Science notes, chapter 6

Periodic Table

Textbook Science Form 1 (DLP), pages 162–193

By the end of this chapter you can

Describe atoms, subatomic particles and molecules; tell elements from compounds; locate metals, non-metals and inert gases in the Periodic Table and compare metals with non-metals (6.1)
Give examples of mixtures and choose a physical method to separate them — filtration, distillation, magnet, sedimentation, floatation, chromatography (6.2)
Explain how compounds form, show that mass is conserved, separate compounds by electrolysis, and compare physical with chemical changes and mixtures with compounds (6.3)

6.1Classification of Elements

In Chapter 5 you learned that matter is anything that has mass and occupies space. All matter is made of tiny, separate particles called atoms. A metal bracelet is made of atoms packed tightly together; the hot air inside a hot air balloon is made of particles that are far apart.

Atom — the smallest particle of an element. Atoms are far too small to see with our eyes; we need an electron microscope, which can enlarge them about a million times.

To imagine how small an atom is: if an orange were an atom, the electron microscope would have to make it as big as the Earth for us to see it.

Inside an atom

Model of an atom: electrons circling a nucleus of protons and neutrons — The structure of an atom. Protons and neutrons sit in the nucleus; electrons move around it.

Textbook Figure 6.3, p. 165

Subatomic particle	Charge	Where it is
Proton	Positive (+)	In the nucleus
Electron	Negative (−)	Moving around the nucleus
Neutron	No charge (neutral)	In the nucleus

Remember the three subatomic particles with PEN: Proton, Electron, Neutron.

The nucleus has an overall positive charge because of its protons. An atom has the same number of electrons as protons, so the positive and negative charges cancel. That is why an atom is neutral.

Molecule — a neutral particle made of two or more atoms joined together. Example: two oxygen atoms join to form one oxygen molecule (O₂).

An atom is not neutral because it has no particles with charge. It is neutral because the number of protons (+) equals the number of electrons (−).

Elements and compounds

Element — the simplest form of a substance. It cannot be broken down into simpler substances, and it has only one type of atom.
Compound — two or more elements combined chemically. It is made by a chemical reaction.

Zinc bucket, oxygen tank, hydrogen tank, iron nails and pencil carbon each shown with one type of atom — Each element is made of one type of atom only: zinc (Zn), oxygen (O), hydrogen (H), iron (Fe) and carbon (C).

Textbook Figure 6.5, p. 166

	Examples
Elements	Iron, oxygen, hydrogen, aluminium, carbon, copper (oxygen is the most abundant element on Earth)
Compounds	Salt (sodium + chlorine), sugar (carbon + hydrogen + oxygen), water (hydrogen + oxygen), shell (calcium + carbon + oxygen), aluminium oxide, zinc sulphide, iron chloride

The parts of a compound cannot be separated by physical methods. They can only be separated chemically, for example by passing electricity through the compound (electrolysis).

Some metals and non-metals such as carbon and helium exist as single atoms. Is helium an element or a compound? Explain.

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An element — it is made of only one type of atom (helium atoms) and cannot be broken down into simpler substances.

The Periodic Table

In the 18th and 19th centuries scientists discovered many elements. In 1869 the Russian chemist Dmitri Mendeleev arranged the 63 known elements in a table and left gaps for elements not yet found. When germanium was discovered in 1886 and matched his prediction, scientists accepted his idea. Today's Periodic Table arranges the elements in an orderly, systematic way according to proton number. As of 2016, 118 elements had been discovered.

Textbook Figure 6.6, p. 167

Type	Position in the Periodic Table	Examples
Metals	Left side and middle (the largest group)	Sodium, magnesium, aluminium, iron, copper, zinc, gold, mercury
Non-metals	Right side (and hydrogen at the top left)	Hydrogen, carbon, nitrogen, oxygen, sulphur, chlorine, iodine
Semi-metals	A "staircase" between metals and non-metals	Boron, silicon, germanium, arsenic
Inert gases	Last column on the right (Group 18) — they are non-metals	Helium, neon, argon, krypton, xenon

How elements are named: some old names end in "-gen" (from "generator"): hydrogen means "water generator". New names must be approved by IUPAC (International Union of Pure and Applied Chemistry); many honour a scientist or a place, e.g. rutherfordium (Ernest Rutherford), bohrium (Niels Bohr), darmstadtium (Darmstadt, Germany).

