chemistry 1

atoms , elements and compounds

ATOM = smallest part of an element that can exist COMPOUNDS = 2 or more elements chemically bonded in fixed proportions , they can only be separated through chemical reactions and it is hard to separate them MIXTURE = 2 or more elements that are not chemically bonded , mixtures can be seperate using physical seperation techniques such as filtration , distilation etc.

physical separation techniques - mixtures

FILTRATION is used to separate an insoluble solid from a liquid. Filtration works because the filter paper has tiny holes, or pores, in it. These are large enough to let small molecules and dissolved ions through. CRYSTALLISATION is used to produce solid crystals from a solution. When the solution is warmed, some of the solvent evaporates leaving behind a more concentrated solution. SIMPLE DISTILLATION is used to separate a solvent from a solution. It is useful for producing water from salt solution. Simple distillation works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour evaporates from the solution. The gas moves away and is cooled and condensed . The remaining solution becomes more concentrated in solute as the amount of solvent in it decreases. FRACTIONAL DISTILLATION is used to separate different liquids from a mixture of liquids. It is useful for separating ethanol from a mixture of ethanol and water, and for separating different fractions from crude oil. Fractional distillation works because the different liquids have different boiling points. When the mixture is heated: - vapours rise through a column which is hot at the bottom, and cold at the top - vapours condense when they reach a part of the column that is below the temperature of their boiling point the liquid flows out of the column Different liquids can be obtained from different parts of the column, or by continuing to heat the mixture to increase the temperatures in the column. PAPER CHROMOTOGRAPHY is a technique used to separate and identify mixtures of soluble substances, such as inks and food colourings. It works based on the principle that different substances have different solubilities in a given solvent. The separation process relies on two key components: STATIONARY PHASE : The chromatography paper, which does not move. MOBILE PHASE: The solvent (e.g., water or ethanol) that moves up the paper, carrying the mixture with it. The paper is placed in a beaker with a shallow layer of solvent. The solvent level must be below the pencil line. As the solvent moves up the paper by capillary action, it carries the different substances in the mixture with it. Each substance in the mixture will travel at a different rate, depending on its solubility and attraction to the paper. The more soluble substances travel further up the paper.

the development of the model of the atom

BEFORE THE DISCOVERY OF ELECTRONS atoms were thought to be tiny sphere which couldn't be divided the PPM was discovered due to the discovery of electrons.The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it. the ALPHA PARTICLE SCATTERING experiment led to the conclusion that the mass of the atom was concentrated at the center leading to discovery of the nucleus and so the nuclear model replaced the PPM NEIL BOHR adapted the nuclear model and suggested that electrons orbit the nucleus in shells PROTONS were later discovered as smaller particles of the positive charge. JAMES CHADWICK provided the evidence to show the existence of neutrons within the nucleus

relative charge of subatomic particles

In an atom, the NUMBER OF ELECTRONS ARE equal to the number of protons in the nucleus. Atoms have no overall electrical charge. ATOMIC NUMBER The number of protons in an atom of an element . All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

size and mass of atoms

Almost all of the mass of an atom is in the nucleus. MASS NUMBER is the sum of protons and neutrons ISOTOPES same element with different number of neutrons

electronic configuration

2 , 8 ,8

periodic table

elements are ordered in the put in order of atomic (proton) number s 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.

development of the periodic table

BEFORE THE DICOVERY 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.

metals and non metals

POSITIVE IONS - metals NEGATIVE IONS - non metals METALS ARE ON THE LEFT because the elements there have fewer valence electrons and prefer to lose them to achieve a stable configuration. NON METALS ARE ON THE RIGHT because their atoms are close to having full outer shells and prefer to gain or share electrons rather than lose them. METALS are shiny , solid , high mp bp and density , good conductors , malleable , ductile , sonorous NON METALS are dull , gas , low mp bp and density , poor conductors , brittle

group 0

the noble gases. They are unreactive and do not easily form ions because their atoms have stable arrangements of electrons. The noble gases have full outer shell, The boiling points of the noble gases increase with increasing relative atomic mass (going down the group).

group 1

alkali metals single electron in outer shell so loses electrons (+ions) Group 1, the reactivity of the elements increases going down the group. very reactive OXYGEN Lithium and oxygen produce lithium oxide → red flame → white solid forms Sodium and oxygen produce sodium oxide (and some sodium peroxide) → yellow/orange flame → white solid forms Potassium and oxygen produce potassium peroxide → lilac flame → white solid forms CHLORINE Lithium and chlorine produce lithium chloride → bright flame → white solid forms Sodium and chlorine produce sodium chloride → yellow flame → white solid forms Potassium and chlorine produce potassium chloride → lilac flame → white solid forms WATER Lithium and chlorine produce lithium chloride → bright flame → white solid forms Sodium and chlorine produce sodium chloride → yellow flame → white solid forms Potassium and chlorine produce potassium chloride → lilac flame → white solid forms Alkali metals: soft, low melting, very reactive metals. Transition metals: hard, dense, high melting metals; less reactive, form colored compounds, and often used as catalysts.

group 7

halogens and have similar reactions because they all have seven electrons in their outer shell. Halogens are non-metals, and their molecules come in pairs of atoms. Cl₂ 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 replace a less reactive halogen from its salt dissolved in water. Chlorine can displace bromine from bromide solution. When chlorine, bromine and iodine react with metals, they form ionic compounds called metal halides. These are solid crystals with high melting points, made when the metal loses electrons and the halogen gains electrons. When they react with non-metals, they form covalent compounds where atoms share electrons. These are usually molecules with low melting and boiling points, often gases or liquids at room temperature.