Category Chemistry

The ability of an element to take part in a chemical reaction is called reactivity. Metals vary in their reactivity. Some alkali metals, such as sodium and potassium, are so reactive that they have to be stored in oil. They would react strongly with the oxygen in air or water. The least reactive metal is gold.

The metal reactivity series is a commonly taught concept in chemistry, placing the metals, as its name suggests, in order of reactivity from most reactive to least reactive. It’s also a useful tool in predicting the products of simple displacement reactions involving two different metals, as well as providing an insight into why different metals are extracted from their ores in different manners. This graphic places a selection of common metals into order of reactivity, as well as showing their reactions with air, water and steam.

Metals have a range of relativities – to illustrate this, you have to look no further than the classic alkali metals in water demonstration commonly used in chemistry classes. In this demonstration, small pieces of three different metals from group 1 of the periodic table are dropped into a large bowl of water. Lithium fizzes gently, sodium fizzes vigorously, and potassium’s reaction is so energetic it bursts into a lilac flame as it zips across the water’s surface. Cesium, the most reactive metal in the periodic table, reacts extremely violently – hence why it can’t be demonstrated in a classroom! This can be compared to other common metals, such as iron and copper, which produce no reaction when dropped into water.

The reactivity series offers a ranking of the metals in order of their reactivity. Group 1 metals, the most reactive metals in the periodic table, head up the rankings. They’re closely followed by the marginally less reactive group two metals. The metals designated as the transition metals in the periodic table are much less reactive, and metals such as gold and platinum prop up the bottom of the series, exhibiting little in the way of chemical reaction with any everyday reagents.

Tin cans are made of steel, not tin, but they do have a coating of tin to stop food inside corroding the steel. Drinks cans are often made of aluminium.

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There are over 80 different metals. They tend to conduct heat and electricity well, and many of them can be shaped by pulling, beating, or melting and pouring into a mould. Metals with similar properties are often grouped together, although a metal may sometimes appear in more than one group, as these pages show. Unlike most non-metals, metals are shiny when cut. Metals have played an enormous part in the history of human activity, which is why some periods, such as the Iron Age and the Bronze Age, are known by the names of metals. Some people say that our present period should be called the Silicon Age. But silicon is what is known as a semi-metal, having some but not all of the properties of metals.

Hard, shiny, and tough—metals are the macho poster boys of the material world. Learning how to extract these substances from the Earth and turn them into all kinds of useful materials was one of the most important developments in human civilization, spawning tools, jewelry, engines, machines, and giant static constructions like briges and skyscrapers. Having said that, “metal” is an almost impossibly broad term that takes in everything from lead (a super-heavy metal) and aluminum (a super-light one) to mercury (a metal that’s normally a liquid) and sodium (a metal soft enough to cut like cheese that, fused with chlorine, you can sprinkle on your food—as salt!).

When we talk about nonmetals, it ought to mean everything else—although things are a bit more complex than that. Sometimes you’ll hear people refer to semi–metals or metalloids, which are elements whose physical properties (whether they’re hard and soft, how they carry electricity and heat) and chemical properties (how they behave when they meet other elements in chemical reactions) are somewhere in between those of metals and nonmetals. Semi-metals include such elements as silicon and germanium—semiconductors (materials that conduct electricity only under special conditions) used to make integrated circuits in computer chips and solar cells. Other semi-metals include arsenic, boron, and antimony (all of which have been used in the preparation—”doping”— of semiconductors).

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Ionic bonds happen when one atom gains one or more electrons from another atom. The electrons in an atom have a negative charge and are equal in number to the positively charged protons in the nucleus. When an atom gains or loses electrons, the balance of charges is broken, so the atom becomes either positively or negatively charged. It is called an ion. An atom that has gained electrons has a negative charge and is called an anion. One that has lost electrons has a positive charge and is called a cation. As opposite charges attract each other, the two atoms that have gained and lost electrons are pulled together into a bond.

When two atoms combine, they form a compound or molecule in a chemical bond, which links them together. This bond can be ionic or covalent. In an ionic bond, one atom donates an electron to the other to stabilize it. In a covalent bond, the atoms are shared by the electrons.

In the chemistry world, an ionic bond is made from atoms with different electronegativity values. It is considered a polar bond if the attraction is between two oppositely charged ions. This works much in the same way as magnets that attract each other. If two atoms have different electronegativity values, they will make an ionic bond.

