GCSE CHEMISTRY - ATOMIC STRUCTURE AND IONS

What is an element?

An element is a substance of only one type of atom

What is an atom?

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

Elements can be classified into two groups based on their properties; what are these?
groups?

Metals and non-metals

Elements may combine through chemical reactions to form new products; what are
these new substances called?

Compounds

What is a compound?

Two or more elements combined chemically in fixed proportions which can be represented by formulae

Do compounds have the same properties as their constituent elements?

No, they have different properties.

What is a mixture? Does it have the same chemical properties as its constituent materials?

A mixture consists of two or more elements or compounds not chemically combined together; it does have the same chemical properties

What are the methods through which mixtures can be separated (there are five)? Do these involve chemical reactions?

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

Describe and explain simple distillation.

Simple distillation is used to separate liquid from a solution – the liquid boils off and condenses in the condenser. The thermometer will read the boiling point of the pure liquid. Contrary to evaporation, we get to keep the liquid.

Describe and explain crystallisation/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 is similar, but we only remove some of the solvent by evaporation to form a saturated solution (the one where no more solid can be dissolved). Then, we cool down the solution. As we do it, the solid starts to crystallise, as it becomes less soluble at lower temperatures. The crystals can be collected and separated from the solvent via filtration.

Describe and explain fractional distillation

Fractional distillation is a technique for separation of a mixture of liquids. It works when liquids have different boiling points.
The apparatus is similar to the one of simple distillation apparatus, with the additional fractionating column placed on top of the heated flask.
The fractionating column contains glass beads. It helps to separate the compounds.
In industry, mixtures are repeatedly condensed and vapourised. The column is hot at the bottom and cold at the top. The liquids will condense at different heights of the column.

Describe and explain filtration

Filtration is used to separate an insoluble solid is 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. Apparatus: filter paper + funnel.

Describe the plum-pudding model

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

Describe the Bohr/nuclear model and how it came about

The nuclear model suggests that electrons orbit the nucleus at specific distances (shells) – it came about from the alpha scattering experiments

What did the work of James Chadwick provide evidence for?

The existence of neutrons in the nucleus

Describe the structure of an atom

The atom has a small central nucleus (made up of protons and neutrons) around which there are electrons

State the relative masses and relative charges of the proton, neutron and electron

Masses: 1, 1, very small
Charges: 1, 0 , -1 (respectively)

Explain why atoms are electrically neutral.

They have the same number of electrons and protons

What is the radius of an atom?

0.1 nm

What is the radius of a nucleus and
what is it compared to that of the
atom?

1 x 10-14 m and 1/10000

What name is given to the number of protons in the nucleus?

Atomic number

Atoms of the same element have the same number of which particle in the nucleus?

Protons

What is an isotope? Do isotopes of a certain element have the same
chemical properties?

Atoms of the same element (same proton number) that have a different number of neutrons.
They have the same chemical properties as they have the same
electronic structure

What are ions?

Ions are charged particles. They are formed when atoms lose electrons (positive ions) or gain (negative ions) electrons.

What is formed when a metal reacts with a non-metal?

An ionic compound (made of positive and negative ions).

What is formed when a non-metal reacts with a non-metal?

A molecular compound containing covalently bonded atoms.

Early periodic tables were incomplete and elements were placed in inappropriate groups if what was to be followed?

The strict order of atomic weights

Mendeleev overcame some problems with the table by doing what? He also changed the order of some elements based on what?

Leaving gaps; atomic weights

Elements in Group 1 are known as…?

The alkali metals

State three characteristics of the Alkali Metals

All have one electron in their outer shell; have low density; are stored
under oil (to prevent reactions with oxygen or water); are soft (can be cut with knife).

How do Group 1 elements react with non-metals? Why are these reactions similar for the different Group 1 elements?

They form ionic compounds which are soluble white solids which form colourless solutions – they all have one electron in their outer shell.

How do Group 1 elements react with water?

They release hydrogen and form hydroxides which dissolve to form
alkaline solutions; react vigorously with water fizzing and moving around on the surface of the water.

How does the reactivity change moving down Group 1? Why?

Reactivity increases as the atoms get larger and the distance between the nucleus and the outer electrons increases and thus attraction from the nucleus decreases, allowing them to more easily lose electrons.

State five characteristics of Group 7

- 7 electrons in outer shell
- Coloured vapours
- Diatomic molecules
- Form ionic salts with metals
- Form molecular compounds with non-metals

State Group 7 elements and their states of matter.

- Fluorine, F. F2 is a pale yellow gas.
- Chlorine, Cl. Cl2 is a pale green gas.
- Bromine, Br. Br2 is dark brown liquid
- Iodine, I. I2 is a grey solid.

State three changes that occur in Group 7 as one moves down the group

- Higher relative molecular mass
- Higher melting and boiling point
- Less reactive – less easily gain electrons

A more reactive halogen displaces a less
reactive one from an aqueous solution of its salt; explain the trend in reactivity of halogens in these reactions

Reactivity decreases down the group. As we go down the group, the atoms get larger, so an incoming electron will be less tightly held by the attractive forces from the nucleus. That’s why Cl2 displaces Br– and I–.

Compare Group 1 metals and transition metals

Group 1 metals and transition metals are heat and electricity conductors.
They are shiny when polished and form ionic compounds with non metals.
Transition metals have higher densities and higher melting points than Group 1 metals. They are less reactive and harder than Group 1 metals.

State three common characteristics of transition metals

- Ions with different charges
- Coloured compounds
- Catalytic properties

What is a catalyst?

A catalyst is a chemical substance that increases the rate of a
chemical reaction.
It is not used up over the course of the reaction

Describe the properties of noble gases. Discuss the trend in boiling point down the group.

Non-metals, gases, low boiling points, unreactive (full outer shell; they
don’t easily accept or lose electrons).
The boiling point increases down the group, as the atoms get heavier.