atoms, elements and compounds

Question 1

Element

Answer 1

A substance made of atoms that all contain the same number of protons (one type of atom) and cannot be split into anything simpler.

There is a limited number of elements and all elements are found on the Periodic Table.

E.g. hydrogen, carbon, nitrogen.

Question 2

Compound

Answer 2

A pure substance made up of two or more elements chemically combined together.

There is an unlimited number of compounds.

Compounds cannot be separated into their elements by physical means.

E.g. copper (II) sulphate (CuSO4), calcium carbonate (CaCO3), carbon dioxide (CO2).

Question 3

Mixture

Answer 3

A combination of two or more substances (elements and/or compounds) that are not chemically combined.

Mixtures can be separated by physical methods such as filtration or evaporation.

E.g. sand and water, oil and water, sulphur powder and iron filings.

Question 4

Metals and nonmetals

Answer 4

The Periodic Table contains over 100 different elements.

They can be divided into two broad types: metals and nonmetals.

Most of the elements are metals and a small number of elements display properties of both types. These elements are called metalloids or semimetals.

Question 6

Properties of metals

Answer 6

Conduct heat and electricity.

Are malleable and ductile (can be hammered and pulled into different shapes).

Tend to be lustrous (shiny).

Have high density and usually have high melting points.

Form positive ions through electron loss.

Form basic oxides.

Question 7

Describing Alloys

Answer 7

Alloys are mixtures of metals, where the metals are mixed together but are not chemically combined.

They can be made from metals mixed with nonmetals such as carbon.

Alloys often have properties that can be very different to the metals they contain, for example they can have more strength, hardness or resistance to corrosion or extreme temperatures.

Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms.

This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal.

Brass is a common example of an alloy which contains 70% copper and 30% zinc.

Question 8

Properties of nonmetals

Answer 8

Do not conduct heat and electricity.

Are brittle and delicate when solid and easily break up.

Tend to be dull and nonreflective.

Have low density and low melting points (many are gases at room temperature).

Form negative ions through electron gain (except for hydrogen).

Form acidic oxides.

Question 10

Ionisation of metals and non-metals

Answer 10

Metals: all metals lose electrons to another atom and become positively charged ions.

Non-metals: all non-metals gain electrons from another atom to become negatively charged ions.

Question 11

Electrostatic attraction

Answer 11

The positive and negative charges are held together by the strong electrostatic forces of attraction between opposite charges.

This is what holds ionic compounds together.

Question 12

Describe the formation of ionic bonds between elements from Groups I and VII

Answer 12

Explanation

Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons.

A positive ion with the charge +1 is formed.

Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons.

One electron will be transferred from the outer shell of the Sodium atom to the outer shell of the Chlorine atom.

Chlorine atom will gain an electron to form a negative ion with charge -1.

Formula of Ionic Compound: NaCl

Question 13

Ionic Bonds between Metallic and Non-Metallic Elements

Answer 13

Explanation

Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons.

Example: Magnesium Oxide, MgO

A positive ion with the charge +2 is formed.

Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons.

Two electrons will be transferred from the outer shell of the Magnesium atom to the outer shell of the Oxygen atom.

Oxygen atom will gain two electrons to form a negative ion with charge -2.

Formula of ionic compound: MgO

Question 14

The Lattice Structure of Ionic Compounds

Answer 14

Lattice structure refers to the arrangement of the atoms of a substance in 3D space.

In these structures the atoms are arranged in an ordered and repeating fashion.

The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions.

Question 16

Ions

Answer 16

An ion is an electrically charged atom or a group of atoms formed by the loss or gain of electrons.

This loss or gain of electrons takes place to gain a full outer shell of electrons.

The electronic structure of an ion will be the same as that of a noble gas – such as helium, neon and argon.

Question 17

Covalent compounds

Answer 17

Covalent compounds are formed when electrons are shared between atoms.

Only non-metal elements participate in covalent bonding.

As in ionic bonding, each atom gains a full outer shell of electrons.

When two or more atoms are chemically bonded together, we describe them as ‘molecules’.

Question 18

draw covalent bonding hydrogen

Answer 18

Question 19

draw covalent bonding chlorine

Answer 19

Question 20

draw covalent bonding H2O

Answer 20

Question 21

draw covalent bonding CH4

Answer 21

Question 22

draw covalent bonding NH3

Answer 22

Question 23

draw covalent bonding HCl

Answer 23

Question 24

Ionic compounds:

Answer 24

Have high melting and boiling points so ionic compounds are usually solid at room temperature.

Not volatile so they don’t evaporate easily.

Usually water soluble as both ionic compounds and water are polar (see polarity in Glossary).

Conduct electricity in molten state or in solution as they have ions that can move and carry charge.

Question 25

Covalent compounds:

Answer 25

Have low melting and boiling points so covalent compounds are usually liquids or gases at room temperature.

Usually volatile which is why many covalent organic compounds have distinct aromas.