Metals and non-metals

Both metals and non-metals are elements. They differ in their physical characteristics:

Characteristic	Metals	Non-metals
Appearance	Shiny	Dull
Ductility (can be pulled / bent into wires)	Ductile	Brittle
Malleability (can be hammered into shape)	Malleable	Not malleable
Tensile strength	High	Low (break easily)
Electrical conductivity	Good	Poor (except carbon)
Heat conductivity	Good	Poor
Density	High	Low
Melting and boiling points	High	Low

Based on textbook Figure 6.8, p. 169

Aluminium
Malleable, strong and light, grey and shiny — used for foil.

Iron
Strong, malleable, magnetic, good conductor — used for hoes.

Copper
Ductile, rustproof, good conductor, brown — used for electric wires.

Zinc
Strong, grey, good conductor — used for roofs.

Iodine
Black crystals, poisonous, antiseptic.

Chlorine
Greenish-yellow gas, poisonous, bleaching agent.

Sulphur
Yellow powder, poisonous — used to harden rubber tyres.

Carbon
Black, smooth, light; the only non-metal that conducts electricity — pencil lead, racquets.

Experiment 6.2 — testing metals and non-metals

The manipulated variable in every test is the type of material; the size of material is kept constant.
• Appearance: rub copper and carbon rods with sandpaper → copper is shiny, carbon is dull.
• Ductility: bend copper wire and pencil lead → copper wire bends into a circle; pencil lead snaps.
• Malleability: hammer iron, copper and sulphur → iron and copper flatten; sulphur shatters.
• Electrical conductivity: connect iron, carbon and sulphur rods in a circuit → the ammeter needle deflects for iron and carbon, not sulphur.
• Heat conductivity: heat one end of a rod with a thumbtack waxed to the other end → the thumbtack drops first on the best heat conductor (copper / iron), not on carbon.
• Melting point: heat tin and sulphur powder → sulphur melts first; tin (metal) has the higher melting point.

Some elements, such as germanium and silicon, have characteristics of both metals and non-metals. They are called semi-metals. Antoine Lavoisier was the first scientist to classify elements as metals and non-metals.

Classify these elements as metal or non-metal: carbon, magnesium, iodine, neon, copper, mercury.

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Metals: magnesium, copper, mercury. Non-metals: carbon, iodine, neon (neon is also an inert gas).

6.2Mixtures

Mixture — two or more elements or compounds mixed physically (not chemically joined). Examples: air, cocktail drinks, air batu campur, salad, sandwiches.

Because a mixture is made physically, it can be separated by physical methods. Which method to use depends on:

the physical properties of the substances in the mixture (size, solubility, density, boiling point, magnetism), and
the substance you want to obtain from the mixture.

Method	Used to separate	Works because	Example
Filtration	An insoluble solid from a liquid	Solid particles are too big to pass through the filter paper	Sand and water; coffee powder from coffee
Distillation	Liquids that mix completely (miscible)	The liquids have different boiling points	Water and alcohol; making perfume from rose petals
Magnet	A magnetic solid from a non-magnetic solid	Iron, nickel and cobalt are attracted to a magnet	Iron nails and sand; iron and sulphur powder
Sedimentation	An insoluble solid from a liquid	The denser solid sinks and settles at the bottom	Sand / silt and water
Floatation	Substances that do not mix, by density	The less dense substance floats on top	Oil and water (separating funnel)
Chromatography	Small amounts of dissolved substances, e.g. dyes in ink	Different substances travel different distances up the paper	Colours in ink; harmful food colouring; drug tests on urine
Sieving	Small particles from larger ones	Particle size	Removing impurities from flour

(a) Filtration

Filtration apparatus with filter funnel, filter paper, residue and filtrate — The solid left on the filter paper is the **residue** (sand). The liquid that passes through is the **filtrate** (water).

Textbook Figure 6.20, p. 178

(b) Distillation

Distillation apparatus with round-bottom flask, thermometer and Liebig condenser — Heating a water–alcohol mixture. Alcohol boils first; its vapour cools in the Liebig condenser and drips into the beaker.

Textbook Figure 6.21, p. 179

The thermometer bulb is at the side arm and measures the boiling point of the vapour leaving the flask.
Cold water flows in at the bottom and out at the top of the Liebig condenser to cool the vapour back into a liquid.
Porcelain chips make the boiling smooth (they stop the mixture from boiling up suddenly).

(c) Separation using a magnet

Magnet bar held over paper covering a Petri dish of iron and sulphur powder — Iron powder is attracted to the magnet; sulphur powder stays in the dish.

Textbook Figure 6.23, p. 181

Magnetic metals: iron, nickel, cobalt. Non-magnetic metals: gold, bronze, aluminium.
Food factories use magnet separators to remove iron particles; scrapyards use strong magnets to collect iron and steel.

