Atoms, elements and molecular models

A sealed sample is stated to contain only oxygen, with no other substance present. Which description follows?

Pure describes composition, not the amount or appearance

Distinguish an atom from an element

An atom is the smallest unit that retains the identity of a chemical element. An element contains only one type of atom, such as helium atoms or oxygen atoms

An element is not defined by how many atoms are present. Four helium atoms and a much larger pure helium sample contain the same element

Atoms can occur separately or be chemically bonded into groups. A molecule is a discrete group of two or more atoms bonded together; it may contain one element or more than one

The earlier particle models did not identify the atoms inside each particle. Here the symbols on the spheres identify atoms and the connections show which atoms form each molecule

Read the key before interpreting a model

Each He-labelled sphere represents one helium atom. The colour helps distinguish element types, but the written symbols also work without colour

Symbols identify the atoms; colours are a key, not observed atom colours. Sizes, spacing and sample population are schematic, not to scale or a bulk measurement. Lines represent bonds, not physical sticks. This is not a state, density or electron-cloud model

A connection represents a chemical bond within a molecule. One pair of parallel lines represents a double bond between the same two atoms, not an extra pair of atoms. Detailed bonding theory is not needed for the counts here

Classify the helium specimen

The selected helium specimen contains only the labelled atoms shown; no bonds join them

Symbols identify the atoms; colours are a key, not observed atom colours. Sizes, spacing and sample population are schematic, not to scale or a bulk measurement. Lines represent bonds, not physical sticks. This is not a state, density or electron-cloud model

Explain the classification from the atom labels and absence of connections

All four atom labels are He, so there is one type of atom

The specimen is an element represented by separate atoms; the four drawn atoms are a selected sample, not the population in a real container

Interpret oxygen particles

Keep the letters and case of element symbols

Element symbols are short labels with a capital first letter. If there is a second letter, it is lower-case. In this catalogue H means hydrogen, He means helium, C means carbon and O means oxygen

He is one symbol, not hydrogen plus another element. H and He name different elements. The capital and lower-case letters are part of the notation, not optional styling

A symbol names an element type; a formula can state the composition of a molecule. O is the oxygen element symbol, while O₂ describes a molecule containing two oxygen atoms

Select the helium symbol

Count atoms, not just molecules

A compound contains chemically combined elements

A compound is a pure substance in which atoms of two or more elements are chemically combined in a fixed composition. Water and carbon dioxide are molecular compounds: they consist of discrete molecules

A water molecule contains two hydrogen atoms bonded to one oxygen atom. The water substance is not a mixture of separate hydrogen gas and oxygen gas

Not every molecule is a compound: an oxygen molecule contains only oxygen. Not every compound consists of separate molecules: nonmolecular compounds have extended structures, which these molecular pictures do not show

Use connections to identify a compound

The water specimen contains only the connected groups shown, each with the same composition

Symbols identify the atoms; colours are a key, not observed atom colours. Sizes, spacing and sample population are schematic, not to scale or a bulk measurement. Lines represent bonds, not physical sticks. This is not a state, density or electron-cloud model

Explain why this water specimen represents one compound rather than an element or a mixture

Every group contains H and O atoms connected in the same arrangement, with two H atoms for each O atom

The model represents one pure compound, water, rather than a mixture of uncombined hydrogen and oxygen substances

Read a formula for one molecule

In a molecular formula, a small subscript after an element symbol gives the number of that element's atoms in one molecule. No written subscript means one atom

H₂O records two hydrogen atoms and one oxygen atom in one water molecule. CO₂ records one carbon atom and two oxygen atoms in one carbon dioxide molecule

Count within one molecule before selecting its formula. Showing several identical molecules changes the sample totals, not the formula for each molecule

A formula gives composition but is not a complete picture of shape or bonding. A flat connectivity model and a spatial model show additional information

Match a formula to one molecule

Separate two symbols from a two-letter symbol

The carbon dioxide model has C and O labels. The capital C and capital O are two separate element symbols; the lower-case o in Co would instead make the single symbol for cobalt, a different element not shown in this catalogue

Only CO₂ matches one carbon and two oxygen atoms. Do not use the number of bond lines as an atom count: the two lines on each side still connect the same two atoms

Read carbon dioxide composition

Keep three different counts separate

Element-type count asks how many different element identities occur. Molecule count asks how many separate bonded groups occur. Atom count includes every constituent atom, even those bonded inside molecules

In the selected carbon dioxide specimen there are two element types, three molecules and nine atoms in total. Of those atoms, three are carbon and six are oxygen

Count only the element requested when a question asks for hydrogen or oxygen atoms. These are whole-number counts of the displayed model, not a measurement of a real sample

Count hydrogen across the water specimen

Change the representation, not the substance

A flat connectivity diagram places labelled atoms and their connections on a plane. The spatial ball-and-stick model projects three-coordinate positions onto the screen; a different orientation gives another view of the same selected specimen

The water model shows bent molecules and the carbon dioxide model shows linear molecules. A projected image can change apparent distances and angles, so do not measure the screen as if it were a calibrated molecule

Turning or tilting the view does not make a reaction, split molecules, alter the element identities or change the selected population. Orientation controls affect only the spatial view; the flat diagram has fixed positions

Predict what will stay the same. Begin with the flat connectivity model, then select the spatial ball-and-stick model and compare Front, Turned and Tilted. Follow one O atom and its two connected H atoms through the views, then check every molecule. Switching back to the flat model makes orientation changes inactive for its geometry

Which observation is consistent across these views?

Compare the two actual representations

Different elements together do not always form a compound

This specified non-reacting mixture contains helium atoms and oxygen molecules together. The helium and oxygen do not become a single substance merely by sharing the picture

The model has no bonds between helium and oxygen. Each oxygen molecule keeps its two O atoms. Unlike a molecular compound, the mixture has no single molecular formula describing all its particles as one molecule

The particular sample contains two helium atoms and two oxygen molecules, but this is not a required mixing ratio. Mixtures can have different proportions; the composition of each oxygen molecule stays fixed

When a question uses the word particle, read its stated counting rule. Here a separate helium atom is one particle and a complete oxygen molecule is one particle; an oxygen atom inside a molecule still counts as an atom

Interpret the combined specimen

Apply the stated particle-count rule

A pure oxygen sample consists only of molecules with two O atoms each, and pure water has molecules with two H atoms bonded to one O. Compare these records with the displayed helium and oxygen specimen. Which classification report is supported?

Use atom identities within each molecule and check whether different substances in the displayed specimen are joined together

Identify what these models leave out

The catalogue shows discrete atoms and molecules only. Some compounds have repeating extended structures rather than separate molecules. Those nonmolecular structures need a different reviewed model; the pictures here are not examples of them

For molecular examples a formula gives atoms per molecule. For a nonmolecular compound a formula describes a composition ratio, not a count inside an invented separate molecule

These models do not show electrons, electron clouds, real atom colours or a bulk material's state, pressure or density. The sphere sizes, distances and selected counts are not calibrated measurements

A useful explanation says both what a model represents and what it omits. Do not conclude that all compounds are molecular from a catalogue containing only molecular compounds

Evaluate the model's scope

Construct and explain your own representations

Drawing, physical-model and notation task, not automatically marked

On paper, draw two water molecules, three separate helium atoms and two oxygen molecules in three separately labelled samples. Write the appropriate symbol or single-molecule formula beside each, and include an atom-identity key. State the element types, molecule count and total atom count for each sample

Using large labelled modelling pieces or your own digital modelling tool, construct the two water molecules with bent arrangements. Keep the same connections while comparing the model from two directions, and sketch both views. No chemicals or practical heating are involved

Draw a fourth sample containing the three helium atoms mixed with the two oxygen molecules without adding bonds between the substances. Classify each sample and explain why the fourth is a mixture rather than a single molecule

State two things your models do not establish, including why they do not represent a nonmolecular extended structure. Ask a teacher to check your written symbols, formulas, constructions and explanations

Compare with a complete model explanation

The first sample has two separate bent H–O–H groups, each with two H atoms connected to one O atom. Its formula is H₂O, its element types are hydrogen and oxygen, and it contains two molecules with six atoms altogether. It is a molecular compound

The helium sample has three separate He-labelled atoms, one element type and no molecules. The oxygen sample has two separate O–O groups, formula O₂, one element type, two molecules and four atoms. Both samples are elements

The mixed sample keeps three separate He atoms and two O₂ molecules. It contains two element types and seven constituent atoms in five particles under the atom-or-complete-molecule counting rule. It is a mixture, with no bonds added between helium and oxygen

The two water views may differ in apparent position, but each keeps two H atoms connected to each O atom. Labelled pieces and drawn lines are representations, not atoms and physical sticks. Their size, colour and distance are not real measurements, and separate molecular groups do not model a repeating nonmolecular structure

Read identity, connections and scope together

Identify the element symbols first, then use connections to distinguish separate atoms, molecules and substances. One element can have molecular particles, while a compound contains chemically combined elements

Keep a single molecule's formula separate from sample totals. Preserve letter case and read subscripts as atom counts for the preceding symbol

Compare flat and spatial models without changing the specimen's identity. State the counting rule and model limits; a small molecular catalogue does not establish every possible compound structure