Atomic Structure and Periodic Table

Atom

An atom is the smallest part of an element that can exist.

Element

An element is a substance of only one type of atom.

Periodic Table

Table listing all existing elements; there are approximately 100.

What are the two types of elements?

Metals and non-metals.

Compound

Two or more elements chemically bonded.

Properties of Compounds after bonding

They have different properties.

Mixture

Two or more elements or compounds not chemically bonded; it does have the same chemical properties.

Separation Methods for Mixtures

Filtration, crystallisation, simple distillation, fractional distillation and chromatography; they do not involve chemical reactions.

Simple Distillation

Simple distillation is used to separate liquid from a solution.

• The liquid boils off and condenses in the condenser

Evaporation

Evaporation is a technique for separation of a solid dissolved in a solvent from a solvent (e.g. salt from H2O).

• The solution is heated until all the solvent evaporates; the solids stays in the vessel.

Crystallisation

Crystallisation is the separation of a soluble solid from a solution.

1. Gently heat solution in an evaporating basin with a bunsen burner/water bath

2. Solution evaporates

3. Solid crystals left behind

Fractional Distillation

Fractional distillation is a technique for separation of a liquid in a mixture of liquids. It works when liquids have different boiling points.

1. Heat mixture of liquids in a flask attavched to fractionating column

2. Liquid with lowest boiling point evaporates first

3. It condenses at the top and is collected

4. After the first liquid id collected, temperature of the column can be increased

5. Liquid with next highest boiling point to be collected.

Industrial Fractional Distillation

Mixtures are repeatedly condensed and vapourised.

Filtration

Filtration is used to separate an insoluble solid suspended in a liquid.

• The insoluble solid (called a residue) gets caught in the filter paper because the particles are too big to fit through the holes in the paper.

• The filtrate is the substance that comes through the filter paper.

Chromatography

Chromatography is used to separate a mixture of substances dissolved in a solvent.

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Plum-Pudding Model - J.J. Thompson

The atom is a ball of positive charge with negative electrons embedded in it.

Nuclear Model - Bohr

The nuclear model suggests that electrons orbit the nucleus at specific distances (shells)

• Came about from the alpha scattering experiments.

Protons

Sub-atomic particle with a positive charge; situated in the nucleus.

Neutrons

Sub-atomic particle with no charge; situated in the nucleus

• James Chadwick provided evidence for the existence of neutrons in the nucleus

Relative Masses and Charges

(Protons : Neutrons : Electrons)

Masses: 1, 1, very small;

Charges: 1, 0, -1

Neutrality of Atoms

Atoms are electrically neutral because they have the same number of electrons and protons.

Radius of an Atom

0.1 nm.

Atomic Number

The number of protons or electrons in an atom.

Protons in Elements

Atoms of the same element have the same number of protons.

Where is the Majority Mass of an Atom Located?

In the nucleus.

Mass Number

Total number of protons and neutrons in the nucleus.

Calculating number of neutrons

Mass number - Atomic number

Isotope

Atoms of the same element (same proton number) that have a different number of neutrons.

Chemical properties of isotopes

They have the same chemical properties as they have the same electronic structure.

Relative atomic mass

The average mass value which takes the mass and abundance of isotopes of an element into account, on a scale where the mass of carbon (C) is 12.

Max number of electrons per shell - electron configuration

1st shell: 2

2nd shell: 8

3rd shell: 8

4th shell: 2

Ions

Charged particles formed when atoms lose electrons (positive ions) or gain electrons (negative ions).

Example of a positive ion

• Sodium

Sodium atom: 11 electrons (atomic number) - Configuration: 2, 8, 1

Sodium ion: New configuration: 2, 8

→ Loses one electron from 3rd shell to have a full 2nd shell: Positive ion

Properties of metals

• High boiling/melting point

• Conduct heat and electricity

• Shiny appearance

• malleable

• high density

• form basic oxides.

Properties of non-metals

• Low boiling/melting point

• do not conduct heat and electricity (with exception of graphite)

• dull appearance

• brittle

• low density

• form acidic oxides.

Ionic bonding

The electrostatic attraction between oppositely charged ions.

Covalent bonding

Shared pair of electrons and the sea of delocalised electrons.

Metallic bonding

The electrostatic attraction between positively charged ions and the sea of delocalised electrons.

Definition of solute

A substance that is dissolved in a solvent.

Definition of solvent

A substance that dissolves a solute to form a solution.

Definition of solution

A mixture formed by dissolving a solute in a solvent.

Definition of soluble

A substance that can be dissolved in a solvent, e.g. salt in water.

Definition of insoluble

A substance that won't dissolve in a particular solvent.

Name of columns of the periodic table + Trend

Groups.

→ Groups show the number of electrons in outer shell

Name for rows of the periodic table + Trend

Periods.

→ Periods show the number of shells in an element

Elements in the same group: Properties

They may have similar chemical properties as they have the same number of outer shell electrons.

Differences between elements of the same period

Differences in energy levels (shells).

Name for Group 0 elements

Noble gases.

Change in shell number going down a group

Number of shells increases as one moves down a group.

Early periodic tables

Early periodic tables were incomplete and elements were placed in inappropriate groups if the strict order of atomic weights was to be followed.

Isotopes in the making of the Periodic table

Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.

Mendeleev's improvements

Mendeleev overcame some problems with the table by:

• leaving gaps

• changing the order of some elements based on atomic weights.

What type of elements form positive ions?

Metals

What type of elements form negative ions?

Non-metals.

Name for Group 1 elements

Alkali metals.

Characteristics of Alkali Metals

• All have one electron in their outer shell (group 1)

• have low density

• are stored under oil (to prevent reactions with oxygen or water)

• are soft (can be cut with a knife).

Group 1 elements and water

Group 1 elements react with water to release hydrogen and form hydroxides which dissolve to form alkaline solutions

• They react vigorously with water, fizzing and moving around on the surface.

Reactivity trends in Group 1

Reactivity increases going down the group.

→ As the atoms get larger and the distance and attraction between the nucleus and the outer electrons increases: allowing them to more easily lose electrons.

Characteristics of Group 7 (Halogens)

• 7 electrons in outer shell

• coloured vapours

• diatomic molecules (exist as pairs: e.g. oxygen → O2)

• form ionic salts with metals

• form molecular compounds with non-metals.

Group 7 Elements and States

Fluorine (F): pale yellow gas

Chlorine (Cl): pale green gas

Bromine (Br): dark brown liquid

Iodine (I): grey solid.

Changes in Group 7 Down the Group - Trends

• Higher relative molecular mass

• higher melting and boiling point

• less reactive - less easily gain electrons.

Reactivity Trend of Halogens

Reactivity decreases down the group.

→ As the atoms get larger, incoming electrons less tightly held by the nucleus.

Comparison of Group 1 and Transition Metals

Group 1 metals and transition metals both:

• heat and electricity conductors

• shiny when polished

• form ionic compounds with non-metals.

Transition metals have:

• higher densities and higher melting points

• are less reactive

• harder.

Common Characteristics of Transition Metals

• Ions with different charges

• coloured compounds

• catalytic properties.

Definition of a Catalyst

Chemical substance that increases the rate of a chemical reaction without being used up.

Flame Colors of Lithium, Sodium, and Potassium

Li: Crimson-red

Na: yellow-orange

K: lilac

Properties of Noble Gases (Group 0)

• Non-metals

• gases

• low boiling points

• unreactive due to full outer shell; they don't easily accept or lose electrons.

Boiling Point Trend in Noble Gases

The boiling point increases down the group as the atoms get heavier.