What would a dot and cross diagram of an oxygen molecule show?

It would show two oxygen atoms sharing two pairs of electrons as a double bond, with two non-bonding pairs left on each atom.

What would a dot and cross diagram of methane show?

It would show a central carbon atom sharing one pair of electrons with each of four hydrogen atoms, giving four single covalent bonds.

What would a dot and cross diagram of sodium chloride show?

It would show a sodium ion with an empty outer shell and a single positive charge, and a chloride ion with eight outer electrons and a single negative charge, each drawn in square brackets.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Elements, Compounds and Mixtures

Dot and cross diagrams, and the limits of models

3 things to know
Paper 3 (Chemistry)

Overview

Why dots and crosses are used, the diagrams for hydrogen chloride, oxygen, methane, sodium chloride and magnesium oxide, and what dot and cross, ball and stick and flat drawings each fail to show.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why are dots used for the electrons of one atom and crosses for the electrons of the other?

Key term

In a dot and cross diagram of hydrogen chloride, how many electrons are shared and which atoms supply them?

Key term

How does a dot and cross diagram of an ionic compound differ from one of a covalent molecule?

Questions from this subtopic

Each one with the model answer underneath.

1
Why are dots used for the electrons of one atom and crosses for the electrons of the other?
Model answer
To show which atom each electron came from, even though in reality all electrons are identical.
2
In a dot and cross diagram of hydrogen chloride, how many electrons are shared and which atoms supply them?
Model answer
Two electrons are shared, one from the hydrogen atom and one from the chlorine atom, making a single covalent bond.
3
What would a dot and cross diagram of an oxygen molecule show?
Model answer
It would show two oxygen atoms sharing two pairs of electrons as a double bond, with two non-bonding pairs left on each atom.
4
What would a dot and cross diagram of methane show?
Model answer
It would show a central carbon atom sharing one pair of electrons with each of four hydrogen atoms, giving four single covalent bonds.
5
How does a dot and cross diagram of an ionic compound differ from one of a covalent molecule?
Model answer
The ionic diagram shows separate ions written in square brackets with their charges and no shared electrons, while the covalent one shows overlapping outer shells containing shared pairs.
6
What would a dot and cross diagram of sodium chloride show?
Model answer
It would show a sodium ion with an empty outer shell and a single positive charge, and a chloride ion with eight outer electrons and a single negative charge, each drawn in square brackets.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AMetals are on the left and in the centre, and non-metals are on the right-hand side.

BKI, because potassium loses one electron and iodine gains one.

CCaCl₂, because a calcium atom loses two electrons and each chlorine atom gains one.

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Sodium ions give a bright yellow flame colour.

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