بالغة الأنجليزية (Atoms and Elements)

Atoms

Each element is made up of one kind of atom. We can define an atom as the smallest part of an element that can enter into combination with other elements.
Structure of the atom

Each atom consists of
a small, dense, positively-charged nucleus surrounded by
much lighter, negatively-charged electrons.
The nucleus of the simplest atom, the hydrogen atom (H), consists of
a single positively-charged proton. Because of its single proton, the atom of hydrogen is assigned an atomic number of 1.
a single electron.
The charge of the electron is the same magnitude as that of the proton, so the atom as a whole is electrically neutral. Its proton accounts for almost all the weight of the atom.

The nucleus of the atom of the element helium (He) has
two protons (hence helium has an atomic number of 2) and
two neutrons. Neutrons have the same weight as protons but no electrical charge.
The helium atom has two electrons so that, once again, the atom as a whole is neutral.

The structure of each of the other kinds of atoms follows the same plan. From Lithium (At. No. = 3) to uranium (At. No. = 92), the atoms of each element can be listed in order of increasing atomic number. There are no gaps in the list. Each element has a unique atomic number and its atoms have one more proton and one more electron than the atoms of the element that precedes it in the list.

Elements

Elements consist of only one kind of atom and cannot be decomposed into simpler substances.

Our planet is made up of some 90 elements. (Tiny amounts — sometimes only a few atoms — of additional elements have been made in nuclear physics laboratories, but they play no role in our story). Of these 90, only 25 or so are used to build living things.

The table shows the 11 most prevalent elements in the lithosphere (the earth's crust) and in the human body.

Living matter
uses only a fraction of the elements available to it
but, as the table shows, the relative proportions of those it does acquire from its surroundings are quite different from the proportions in the environment.
So,
the composition of living things is not simply a reflection of the elements available to them
For example, hydrogen, carbon, and nitrogen together represent less than 1% of the atoms found in the earth's crust but some 74% of the atoms in living matter.

one of the properties of life is to take up certain elements that are scarce in the nonliving world and concentrate them within living cells.
Some sea animals accumulate elements like vanadium and iodine within their cells to concentrations a thousand or more times as great as in the surrounding sea water. It has even been proposed that uranium be "mined" from the sea by extracting it from certain algae that can take up uranium from sea water and concentrate it within their cells.

There is still some uncertainty about the exact number of elements required by living things. Some elements, e.g., selenium and aluminum, are found in tiny amounts within living things, but whether they are playing an essential role (selenium is) or are simply an accidental acquisition (aluminum probably) is sometimes difficult to determine

» ما هي اللغة الأنجليزية
» Atoms and molecules definition
» REACTIONS OF THE GROUP 1 ELEMENTS WITH WATER " Periodic Table"
» where m is the particle's mass and v is its velocity. The thermal energy of the gas sample consisting of N atoms is given by the sum of these energies, assuming no losses to the container or the environment: