Elements, compounds & atomic theory

Year 8 answers

Fluency

Fluency

Reasoning

Water is a compound — hydrogen and oxygen are chemically bonded in a fixed 2:1 ratio with entirely new properties. Sea water is a mixture — salt is simply dissolved in water and can be recovered by evaporating the water.
Both diamond and graphite are made of only carbon atoms (one kind of atom), so they are both the element carbon. They look different because the same atoms are arranged in different patterns.
A mixture — filtering is a physical separation method, which only works when substances are not chemically bonded.
Symbols are case-sensitive. Co = cobalt (one element). CO = carbon monoxide (a compound of carbon and oxygen). The capitalisation tells you whether you are reading one symbol or two.

Problem solving

Na: 1, H: 1, C: 1, O: 3. Total atoms = 6.
An element.
Wood contains carbon, hydrogen and oxygen atoms. Burning rearranges them into carbon dioxide and water vapour, which escape as gas. No atoms are lost — they just move into the air, which is why the wood “seems” to disappear.
Argon is in Group 18 (noble gases), which are very unreactive because their atoms are already stable. So argon reacts very little with other elements.

Reasoning

Each formula unit has 2 X and 3 Y. Four formula units: $4 \times 2 = 8$ X atoms and $4 \times 3 = 12$ Y atoms.
A mixture (of elements). Real example: the air we breathe is mostly N $_2$ , O $_2$ and argon.
The core idea of atomic theory is that atoms of an element behave the same way chemically. Isotopes have the same number of protons and so react the same way; they just differ in mass. Atomic theory still holds — it just became more detailed.
Mendeleev’s gaps let him predict the properties of elements that had not yet been discovered (such as gallium and germanium). When those elements were later found with matching properties, it was strong evidence that his table was a real pattern in nature, not an arbitrary sorting.