Chemistry - Unit 2: Atoms, elements and compounds

what is an element?

a substance made up of only one type of atom

what is a compound?

a substance made from 2 or more elements that have reacted with each other and formed chemical bonds between atoms.

what is a mixture?

a substance made up of 2 or more elements or compounds, but aren’t chemically bonded together

describe the structure of an atom.

a central nucleus containing neutrons and protons surrounded by electrons in shells

what is the relative charge of protons?

+1

what is the relative charge of neutrons?

0

what is the relative charge of electrons?

-1

what is the relative mass of protons?

1

what is the relative mass of neutrons?

1

what is the relative mass of electrons?

1/1840

what is atomic/proton number?

the number of protons in the nucleus of an atom

what is mass/nucleon number?

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

what do group VIII noble gases have?

a full outer electron shell

what do the number of outer shell electrons equal to?

the group number in group I to VII

what do the number of occupied shells equate to?

the period number

what are isotopes?

different atoms of the same element that have the same number of protons but different number of neutrons

why do isotopes of the same element have the same chemical properties?

they have the same number of electrons and thus, the same electronic configuration

how are cations (positive ions) formed?

the loss of an electron from a metal results in a cation.

how are anions (negative ions) formed?

the gain of an electron to a non-metal results in an anion.

how are ionic compounds formed?

an ionic compound has a giant lattice structure which means the cation and anion are arranged alternatively. the ionic lattice is held together by strong electrostatic forces of attraction between the cations and anions.

how are ionic bonds formed?

ionic bonds are formed between a cation and anion, the bond is a strong electrostatic attraction between the oppositely charged ions.

what are the properties of ionic compounds?

• high melting and boiling points

• good electrical conductivity when aqueous or molten and poor when solid

why do ionic compounds have high melting and boiling points?

ionic compounds have a giant ionic lattice structure with many ionic bonds. these bonds have very strong electrostatic forces of attraction so there needs to be a lot of energy to overcome them.

why do ionic compounds have good electric conductivity when in molten/aqueous state?

in molten/aqueous state, the ions are no longer fixed in position and are free to move to carry charge.

why do ionic compounds have poor electrical conductivity when in solid state/

in solid, the ions are fixed in position and are unable to move to carry charge, so solid ionic compounds cannot conduct electricity.

how are covalent bonds formed?

when a pair of electrons is shared between 2 atoms leading to noble gas electronic configuration

what are the characteristics of simple molecular compounds?

• low melting and boiling points

• poor electrical conductivity

why do simple molecular compounds have low boiling and melting point?

simple molecular compounds have low melting and boiling point as the intermolecular forces between the molecules are weak so little energy is needed to overcome them.

why do simple molecular bonds have poor electrical conductivity?

due to the absence of ions (charged particles) to carry any charge.

what are example of giant covalent structures?

• graphite

• diamond

• silicone(IV) oxide, SiO²

describe the structure and properties of a diamond

each carbon atom is covalently bonded to 4 other carbon atoms and arranged in a tetrahedral 3D shape.

• diamonds are very hard due to many strong covalent bonds

• can’t conduct electricity as there are no delocalised electrons

describe the structure and properties of graphite.

in graphite, each carbon atom is covalently bonded to 3 other carbons and its structure consists of layers of hexagonal rings with no covalent bonds between the layers

• graphite is very soft, since the layers can slide over each other

• graphite can conduct electricity since each carbon atom has a delocalised electron

explain the uses of diamonds, using its structure and bonding

diamond can be used in cutting tools due to its hardness through its tetrahedral, rigid arrangement of atoms bonded with many strong covalent bonds

explain the uses of graphite, using its structure and bonding

• lubricant - layers slide over each other due to weak intermolecular forces between layers

• conductor - can conduct electricity due to delocalised electrons

explain the uses of diamond, using its structure and bonding?

diamonds can be used in cutting tools due to its hardness through its tetrahedral, rigid arrangement of atoms bonded with many strong covalent bonds

describe the structure of silicon(IV) oxide, SiO²

• each silicon atom is covalently bonded to 4 oxygen atoms

• each oxygen atom is covalently bonded to 2 silicon atoms

• thus, the formula is SiO² (as Si²O⁴ simplified is SiO²)

describe the similarities between diamond and silicon(IV) oxide

• in silicon (IV) oxide, each silicon atom is covalently bonded to 4 oxygen atoms and in diamond, each carbon atom is covalently bonded to 4 carbon atoms

• high melting and boiling points - both have strong covalent bonds

• atoms bonded in a tetrahedral arrangement - both structures are very hard and rigid