to calculate the amount in moles in a given mass of that substance and vice versa, giving the answer in the appropriate units.

MS 3b, 3c

Change the subject of a mathematical equation.

WS 4.6, MS 2a

Provide answers to an appropriate number of significant figures.

MS 1b

Calculate with numbers written in standard form when using the Avogadro constant.

MS 3a

Understand and use the symbols: =, <, <<, >>, >, ∝ , ~

Deduce the stoichiometry of an equation from the masses of reactants and products and explain the effect of a limiting quantity of a reactant.

The balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios.

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant.

MS 3c, 3d

Balance an equation given the masses of reactants and products.

Explain the effect of a limiting quantity of a reactant on the amount of products it is possible to obtain in terms of amounts in moles or masses in grams.

Use a balanced equation to calculate masses of reactants or products.

The masses of reactants and products can be calculated from balanced symbol equations.

Chemical equations can be interpreted in terms of moles. For example:

$\mathsf{M}\mathsf{g}\mathsf{ }\mathsf{ }\mathsf{+}\mathsf{ }\mathsf{ }\mathsf{2}\mathsf{H}\mathsf{C}\mathsf{l}\mathsf{ }\mathsf{ }\mathsf{\to }\mathsf{ }\mathsf{ }\mathsf{M}\mathsf{g}\mathsf{C}{\mathsf{l}}_{\mathsf{2}}\mathsf{ }\mathsf{ }\mathsf{+}\mathsf{ }\mathsf{ }{\mathsf{H}}_{\mathsf{2}}$

shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.

MS 1a, 1c, 3c, 3d

Calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.

WS 4.6, MS 2a

Provide answers to an appropriate number of significant figures.

(HT only) Explain how the mass of a solute and the volume of the solution is related to the concentration of the solution.

Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm³ (g/dm³ ).

MS 1c, 3c

Calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution.