gcse chemistry topic 1

what are the features of an element

In an element all of the atoms are the same. Every element has a symbol.

What are the features of a mixture

In a mixture there are different elements or compounds but they are not chemically combined. To separate a mixture you can use a physical technique.

Molecule

Molecules contain any elements chemically combined, even the same ones.

Compound

Compounds contain two or more different elements chemically combined in fixed proportions. Compounds have totally different properties to the elements that they're made from. To separate a compound back into its elements, you have to use a chemical reaction.

Filtration

Used to separate insoluble solids from a liquid.

Filtration Process

Mixture is filtered with filter paper.

Filtrate

The filtered liquid.

Simple Distillation

Used to separate a soluble solid from a liquid without losing the liquid.

Simple Distillation Process

First the liquid is evaporated by heating and turned into vapour. Next it rises through a glass tube into the condenser. The condenser is cooled by circulating tap water around it, which causes the vapour to condense back to a liquid.

Fractional Distillation

Used to separate a mixture of different liquids with different boiling points.

Fractionating prossess

The vapour rises up through the fractionating column which contains hundreds of glass beads.

Crystallisation

Used to separate a soluble solid from a liquid.

Crystallisation Process

Solution is left until the liquid evaporates and leaves behind crystals of solid.

Paper Chromatography

Used to separate substances based on their different solubilities.

Chromatography Process

A straight line is drawn with pencil on chromatography paper and dots are drawn with pens on the line. The paper is then lowered into a solvent. The colours forming a single spot.

Stationary phase and mobile phase in paper chromatography

stationary: paper, mobile: solvent

results of paper chromatography

The colours forming a single spot are pure mixtures and the ones forming more than one spot are a mixture of two different colours

Alpha scattering experiment

An experiment to test the structure of atoms by firing alpha particles at gold foil.

Plum pudding model

A model suggesting the atom is a ball of positive charge with negative electrons embedded in it.

Alpha particles

Tiny particles with a positive charge used in the alpha scattering experiment.

what greeks believed atoms were

tiny spheres that cannot be divided.

results of alpha scattering experiment

molecules passing through without changing direction: atoms are mostly empty space. molecules deflecting: the centre atom must have a positive charge. molecules bouncing back: the center of an atom (nucleus) must have a great deal of mass.

Nuclear model

• most of an atom is empty space

• in the centre is a positive nucleus which contains most of the mass of the atom

• around the edges are negative electrons

Electrons

Negative particles that orbit the nucleus at specific distances, energy levels, or shells.

Protons

Tiny positive particles in the nucleus that give it a positive charge.

Neutrons

Neutral particles discovered by James Chadwick that are also found in the nucleus.

Niels Bohr

Suggested that electrons orbit the nucleus at specific distances.

Structure of the nucleus

Contains protons and neutrons.

dobereiner's triads

elements with similar chemical properties

newlands' law of octaves

when elements are arranged in order of increasing atomic weight, every eighth element reacts in a similar way, however by always sticking to the exact order of atomic weight, sometimes elements were grouped together when they had different properties

dmitri mendeleev

developed the first modern periodic table

• he started by arranging the elements in increasing atomic weight, but he switched the order of specific elements so they could be grouped together

• he also left gaps in the periodic table for undiscovered elements and predicted the properties of these elements based on other elements in the same group

the modern periodic table

the elements are arranged in order of atomic number (number of protons)

radius of an atom

0.1 nanometres = 1×10⁻^10 m

radius of the nucleus

1×10⁻^14 m

isotopes

atoms of the same element with different numbers of neutrons

ions

atoms with an overall charge, ions have lost or gained electrons

mass number

protons + neutrons

atomic number

protons/electrons

first energy level

2

second energy level

8

third energy level

8

group 0 (the noble gases)

they are unreactive gases with a full outer energy level and boiling points lower than room temperature

group 1 metals (alkali metals)

all group 1 metals have 1 electron in their outer energy level and react rapidly with oxygen, chlorine and water

metals

when metals react they lose electrons to achieve a full outer energy level, forming positive ions

group 7 (the halogens)

group 7 elements are non-metals with 7 electrons in their outer energy level

covalent bond

a bond formed when two atoms share electrons

ionic compounds

formed when halogens react with metals

displacement reaction

a more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt

transition elements

hard and strong metals with high melting points and density, much less reactive to oxygen, chlorine and water than group 1 metals

why do metals become more reactive as we go down group 1

as we move down group 1, the radius of the atom increases and the outer electron is also repelled by the electrons in the internal energy(shielding) levels this causes the outer electrons to be less attracted to the positive nucleus and makes them easier to lose and makes them more reactive

why do halogens become less reactive as we move down group7

• as we move down group 7, the radius of the atom increases and there is greater shielding of the outer electron this causes the outer electrons to be less attracted to the positive nucleus and makes it harder to gain electrons and less reactive