# 3.1-3.7: Atoms, Elements and Compounds

Atoms are the smallest particles of matter, that we cannot break down further by chemical means.

Elements Contain only one kind of atom, example sodium.

## Defining Proton Number

• The atomic number (or proton number) is the number of protons in the nucleus of an atom

• The symbol for atomic number is Z

• It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table.

## Defining Nucleon Number

• Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom

• The symbol for nucleon number is A

how to find neutrons number: The nucleon number minus the proton number gives you the number of neutrons of an atom

• protons + neutrons are called nucleons.

• The atomic number and mass number for every element is on the Periodic Table

## Electrons

• The symbol for an electron is e-, but other symbols are used such as x and ⦁ in bonding diagrams to make it easier to see where electrons come from.

• These subatomic particles move very fast around the nucleus.

• They move in orbital paths called shells

• The mass of the electron is negligible, hence the mass of an atom is concentrated in the nucleus where the neutrons and protons are found.

## Protons neutrons and elements

• Elements are made of tiny particles of matter called atoms

• Each atom is made of subatomic particles called protonsneutrons and electrons

• Their size is so tiny that we can't really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used

• One relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom.

• All other elements are measured relative to the mass of a carbon-12 atom, so relative atomic mass has no units (although sometimes you may come across a unit Da or u which stands for a Dalton and it means the same thing).

• Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

• The relative mass and charge of the subatomic particles are shown below:

## Types of Substances and there Properties

Compound

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

• There is an unlimited number of compounds

• Compounds cannot be separated into their elements by physical means.

example: Copper sulfate and calcium carbonate Mixture

• 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

• Eg: sand and water, oil and water, sulphur powder and iron filings

## Electron Shells

We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronicstructure

## Electron shell diagrams

• Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it

• The further away from the nucleus then the more energy a shell has

• Electrons occupy the shell closest to the nucleus which can hold only 2 electrons

• When a shell becomes full electrons then fill the next shell

• The second shell can hold 8 electrons and the third shell can also hold 8 electrons and the electrons organise themselves in pairs in these shells

• The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons

• Elements in the same group have the same number of outer shell electrons

Period: The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 shells of electrons

Group: The final notation, which is 7 in the example, shows that a chlorine atom has 7 outer electrons

• Noble gases= All of the noble gases are unreactive as they have full outer shells and are thus very stable

• located 8/0

• Below is the electronic Configuration of the first 20 elements

## The Bases of the Periodic table

• Elements are arranged on the Periodic Table in order of increasing atomic number where each element has one proton more than the element preceding it

• Hydrogen has 1 proton, helium has 2 protons, lithium has 3, etc.

• The table is arranged in vertical columns called Groupsnumbered I - VIII and in rows called Periods

• Elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemicals properties

# Ions and ionic bonds

ions

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

• An atom will lose or gain electrons to become more stable

• The loss or gain of electrons takes place to gain a full outershell of electrons which is a more stable arrangement of electrons

Ionisation of metals and non-metals

• Metals: all metals can lose electrons to other atoms to become positively charged ions

• Non-metals: all non-metals can gain electrons from other atoms to become negatively charged ions

## Electrostatic attraction

• Ionic compounds are formed when metal atoms react with non-metal atoms

• Metal atoms lose their outer electrons which the non-metal atoms gain to form positive and negative ions

• The positive and negative ions are held together by strong electrostatic forces of attraction between oppositecharges

• This force of attraction is known as an ionic bond and they hold ionic compounds together

This happens because formation of ionic bonds, Group 1 to VII

• Sodium is a Group I metal so will lose one outer electron to another atom to gain a full outer shell of electrons

• A positive sodium ion with the charge 1+ is formed

• Chlorine is a Group VII 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

• A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of 1-

• The oppositely charged ions are held together by strong electrostatic forces of attraction

• The ionic compound has no overall charge

Formula of ionic compound:    NaCl

Ionic bonds happen between metals and non metals

Lattice structures: In lattice structures, the atoms are arranged in an orderedand repeating fashion

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

# Isotopes

Isotopes are atoms of the same element that contain the same number of protonsand electrons but a different number of neutrons.

There are two types of Isotopes:

• Radioactive isotopes (radioisotopes) are unstable due to the imbalance of neutrons and protons, which causes the nucleus to decay over time through nuclear fission and emit radiation

• Examples of radioisotopes include tritium and carbon-14

• Radioactive have a lot of use in the medical field

Example= Cancer treatments, Medical tracers, and Medical instruments and materials are routinely sterilized by exposure to radiation, which kills any bacteria present

• the industrial uses=

Example= Radioactive tracers are deployed in industry to detect leaks in gas or oil pipes, Geiger Counter, and Radiocarbon dating.

• Non-radioactive isotopes are stable atoms which really only

• isotopes share properties, why? how?

Isotopes of the same element display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells

• The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

• The difference in mass affects the physical properties, such as density, boiling point and melting point

# Metals and Non Metals

There are over 100 elements in the Periodic Table

some of them can be classified as 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

## Properties of metals

• Conduct heat and electricity

• Are malleable (can be hammered and made into different shapes) and ductile (can be drawn into wires)

• Tend to be lustrous (shiny)

• Have high density and usually have high melting points

• Form positive ions through electron loss

• Form basic oxides

## Properties of nonmetal elements

• Do not conduct heat and electricity

• Are brittle 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

# Alloys

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

• They can also 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 greater strengthhardness 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

• Metals have high melting and boiling points

• There are many strong metallic bonds in giant metallic structures between the positive metal ion and delocalised electrons

• A lot of heat energy is needed to break these bonds

• Metals conduct electricity

• There are free electrons available to move through the structure 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 outer 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 or drawn into wires without breaking