Personal

 Personal

Main article: English personal pronouns

The personal pronouns of modern standard English are presented in the table above. They are I, you, she, he, it, we, and they. The personal pronouns are so-called not because they apply to persons (which other pronouns also do), but because they participate in the system of grammatical person (1st, 2nd, 3rd).

The second-person forms such as you are used with both singular and plural reference. In the Southern United States, y'all (you all) is used as a plural form, and various other phrases such as you guys are used in other places. An archaic set of second-person pronouns used for singular reference is thou, thee, thyself, thy, thine, which are still used in religious services and can be seen in older works, such as Shakespeare's—in such texts, the you set of pronouns are used for plural reference, or with singular reference as a formal V-form. You can also be used as an indefinite pronoun, referring to a person in general (see generic you), compared to the more formal alternative, one (reflexive oneself, possessive one's).

The third-person singular forms are differentiated according to the gender of the referent. For example, she is used to refer to a woman, sometimes a female animal, and sometimes an object to which feminine characteristics are attributed, such as a ship or a country. A man, and sometimes a male animal, is referred to using he. In other cases, it can be used. (See Gender in English.) The word it can also be used as a dummy subject, concerning abstract ideas like time, weather, etc., or a dummy object of a verb or preposition.

The third-person form they is used with both plural and singular referents. Historically, singular they was restricted to quantificational constructions such as Each employee should clean their desk and referential cases where the referent's gender was unknown. However, it is increasingly used when the referent's gender is irrelevant or when the referent is neither male nor female.

The possessive determiners such as my are used as determiners together with nouns, as in my old man, some of his friends. The second possessive forms like mine are used when they do not qualify a noun: as pronouns, as in mine is bigger than yours, and as predicates, as in this one is mine. Note also the construction a friend of mine (meaning "someone who is my friend"). See English possessive for more details.

Demonstrative

The demonstrative pronouns of English are this (plural these), and that (plural those), as in these are good, I like that. All four words can also be used as determiners (followed by a noun), as in those cars. They can also form the alternative pronominal expressions this/that one, these/those ones.

Interrogative

The interrogative pronouns are who, what, and which (all of them can take the suffix -ever for emphasis). The pronoun who refers to a person or people; it has an oblique form whom (though in informal contexts this is usually replaced by who), and a possessive form (pronoun or determiner) whose. The pronoun what refers to things or abstracts. The word which is used to ask about alternatives from what is seen as a closed set: which (of the books) do you like best? (It can also be an interrogative determiner: which book?; this can form the alternative pronominal expressions which one and which ones.) Which, who, and what can be either singular or plural, although who and what often take a singular verb regardless of any supposed number. For more information see who.

In Old and Middle English, the roles of the three words were different from their roles today. "The interrogative pronoun hwā 'who, what' had only singular forms and also only distinguished between non-neuter and neuter, the neuter nominative form being hwæt".[15] Note that neuter and non-neuter refers to the grammatical gender system of the time, rather than the so-called natural gender system of today. A small holdover of this is the ability of relative (but not interrogative) whose to refer to non-persons (e.g., the car whose door won't open).

Pronouns

Pronouns

Main article: English pronouns

Pronouns are a relatively small, closed class of words that function in the place of nouns or noun phrases. They include personal pronouns, demonstrative pronouns, relative pronouns, interrogative pronouns, and some others, mainly indefinite pronouns. The full set of English pronouns is presented in the following table. Nonstandard, informal and archaic forms are in italics.

			Nominative	Accusative	Reflexive	Independent genitive	Dependent genitive
			(subject)	(object)		(possessive)
First-person	Singular		I	me	myself	mine	my mine (before vowel; archaic) me (esp. BrE)
	Plural		we	us	ourselves ourself	ours	our
Second-person	Singular	Standard	you	you	yourself	yours	your
		Archaic informal	thou	thee	thyself	thine	thy thine (before vowel)
	Plural	Standard	you	you	yourselves	yours	your
		Archaic	ye	you	yourselves	yours	your
		Nonstandard	ye you all y'all youse etc. (see above)	ye you all y'all youse	yeerselves y'all's (or y'alls) selves	yeers y'all's (or y'alls)	yeer y'all's (or y'alls)
Third-person	Singular	Masculine	he	him	himself	his
		Feminine	she	her	herself	hers	her
		Neuter	it	it	itself	its	its
		Epicene	they	them	themselves themself	theirs	their
	Plural		they	them	themselves	theirs	their
Generic	Formal		one	one	oneself		one's
	Informal		you	you	yourself	yours	your
Wh-	Relative and interrogative	For persons	who	whom who		whose†	whose
		Non-personal	what	what
	Relative only		which	which			whose
Reciprocal				each other one another
Dummy			there it	it

