Elements and compounds, chemistry and life

chapter 1 Elements and compounds, chemistry and life



KEY POINTS


image All compounds are made from matter.

image Elements cannot be broken down into other substances by chemical reactions.

image A compound is made from two or more elements which are combined in a fixed ratio.

image Carbon, oxygen, hydrogen and nitrogen make up about 96% of the human body.

image Atoms are composed of neutrons, protons and electrons.

image Elements are defined by their number of protons.

image Isotopes are different forms of the same element that only differ in the number of neutrons.

image Some isotopes are unstable (radioactive) and lose particles to form a new isotope or element.

image Atoms have electrons in shells whose energy is relative to their distance from the nucleus.

image Atoms interact by sharing electrons to form molecules through strong covalent and ionic bonds.

image Atoms and molecules also interact through weaker hydrogen bonds and van der Waals interactions.

image Molecules have distinct three-dimensional shapes due to the arrangement of the bonds between atoms.

All physical objects and all compounds in our environment, both living and non-living, are made from matter. Matter can be defined as any substance which has mass and occupies space. This matter is made from the atoms of elements which are combined in a multitude of different ways to produce all the substances found in the environment.


Elements are substances which cannot be broken down into other substances by chemical reactions. There are 92 naturally occurring elements, each of which has a symbol that is usually derived from Latin or German, and that can be abbreviated using one or two letters. For example, the symbol for sodium is Na, from the Latin word natrium, whereas the symbol for tungsten is W, from the German name of the mineral wolframite from which it was isolated.


A compound is a substance made from two or more elements that are combined in a fixed and definite ratio. For example, if one atom of the reactive metal sodium combines with one atom of the poisonous gas chlorine, then sodium chloride (NaCl) is formed. All sodium chloride contains sodium and chloride in a 1:1 ratio. Compounds have characteristics different from their parent elements.



Elements in living organisms


Only a small fraction of the naturally occurring elements are found in living matter. The major elements—carbon, oxygen, hydrogen and nitrogen—make up approximately 96% of the human body (Table 1-1). Seven lesser elements make up the remaining 4%. Other elements, referred to as trace elements, are required in very small amounts. For example, calcium, a lesser element, makes up about 1.5% of the human body whereas trace elements like selenium, copper and zinc make up less than 0.01% of the body’s mass.



TABLE 1-1 Elements in the human body. Four elements make up the majority of the human body

image


Why are elements the way they are?


All elements are made from atoms, the smallest unit of matter that still retains the properties of the element. The symbol for an element can also represent a single atom. Atoms are very small, ranging in size from 0.04–0.24 × 10−9 m (0.04–0.24 nm). While there is little variability in the sizes of different atoms, their masses vary considerably. In fact, atoms of one element are distinguished from those of a second element by the fact that they have different masses.


Each atom is composed of smaller components called subatomic particles which consist of neutrons, protons and electrons. Each proton has one unit of positive charge, an electron has one unit of negative charge, and a neutron, as its name suggests, does not carry a charge. Protons and neutrons are packed together at the centre of atoms to form the atomic nucleus. The electrons circulate around the nucleus, with the attraction between opposite charges keeping the electrons close to the nucleus (Fig 1-1). Neutrons and protons have roughly the same mass of about 1.7 × 10−24 g. However, such measurements are not very useful so we use a unit called a dalton instead. Thus, neutrons and protons are said to have a mass of 1 dalton (also called an atomic mass unit—amu). Electrons have negligible mass (1/2000th that of a proton/neutron) and are not used to calculate the total mass of an atom.


image

FIGURE 1-1 A simple representation of an atom. Atoms are made of three types of subatomic particles: neutrons and protons in the central nucleus, and electrons that orbit the nucleus. The positive charge on the protons attracts the negatively charged electrons and keeps them associated with, but not part of, the nucleus.



Atomic number and atomic mass


The atoms of different elements have different numbers of subatomic particles but all the atoms of a single element have the same number of protons in their nuclei. The number of protons is unique for each element and is referred to as its atomic number. The atomic number is written as a subscript on the left of the symbol for the element. Thus, 6C tells us that an atom of carbon has six protons in its nucleus. Since atoms do not carry a net charge the carbon atom must also have six electrons.


The mass number of an element allows us to determine the number of neutrons in the nucleus. The mass number is written as a superscript to the left of the element’s symbol. So, using carbon as an example 126C tells us that the nucleus of a carbon atom contains six protons (from its atomic number) and six neutrons (mass number – atomic number). The mass number also gives us a close approximation of the atomic mass (in daltons).



Isotopes


All the atoms of an element have the same number of protons, but sometimes the number of neutrons varies. These different forms of a single element are referred to as isotopes. For example, carbon naturally occurs as a mixture of three isotopes with atomic masses of 12, 13 and 14. The most common form is 126C which accounts for 99% of naturally occurring carbon. The remaining 1% consists mainly of 136C (6 protons and 7 neutrons) with a small amount of 146C (6 protons and 8 neutrons).


Both carbon-12 and carbon-13 are stable isotopes whose nuclei do not lose particles. However, carbon-14 is unstable and is radioactive. Radioactive isotopes have nuclei which spontaneously lose particles and give off energy. This process is often referred to as radioactive decay, and can result in a change in the atomic number such that a different element is formed. For example, 14C decays to produce stable nitrogen.



image

In this decay reaction a neutron becomes a proton, which remains in the nucleus, an electron, and excess energy, which is released.


Radioactive isotopes can be useful in clinical diagnosis and in therapy. Isotopes that are intentionally introduced into the body are called radiopharmaceuticals. Depending on the type, the isotope will collect in one or more areas of the body. Since the isotope emits radiation, it is easily tracked and can be followed through the body and used to check if organs are healthy. Radioactive isotopes are also given to cancer patients in an attempt to damage cancerous tissue. However, radiation from decaying isotopes can also damage healthy tissues leading to cellular injury, often resulting in cellular death.



How are electrons organised in atoms?

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Jun 11, 2016 | Posted by in BIOCHEMISTRY | Comments Off on Elements and compounds, chemistry and life

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