The combination of sodium (Na) and chloride (Cl) forms NaCl or common table salt, and this is an example of an ionic bond. Sulfuric acid is also an ionic bond, combining hydrogen and sulfur oxide, and it is written as H2SO4.

Ionic bonds take more energy to break than covalent bonds, so ionic bonds are stronger. The amount of energy needed to break a bond is known as bond dissociation energy, which is basically the force it takes to break bonds of any type.

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It is useful to think of an atom as having electrons circling in layers around its nucleus. These layers are known as “shells”. Each layer can only have a certain number of electrons before a new shell must be started. Atoms that have as many electrons as possible in the outer shell (or some other particular numbers) are said to be stable. They do not easily form bonds with other atoms. Atoms that are not stable try to become so by sharing electrons with, or borrowing electrons from, or giving electrons to, another atom. The number of electrons that an atom needs to give or gain to achieve a stable outer shell is called its valency.

The combining capacity of an atom is called its valency. Actually it can be defined as the number of electrons that an atom may lose (or) gain during a chemical reaction (or) the number of electrons shared. The numbers of electrons in outermost shell (or) valency shell of an atom are called valency electrons. 

The valency of an element is the number of atoms lost or gained by the atom of an element. Valence electrons are the number of electrons present in the outermost shell of an atom valency of an element depends on the valence electrons valency of atoms having 1, 2, 3 valence electrons = number of valence electrons valency of atoms having 5, 6, 7 valence electrons

= 8 – number of valence electrons

Covalent bond the interatomic linkage that results from the sharing of an electron pair between two atoms. The binding arises from the electrostatic attraction of their nuclei for the same electrons. A covalent bond forms when the bonded atoms have a lower total energy than that of widely separated atoms.

Molecules that have covalent linkages include the inorganic substances hydrogen, nitrogen, chlorine, water, and ammonia (H2, N2, Cl2, H2O, NH3) together with all organic compounds. In structural representations of molecules, covalent bonds are indicated by solid lines connecting pairs of atoms; e.g.

A single line indicates a bond between two atoms (i.e., involving one electron pair), double lines (=) indicate a double bond between two atoms (i.e., involving two electron pairs), and triple lines (?) represent a triple bond, as found, for example, in carbon monoxide (C?O). Single bonds consist of one sigma (?) bond, double bonds have one ? and one pi (?) bond, and triple bonds have one ? and two ? bonds.

The idea that two electrons can be shared between two atoms and serve as the link between them was first introduced in 1916 by the American chemist G.N. Lewis, who described the formation of such bonds as resulting from the tendencies of certain atoms to combine with one another in order for both to have the electronic structure of a corresponding noble-gas atom.

Covalent bonds are directional, meaning that atoms so bonded prefer specific orientations relative to one another; this in turn gives molecules definite shapes, as in the angular (bent) structure of the H2O molecule. Covalent bonds between identical atoms (as in H2) are nonpolar—i.e., electrically uniform—while those between unlike atoms are polar—i.e., one atom is slightly negatively charged and the other is slightly positively charged. This partial ionic character of covalent bonds increases with the difference in the electronegativity’s of the two atoms. 

A compound is a substance that is created when two or more elements are bonded by a chemical reaction. It is difficult to split a compound back into its original elements. Compounds do not necessarily take on the characteristics of the elements that form them. For example, sodium is a metal and chlorine is a gas. Together they form a compound called sodium chloride, which is not like either of them. In fact, sodium chloride is the chemical name for the salt that we put on our food.

In chemistry, a compound is a substance that results from a combination of two or more different chemical elements, in such a way that the atom s of the different elements is held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule.

A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).

Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure.

The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine.

1: Pure water is a compound made from two elements – hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always. Each molecule of water contains two hydrogen atoms bonded to a single oxygen atom.

2: Pure table salt is a compound made from two elements – sodium and chlorine. The ratio of sodium ions to chloride ions in sodium chloride is always.

3: Pure methane is a compound made from two elements – carbon and hydrogen. The ratio of hydrogen to carbon in methane is always.

4: Pure glucose is a compound made from three elements – carbon, hydrogen, and oxygen. The ratio of hydrogen to carbon and oxygen in glucose is always.