Biochemistry, polymers, and technology

 Biochemistry, polymers, and technology

Organic chemistry, of course, looks not only in the direction of physics and physical chemistry but also, and even more essentially, in the direction of biology. Biochemistry began with studies of substances derived from plants and animals. By about 1800 many such substances were known, and chemistry had begun to assist physiology in understanding biological function. The nature of the principal chemical categories of foods—proteins, lipids, and carbohydrates—began to be studied in the first half of the century. By the end of the century, the role of enzymes as organic catalysts was clarified, and amino acids were perceived as constituents of proteins. The brilliant German chemist Emil Fischer determined the nature and structure of many carbohydrates and proteins. The announcement of the discovery (1912) of vitamins, independently by the Polish-born American biochemist Casimir Funk and the British biochemist Frederick Hopkins, precipitated a revolution in both biochemistry and human nutrition. Gradually, the details of intermediary metabolism—the way the body uses nutrient substances for energy, growth, and tissue repair—were unraveled. Perhaps the most representative example of this kind of work was the German-born British biochemist Hans Krebs’s establishment of the tricarboxylic acid cycle, or Krebs cycle, in the 1930s.

But the most dramatic discovery in the history of 20th-century biochemistry was surely the structure of DNA (deoxyribonucleic acid), revealed by American geneticist James Watson and British biophysicist Francis Crick in 1953—the famous double helix. The new understanding of the molecule that incorporates the genetic code provided an essential link between chemistry and biology, a bridge over which much traffic continues to flow. The individual “letters” that make the code—four nucleotides named adenine, guanine, cytosine, and thymine—were discovered a century ago, but only at the close of the 20th century could the sequence of these letters in the genes that make up DNA be determined en masse. In June 2000, representatives from the publicly funded U.S. Human Genome Project and from Celera Genomics, a private company in Rockville, Md., simultaneously announced the independent and nearly complete sequencing of the more than three billion nucleotides in the human genome. However, both groups emphasized that this monumental accomplishment was, in a broader perspective, only the end of a race to the starting line.

DNA is, of course, a macromolecule, and an understanding of this centrally important category of chemical compounds was a precondition for the events just described. Starch, cellulose, proteins, and rubber are other examples of natural macromolecules, or very large polymers. The word polymer (meaning “multiple parts”) was coined by Berzelius about 1830, but in the 19th century it was only applied to special cases such as ethylene (C2H4) versus butylene (C4H8). Only in the 1920s did the German chemist Hermann Staudinger definitely assert that complex carbohydrates and rubber had huge molecules. He coined the word macromolecule, viewing polymers as consisting of similar units joined head to tail by the hundreds and connected by ordinary chemical bonds.

The instrumental revolution

 The instrumental revolution

As far as the daily practice of chemical research is concerned, probably the most dramatic change during the 20th century was the revolution in methods of analysis. In 1930 chemists still used “wet-chemical,” or test-tube, methods that had changed little in the previous hundred years: reagent tests, titrations, determination of boiling and melting points, elemental combustion analysis, synthetic and analytic structural arguments, and so on. Starting with commercial labs that provided an out-source for routine analyses and with pH meters that displaced chemical indicators, chemists increasingly began to rely on physical instrumentation and specialists rather than personally administered wet-chemical methods. Physical instrumentation provides the sharp “eyes” that can see to the atomic-molecular level.

In the 1910s J.J. Thomson and his assistant Francis Aston had developed the mass spectrograph to measure atomic and molecular weights with high accuracy. It was gradually improved, so that by the 1940s the mass spectrograph had been transformed into the mass spectrometer—no longer a machine for atomic weight research but rather an analytical instrument for the routine identification of complex unknown compounds (see mass spectrometry). Similarly, colorimetry had a long history, dating back well into the previous century. In the 1940s colorimetric principles were applied to sophisticated instrumentation to create a range of usable spectrophotometers, including visible, infrared, ultraviolet, and Raman spectroscopy. The later addition of laser and computer technology to analytical spectrometers provided further sophistication and also offered important tools for studies of the kinetics and mechanisms of reactions.

Chromatography, used for generations to separate mixtures and identify the presence of a target substance, was ever more impressively automated, and gas chromatography (GC) in particular experienced vigorous development. Nuclear magnetic resonance (NMR), which uses radio waves interacting with a magnetic field to reveal the chemical environments of hydrogen atoms in a compound, was also developed after World War II. Early NMR machines were available in the 1950s; by the 1960s they were workhorses of organic chemical analysis. Also by this time, GC-NMR combinations were introduced, providing chemists unexcelled ability to separate and analyze minute amounts of sample. In the 1980s NMR became well known to the general public, when the technique was applied to medicine—though the name of the application was altered to magnetic resonance imaging (MRI) to avoid the loaded word nuclear.

Many other instrumental methods have seen vigorous development, such as electron paramagnetic resonance and X-ray diffraction. In sum, between 1930 and 1970 the analytical revolution in chemistry utterly transformed the practice of the science and enormously accelerated its progress. Nor did the pace of innovation in analytical chemistry diminish during the final third of the century.

 Organic chemistry in the 20th century

No specialty was more affected by these changes than organic chemistry. The case of the American chemist Robert B. Woodward may be taken as illustrative. Woodward was the finest master of classical organic chemistry, but he was also a leader in aggressively exploiting new instrumentation, especially infrared, ultraviolet, and NMR spectrometry. His stock in trade was “total synthesis,” the creation of a (usually natural) organic substance in the laboratory, beginning with the simplest possible starting materials. Among the compounds that he and his collaborators synthesized were alkaloids such as quinine and strychnine, antibiotics such as tetracycline, and the extremely complex molecule chlorophyll. Woodward’s highest accomplishment in this field actually came six years after his receipt of the Nobel Prize for Chemistry in 1965: the synthesis of vitamin B12, a notable landmark in complexity. Progress continued apace after Woodward’s death. By 1994 a group at Harvard University had succeeded in synthesizing an extraordinarily challenging natural product, called palytoxin, that had more than 60 stereocentres.

These total syntheses have had both practical and scientific spin-offs. Before the “instrumental revolution,” syntheses were often or even usually done to prove molecular structures. Today they are a central element of the search for new drugs. They can also illuminate theory. Together with a young Polish-born American chemical theoretician named Roald Hoffmann, Woodward followed up hints from the B12 synthesis that resulted in the formulation of orbital symmetry rules. These rules seemed to apply to all thermal or photochemical organic reactions that occur in a single step. The simplicity and accuracy of the predictions generated by the new rules, including highly specific stereochemical details of the product of the reaction, provided an invaluable tool for synthetic organic chemists.

Stereochemistry, born toward the end of the 19th century, received steadily increasing attention throughout the 20th century. The three-dimensional details of molecular structure proved to be not only critical to chemical (and biochemical) function but also extraordinarily difficult to analyze and synthesize. Several Nobel Prizes in the second half of the century—those awarded to Derek Barton of Britain, John Cornforth of Australia, Vladimir Prelog of the Soviet Union, and others—were given partially or entirely to honour stereochemical advances. Also important in this regard was the American Elias J. Corey, awarded the Nobel Prize for Chemistry in 1990, who developed what he called retrosynthetic analysis, assisted increasingly by special interactive computer software. This approach transformed synthetic organic chemistry. Another important innovation was combinatorial chemistry, in which scores of compounds are simultaneously prepared—all permutations on a basic type—and then screened for physiological activity.

Chemistry in the 21st century

 Chemistry in the 21st century

Two more innovations of the late 20th century deserve at least brief mention, especially as they are special focuses of the chemical industry in the 21st century. The phenomenon of superconductivity (the ability to conduct electricity with no resistance) was discovered in 1911 at temperatures very close to absolute zero (0 K, −273.15 °C, or −459.67 °F). In 1986 two Swiss chemists discovered that lanthanum copper oxide doped with barium became superconducting at the “high” temperature of 35 K (−238 °C, or −397 °F). Since then, new superconducting materials have been discovered that operate well above the temperature of liquid nitrogen—77 K (−196 °C, or −321 °F). In addition to its purely scientific interest, much research focuses on practical applications of superconductivity.

In 1985 Richard Smalley and Robert Curl at Rice University in Houston, Tex., collaborating with Harold Kroto of the University of Sussex in Brighton, Eng., discovered a fundamental new form of carbon, possessing molecules consisting solely of 60 carbon atoms. They named it buckminsterfullerene (later nicknamed “buckyball”), after Buckminster Fuller, the inventor of the geodesic dome. Research on fullerenes has accelerated since 1990, when a method was announced for producing buckyballs in large quantities and practical applications appeared likely. In 1991 Science magazine named buckminsterfullerene their “molecule of the year.”

Two centuries ago, Lavoisier’s chemical revolution could still be questioned by the English émigré Joseph Priestley. A century ago, the physical reality of the atom was still doubted by some. Today, chemists can maneuver atoms one by one with a scanning tunneling microscope, and other techniques of what has become known as nanotechnology are in rapid development. The history of chemistry is an extraordinary story.

Understanding Hardware and Software

 

Understanding Hardware and Software

To start your computer science journey, you first need to explore some basic concepts. First, we have hardware and software. Hardware refers to the physical components of a computer, like a screen, keyboard, and mouse. Software, on the other hand, is the collection of programs and instructions that tell the computer what to do.

Imagine a computer as a brain, with hardware as the body and software as the mind. The hardware provides the structure and functionality, while the software gives the computer intelligence and the ability to perform tasks. Just like you need both a body and a mind to function, computers rely on hardware and software working together.

Operating System

An operating system is like the boss of a computer. It manages all the tasks and makes sure everything runs smoothly. It helps you to interact with the computer and use different software and tasks. It controls the screen, keyboard, and other parts of the computer.

Example: Windows, macOS, and Android are examples of operating systems that run on computers and smartphones.