Chemistry Topic 1

  • Created by: Kaja Koz
  • Created on: 21-11-18 17:13

4.1.1.1: Atoms, Elements & Compounds

All substances are made of atoms. An atom is the smallest part of an element that can exist. 

Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium. Elements are shown in the periodic table.

Compounds are formed from elements by chemical reactions. Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change. 

Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. Compounds can only be separated into elements by chemical reactions.

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4.1.1.1: Test

Name the first 20 elements in the periodic table.

Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Flourine, Neon, Soduim, Magnesium, Aluminium, Silicon, Phosphorus, Sulphur, Chlorine, Argon, Potassium, Calcium.

Name the elements in Group 1, 7 and 0.

1 - Li, Na, K, Rb, Cs, Fr.

7 - F, Cl, Br, I, At, Ts

0 - He, Ne, Ar, Kr, Xe, Rn, OG

How are compounds formed?

From chemical reactions.

What does a chemical reaction involve?

The formation of one or more new substances, and often a detectable energy change.

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4.1.1.2: Mixtures

A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged.

Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical reactions and no new substances are made.

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4.1.1.2: Test

What is Filtration? Provide examples. Water filters use two different techniques to remove dirt. Physical filtration means straining water to remove larger impurities. Another method of filtering, chemical filtration, involves passing water through an active material that removes impurities chemically as they pass through. E.g. Brewing coffee involves passing hot water through the ground coffee and a filter. What is crystallisation? Provide examples. Crystallization can also refer to the solid-liquid separation and purification technique in which mass transfer occurs from the liquid solution to a pure solid crystalline phase. An example of thiscrystallization process is the production of Glauber's salt, a crystalline form of sodium sulfate What is distillation? Provide examples. Distillation is a method of separating mixtures based on differences in their volatiles in a boiling liquid mixture. Distillationis a unit operation, or a physical separation process, and not a chemical reaction. Water is distilled to remove impurities, such as salt from seawater. What is chromatography? Provide examples. A process in which a chemical mixture carried by a liquid or gas is separated into components as a result of differential distribution of the solutes as they flow around or over a stationary liquid or solid phase.

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4.1.1.3: Development of the atom model

New experimental evidence may lead to a scientific model being changed or replaced. Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.

The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model. Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

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4.1.1.4: Relative electrical charges of subatomic

The relative electrical charges of the particles in atoms are:

Proton: +1

Neutron: 0

Electron: -1

In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge. The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

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4.1.1.5: Size and mass of atoms

Atoms are very small, having a radius of about 0.1 nm. Almost all of the mass of an atom is in the nucleus.The relative masses of protons, neutrons and electrons are:

Proton: 1

Neutron: 1

Electron: 1/2000th

The sum of the protons and neutrons in an atom is its mass number. Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

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4.1.1.7: Electronic Structure

The electrons in an atom occupy the lowest available energy levels (innermost available shells). The electronic structure of an atom can be represented by numbers or by a diagram. For example, the electronic structure of sodium is 2,8,1 (showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level).

(You should be able to represent the electronic structure of the first 20 elements of the periodic table in both forms; diagram & elec. structure.)

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4.1.2.1: The periodic table.

The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similar properties occur at regular intervals.

Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.

• explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number

• predict possible reactions and probable reactivity of elements from their positions in the periodic table.

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4.1.2.2: Development of the periodic table

Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.

Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct. 

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4.1.2.3: Metals & Non-metals

Elements that react to form positive ions are metals. 

Elements that do not form positive ions are non-metals. 

The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table. Non-metals are found towards the right and top of the periodic table.

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4.1.2.4: Group 0

The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons. The noble gases have eight electrons in their outer shell, except for helium, which has only two electrons.

The boiling points of the noble gases increase with increasing relative atomic mass (going down the group).

• explain how properties of the elements in Group 0 depend on the outer shell of electrons of the atoms

 predict properties from given trends down the group.

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4.1.2.5: Group 1

The elements in Group 1 of the periodic table are known as the alkali metals and have characteristic properties because of the single electron in their outer shell. 

In Group 1, the reactivity of the elements increases going down the group.

How the first three elements react with: oxygen, chlorine & water.

explain how properties of the elements in Group 1 depend on the outer shell of electrons of the atoms

• predict properties from given trends down the group.

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4.1.2.6: Group 7

The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell. The halogens are non-metals and consist of molecules made of pairs of atoms.

Describe the nature of the compounds formed when chlorine, bromine and iodine react with metals and non-metals.

In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.

In Group 7, the reactivity of the elements decreases going down the group.

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

• explain how properties of the elements in Group 7 depend on the outer shell of electrons of the atoms

• predict properties from given trends down the group. 

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4.1.3.1: Comparison with Group 1 elements

The transition elements are metals with similar properties which are different from those of the elements in Group 1.

Describe the difference compared with Group 1 in melting points, densities, strength, hardness and reactivity with oxygen, water and halogens.

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4.1.3.2: Typical properties

Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts

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