Usually not water soluble as covalent compounds tend to be nonpolar but can dissolve in organic solvents.

Cannot conduct electricity as all electrons are involved in bonding so there are no free electrons or ions to carry the charge.

Question 26

draw Electron Arrangement Nitrogen

Answer 26

Question 27

draw Electron Arrangement C2H4 (ethene):

Answer 27

Question 28

draw Electron Arrangement CH3OH (methanol):

Answer 28

Question 29

draw Electron Arrangement CO2

Answer 29

Question 30

Melting and Boiling Points of Ionic and Covalent Compounds

Answer 30

Ionic compounds have high melting and boiling points.

This is because the ions in the lattice structure are attracted to each other by strong electrostatic forces which hold them firmly in place.

Large amounts of energy are needed to overcome these forces so the m.p. and b.p. are high.

Covalent substances have very strong covalent bonds between the atoms, but much weaker intermolecular forces holding the molecules together.

When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds.

So less energy is needed to break the molecules apart so they have lower m.p. and b.p than ionic compounds.

Question 31

Describe the giant covalent structure of graphite and diamond

Answer 31

Diamond and graphite are allotropes of carbon which have giant covalent structures.

These classes of substance contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds forming a giant lattice structure.

Giant covalent structures have high melting and boiling points as they have many strong covalent bonds that need to be broken down.

Large amounts of heat energy are needed to overcome these forces and break down bonds.

Question 32

Relate their structures to their uses

graphite and diamond

Answer 32

Diamond

Each carbon atom bonds with four other carbons, forming a tetrahedron.

All the covalent bonds are identical and strong with no weak intermolecular forces.

Diamond thus:

Does not conduct electricity.

Has a very high melting point.

Is extremely hard and dense (3.51 g/cm3).

Diamond is used in jewellery and as cutting tools.

The cutting edges of discs used to cut bricks and concrete are tipped with diamonds.

Heavy-duty drill bits and tooling equipment are also diamond tipped.

Graphite

Each carbon atom is bonded to three others forming layers of hexagonal shaped forms, leaving one free electron per carbon atom.

These free electrons exist in between the layers and are free to move and carry charge, hence graphite can conduct electricity.

The covalent bonds within the layers are very strong but the layers are connected to each other by weak intermolecular forces only, hence the layers can slide over each other making graphite slippery and smooth.

Graphite thus:

Conducts electricity.

Has a very high melting point.

Is soft and slippery, less dense than diamond (2.25 g/cm3).

Graphite is used in pencils and as an industrial lubricant, in engines and in locks.

It is also used to make non-reactive electrodes for electrolysis.

Question 33

Describe the macromolecular structure of silicon(IV) oxide (silicon dioxide)

Answer 33

SiO2 is a macromolecular compound which occurs naturally as sand and quartz.

Each oxygen atom forms covalent bonds with 2 silicon atoms and each silicon atom in turn forms covalent bonds with 4 oxygen atoms.

A tetrahedron is formed with one silicon atom and four oxygen atoms, similar as in diamond.

Question 34

Diamond and Silicon(IV) Properties

Answer 34

SiO2 has lots of very strong covalent bonds and no intermolecular forces so it has similar properties as diamond.

It is very hard, has a very high boiling point, is insoluble in water and does not conduct electricity.

SiO2 is cheap since it is available naturally and is used to make sandpaper and to line the inside of furnaces.

Question 35

Electrical Conductivity and Malleability of Metals

Answer 35

Metal atoms are held together strongly by metallic bonding.

Within the metal lattice, the atoms lose their valence electrons and become positively charged.

The valence electrons no longer belong to any metal atom and are said to be delocalised.

They move freely between the positive metal ions like a sea of electrons.

Metallic bonds are strong and are a result of the attraction between the positive metal ions and the negatively charged delocalised electrons.

Question 36

The link between metallic bonding and the properties of metals:

Answer 36

Metals have high melting and boiling points:

There are many strong metallic bonds in giant metallic structures.

A lot of heat energy is needed to overcome forces and break these bonds.

Metals conduct electricity:

There are free electrons available to move and carry charge.

Electrons entering one end of the metal cause a delocalised electron to displace itself from the other end.

Hence electrons can flow so electricity is conducted.

Metals are malleable and ductile:

Layers of positive ions can slide over one another and take up different positions.

Metallic bonding is not disrupted as the valence electrons do not belong to any particular metal atom so the delocalised electrons will move with them.

Metallic bonds are thus not broken and as a result metals are strong but flexible.

They can be hammered and bent into different shapes without breaking.

Question 37

State the relative charges and approximate relative masses of protons, neutrons and electrons

Answer 37

Question 39

Define proton number (atomic number) as

Answer 39

the number of protons in the nucleus of an atom

Question 40

proton number is also the number of ..present in an atom and determines the

Answer 40

• electrons
• position of the element on the Periodic Table.

Question 41

Define nucleon number (mass number) as

Answer 41

the total number of protons and neutrons in the nucleus of an atom