VI. Reading

a. Read and translate the text.

Text B

Atomic number, atomic mass and isotopes

Atoms are the fundamental building blocks of all matter and are composed of protons, neutrons and electrons. Because atoms are electrically neutral, the number of positively charged protons must be equal to the number of negatively charged electrons. One of Dalton’s points in his atomic theory was that all atoms of a given element are identical and it is not strictly true, thanks to variability in the number of neutrons that an atom may contain.

The atomic number of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and any atom with a different number of protons would be a different element. The periodic table displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element’s atomic number is indicated above the elemental symbol.

Since atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus.

Not all atoms of a given element are identical. Specifically, the number of neutrons in the nucleus can vary for many elements. As an example, naturally occurring carbon exists in three forms, which are illustrated in Figure below.

While the presence of isotopes affects the mass of an atom, it does not affect its chemical reactivity. Chemical behavior is governed by the number of electrons and the number of protons. Carbon-13 behaves chemically in exactly the same way as the more plentiful carbon-12.

The masses of individual atoms are very, very small. However, using a modern device called a mass spectrometer, it is possible to measure such minuscule masses. An atom of oxygen-16, for example, has a mass of 2.66 × 10⁻²³ g. While comparisons of masses measured in grams would have some usefulness, it is far more practical to have a system that will allow us to more easily compare relative atomic masses. Scientists decided on using the carbon-12 nuclide as the reference standard by which all other masses would be compared. By definition, one atom of carbon-12 is assigned a mass of exactly 12 atomic mass units (amu). An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12. The mass of any isotope of any element is expressed in relation to the carbon-12 standard. For example, one atom of helium-4 has a mass of 4.0026 amu. An atom of sulfur-32 has a mass of 31.972 amu.

Rutherford’s experiment showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number is defined as the total number of protons and neutrons in an atom. Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.

Number of neutrons = mass number - atomic number.

b. Answer the following questions:

1. Why are all atoms electrically neutral?

2. What part of Dalton’s atomic theory is disproved by the existence of isotopes?

3. Which isotope is used as the reference standard for the atomic mass unit?

4. What is the number of protons in an atom?

5. How are electrons arranged in atoms?

c. Match the words with their definitions.

1.stable	a. a substance formed by the chemical union of two or more elements
2.proton	b. sub-atomic particle, with a negative charge and a negligible mass relative to protons and neutrons
3.nucleous	c. atoms of an element with the same number of protons and electrons but different numbers of neutrons
4.reactive	d. An object that does not have a positive or negative charge
5.properties	e. Uncharged sub-atomic particle, with a mass of 1 relative to a proton
6.electron	f. The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom
7.neutral	g. The characteristics of something
8.isotope	h. Sub-atomic particle with a positive charge and a relative mass of 1
9.compound	i. The tendency of a substance to undergo a chemical reaction
10. neutron	j. the condition of atoms when their outer shell contains its maximum number of electrons

d. Complete the text with the given words: protons (2), atoms, mass, solar, particles, electrons, nucleus, charge.

All matter is made up of ……. An atom is like a tiny ……. system. In the centre of the atom is the nucleus which is a cluster of …… and neutrons. The……. have a positive electric ……. while the neutrons are electrically neutral. The nucleus makes up almost all of an atom's ……. or weight. Whirling at fantastic speeds around the nucleus are smaller and lighter ……. called electrons which have a negative electric charge. An atom has the same number of …….. (- ve charge) and protons (+ ve charge) to make the atom electrically neutral. An extremely powerful force, called the nuclear force, holds the protons together in the nucleus as they naturally repelled one another electrically.

e. Translate the sentences into English.

1. Заряд ядра всегда равен порядковому (атомному) номеру элемента в Периодической системе.

2. Атомы одного и того же химического элемента, которые имеют одинаковый заряд ядра, но разную массу (массовое число), называют изотопами.

3. Атомная масса – масса атома в атомных единицах массы (а. е. м.). За единицу а. е. м. принята 1/12 массы изотопа углерод-12.

4. Атомное ядро – сложная положительно заряженная частица, состоящая из нуклонов, связанных между собой ядерными силами.

5. Ядерные частицы протоны и нейтроны в совокупности называют нуклонами.

6. Каждый химический элемент характеризуется определенным атомным номером.

7. Атомный номер элемента совпадает с числом протонов в ядре его атома.

8. Число протонов в ядре атома равно числу электронов, окружающих ядро.

9. Изотопы одного элемента имеют одинаковый атомный номер, но разные массовые числа.

10. Углерод-14 является одним из природных радиоактивных изотопов.