(d) Sedimentation

Sand settling at the bottom of a beaker and the clear water being poured off — Stir the silty water, leave it to settle, then pour the clear water slowly into another beaker. The sand is left behind.

Textbook Figure 6.24, p. 182

(e) Floatation

Separating funnel holding oil above water — Oil is less dense than water, so it floats. Open the tap to let the water run out first, then close it before the oil comes through.

Textbook Figure 6.25, p. 183

(f) Chromatography

Filter paper with a pencil line and ink dots hanging in a beaker of water — Draw a **pencil** line 1.5 cm from the bottom, put ink dots on it, and dip the paper in water so that the water does **not** touch the dots. The colours separate as the water rises.

Textbook Figures 6.26 and 6.27, p. 184

Chromatography is used for very small amounts of mixture — for example to check forged documents (ink colours), detect harmful food colourings and test athletes' urine for illegal drugs.

Use a pencil, not a pen, for the start line — pen ink would also dissolve and spread up the paper.

Can you separate a mixture of rice and sand using filtration? Why?

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No. Filtration separates an insoluble solid from a liquid; rice and sand are both solids. (Sieving would work better, because rice grains are bigger than sand grains.)

6.3Compounds

A compound is formed when two or more elements combine chemically. The new product has its own characteristics, different from the elements it came from. Everyday compounds include salt, sugar, chalk, marble, polythene (a plastic of carbon and hydrogen) and water (hydrogen and oxygen). Rust is a compound formed when iron reacts with oxygen.

How metals and non-metals form compounds

Word equations

magnesium + oxygen → magnesium oxide (also aluminium, zinc, iron, copper → metal oxides)
sodium + water → sodium hydroxide + hydrogen gas (alkali metals: lithium, sodium, potassium)
iron + sulphur → iron sulphide (when heated)

Crucible with lid containing sulphur and iron powder on a pipeclay triangle over heat — Activity 6.11: heat iron and sulphur powder in a covered crucible and weigh it before and after.

Textbook Figure 6.28, p. 186

Mass is conserved in a chemical change: the total mass before heating is the same as the total mass after heating. No mass is gained or lost — the atoms are only rearranged.

Almost all mineral salts in nature exist as compounds, except for gold, silver and platinum, which are found as pure elements.

Separating a compound: electrolysis

Electrolysis — the chemical decomposition (breaking down) of a compound into its elements by passing an electric current through it.

Electrolysis of water producing oxygen at the anode and hydrogen at the cathode — Electrolysis of water (with a little sulphuric acid added).

Textbook Figure 6.29, p. 187

Hydrogen gas collects at the negative electrode (cathode).
Oxygen gas collects at the positive electrode (anode).
Water is broken down into its two elements, so water is a compound.

Physical change and chemical change

	Physical change	Chemical change
New substance?	No new substance formed	A new substance is formed
Properties	Stay the same	Products have different properties
Chemical composition	Stays the same	Changes
Energy	Needs less energy	Needs more energy
Examples	Ice melting, water freezing, water boiling	Rusting of iron, photosynthesis, decaying leaves, cell respiration

Based on textbook Figures 6.30 and 6.31, p. 188

Mixtures and compounds compared

Characteristic	Mixture	Compound
New substance formed?	No	Yes
Chemical bond?	No	Yes
Separation method	Physical	Chemical (electrolysis)
Properties compared with the original substances	Same	Different

Textbook Table 6.2, p. 189

Iron filings mixed with sulphur powder is a mixture — a magnet can still pull out the iron. After heating, iron sulphide is a compound — a magnet can no longer pull the iron out.

State whether each is a physical or chemical change: (a) wax melting, (b) a nail rusting, (c) bread going mouldy.

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(a) Physical — no new substance; the wax can set again. (b) Chemical — rust (iron oxide) is a new substance. (c) Chemical — decay forms new substances.

Summary

Remember	Key fact
Subatomic particles	Proton (+) and neutron (0) in the nucleus; electron (−) around it. "PEN"
Atom is neutral	Number of protons = number of electrons
Molecule	Two or more atoms joined together
Element vs compound	One type of atom vs two or more elements combined chemically
Periodic Table	Arranged by proton number; metals left, non-metals right, inert gases in Group 18
Metals	Shiny, ductile, malleable, good conductors, high melting point (carbon is the non-metal that conducts)
Mixture	Combined physically → separated physically (filtration, distillation, magnet, sedimentation, floatation, chromatography)
Compound	Combined chemically → separated chemically (electrolysis)
Chemical change	Forms a new substance; mass is conserved