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When the object of the following phrasal verbs is a pronoun, the two parts of the phrasal verb must be separated: You have to do it over. Verb Meaning Example blow up explode The terrorists tried to blow up the railroad station. I have a feeling he's not about to give it back. They looked them over carefully.SEE VIDEO BY TOPIC: MEET THE BEAUTIFUL SINGLE LADY IN MY COMPOUND - 2019 Nigerian Full Movies
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Molecule , a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance. The division of a sample of a substance into progressively smaller parts produces no change in either its composition or its chemical properties until parts consisting of single molecules are reached.
Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties. In this latter stage of fragmentation the chemical bonds that hold the atoms together in the molecule are broken. Atoms consist of a single nucleus with a positive charge surrounded by a cloud of negatively charged electrons.
When atoms approach one another closely, the electron clouds interact with each other and with the nuclei. If this interaction is such that the total energy of the system is lowered, then the atoms bond together to form a molecule. Thus, from a structural point of view, a molecule consists of an aggregation of atoms held together by valence forces.
Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical, as in, for example, the oxygen molecule O 2 , they compose a homonuclear diatomic molecule, while if the atoms are different, as in the carbon monoxide molecule CO , they make up a heteronuclear diatomic molecule.
Molecules containing more than two atoms are termed polyatomic molecules , e. Polymer molecules may contain many thousands of component atoms. The ratio of the numbers of atoms that can be bonded together to form molecules is fixed; for example, every water molecule contains two atoms of hydrogen and one atom of oxygen.
It is this feature that distinguishes chemical compounds from solutions and other mechanical mixtures. Thus hydrogen and oxygen may be present in any arbitrary proportions in mechanical mixtures but when sparked will combine only in definite proportions to form the chemical compound water H 2 O.
It is possible for the same kinds of atoms to combine in different but definite proportions to form different molecules; for example, two atoms of hydrogen will chemically bond with one atom of oxygen to yield a water molecule, whereas two atoms of hydrogen can chemically bond with two atoms of oxygen to form a molecule of hydrogen peroxide H 2 O 2. Furthermore, it is possible for atoms to bond together in identical proportions to form different molecules. Such molecules are called isomers and differ only in the arrangement of the atoms within the molecules.
Not all substances are made up of distinct molecular units. Sodium chloride common table salt , for example, consists of sodium ions and chlorine ions arranged in a lattice so that each sodium ion is surrounded by six equidistant chlorine ions and each chlorine ion is surrounded by six equidistant sodium ions. The forces acting between any sodium and any adjacent chlorine ion are equal. Hence, no distinct aggregate identifiable as a molecule of sodium chloride exists.
Consequently, in sodium chloride and in all solids of similar type, the concept of the chemical molecule has no significance. Therefore, the formula for such a compound is given as the simplest ratio of the atoms, called a formula unit—in the case of sodium chloride, NaCl.
Molecules are held together by shared electron pairs, or covalent bonds. Such bonds are directional, meaning that the atoms adopt specific positions relative to one another so as to maximize the bond strengths. As a result, each molecule has a definite, fairly rigid structure, or spatial distribution of its atoms.
Structural chemistry is concerned with valence , which determines how atoms combine in definite ratios and how this is related to the bond directions and bond lengths.
The properties of molecules correlate with their structures; for example, the water molecule is bent structurally and therefore has a dipole moment, whereas the carbon dioxide molecule is linear and has no dipole moment.
The elucidation of the manner in which atoms are reorganized in the course of chemical reactions is important. In some molecules the structure may not be rigid; for example, in ethane H 3 CCH 3 there is virtually free rotation about the carbon-carbon single bond. The nuclear positions in a molecule are determined either from microwave vibration-rotation spectra or by neutron diffraction.
The electron cloud surrounding the nuclei in a molecule can be studied by X-ray diffraction experiments. Further information can be obtained by electron spin resonance or nuclear magnetic resonance techniques. Advances in electron microscopy have enabled visual images of individual molecules and atoms to be produced.
In a molecular structure the bond lengths and bond angles are those for which the molecular energy is the least. The molecular weight of a molecule is the sum of the atomic weights of its component atoms.
If a substance has molecular weight M , then M grams of the substance is termed one mole. Molecular weights can be determined by mass spectrometry and by techniques based on thermodynamics or kinetic transport phenomena. Article Media. Info Print Cite. Submit Feedback. Thank you for your feedback. Home Science Chemistry. See Article History.
Read More on This Topic. Most chemists were confident that atoms really existed long before these sophisticated techniques provided such irrefutable evidence. Get exclusive access to content from our First Edition with your subscription.
Subscribe today. Learn More in these related Britannica articles:. In the 19th century, when the compositions of countless compounds were being determined, it was found that in certain cases different compounds have the same chemical…. For a complete understanding of the liquid state of matter, an understanding of behaviour on the molecular level is necessary. Such behaviour is characterized by two quantities called the intermolecular pair potential function, u , and the radial distribution function, g.
Owing to the poor vacuums available prior to the contributions of Gaede and Langmuir see above , this subject was forced on the attention of early experimenters. They observed masses of 3 and 19, which could not have been produced by simple ionization and…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox!
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Jane Hutcheon's connection to China goes back more than one hundred and fifty years, when her great-great-grand-uncle Phineas, a Scottish tea merchant, was drawn to its shores. When Jane became the ABC's China correspondent in , she began her own journey through an ancient and intriguing culture that is undergoing rapid change. Like learning to eat a dish of slippery braised pig's face, Jane had to learn the etiquette of reporting in China-a country where people often give interviews in lowered voices, or with a glance over their shoulders to see who might be listening. Though China has transformed itself into a heady capitalist republic, the country's new facade covers up a multitude of the same old problems.
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We have to work with money every day. While balancing your checkbook or calculating your monthly expenditures on espresso requires only arithmetic, when we start saving, planning for retirement, or need a loan, we need more mathematics. This could be a starting investment, or the starting amount of a loan. Interest, in its most simple form, is calculated as a percent of the principal.
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Molecule , a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance. The division of a sample of a substance into progressively smaller parts produces no change in either its composition or its chemical properties until parts consisting of single molecules are reached. Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties. In this latter stage of fragmentation the chemical bonds that hold the atoms together in the molecule are broken.
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