PERIODIC TABLE
The periodic table places elements into columns--groups—and rows--periods—that share certain properties. These properties determine an element’s physical state at room temperature—gas, solid, or liquid—as well as its chemical reactivity, the ability to form chemical bonds with other atoms.
The beauty of the periodic table is revealed in the patterns and trends in its rows and columns. Going up or down a column, elements are organized into groups which have similar chemical properties. Moving left or right within a row, the atoms trend smaller or larger in size, and their re activities go up and down. This clip below this in more detail.
In addition to listing the atomic number for each element, the periodic table also displays the element’s relative atomic mass, the weighted average for its naturally occurring isotopes on earth. Looking at hydrogen, for example, its symbol, and name appear, as well as its atomic number of one—in the upper left-hand corner—and its relative atomic mass of 1.01.
The beauty of the periodic table is revealed in the patterns and trends in its rows and columns. Going up or down a column, elements are organized into groups which have similar chemical properties. Moving left or right within a row, the atoms trend smaller or larger in size, and their re activities go up and down. This clip below this in more detail.
In addition to listing the atomic number for each element, the periodic table also displays the element’s relative atomic mass, the weighted average for its naturally occurring isotopes on earth. Looking at hydrogen, for example, its symbol, and name appear, as well as its atomic number of one—in the upper left-hand corner—and its relative atomic mass of 1.01.
Watch this clip to understand the trends of the periodic table
In particular you need to know that each of the ends have highest reactivity.
ELECTRON SHELLS
An atom consists of a central nucleus surrounded by a cloud of electrons. The nucleus makes up most of the atom's mass. It consists of positively charged protons and uncharged neutron.
Each element has a unique number of protons – this is its atomic number. For example, oxygen has an atomic number of 8 so every oxygen atom must have 8 protons in its nucleus.
To balance these positive charges, the atom must have 8 electrons around the nucleus. We then say that the atom is electrically neutral.
The electrons buzzing around the nucleus are not randomly scattered but are confined to shells, which you can think of as like the orbits of the planets as they revolve around the Sun. The electron configuration of an atom is the arrangement of its electrons in shells surrounding the nucleus.
As you'll see, electron configurations are important because they explain the organisation of the periodic table and why atoms bond together to form molecules and crystal lattices.
Atoms, like other things governed by the laws of physics, tend to take on the lowest-energy, most stable configuration they can. Thus, the electron shells of an atom are populated from the inside out, with electrons filling up the low-energy shells closer to the nucleus before they move into the higher-energy shells further out. The shell closest to the nucleus, 1n, can hold two electrons, while the next shell, 2n, can hold eight, and the third shell, 3n, can hold up to eighteen.
Rules for electron structure in atoms.
Each element has a unique number of protons – this is its atomic number. For example, oxygen has an atomic number of 8 so every oxygen atom must have 8 protons in its nucleus.
To balance these positive charges, the atom must have 8 electrons around the nucleus. We then say that the atom is electrically neutral.
The electrons buzzing around the nucleus are not randomly scattered but are confined to shells, which you can think of as like the orbits of the planets as they revolve around the Sun. The electron configuration of an atom is the arrangement of its electrons in shells surrounding the nucleus.
As you'll see, electron configurations are important because they explain the organisation of the periodic table and why atoms bond together to form molecules and crystal lattices.
Atoms, like other things governed by the laws of physics, tend to take on the lowest-energy, most stable configuration they can. Thus, the electron shells of an atom are populated from the inside out, with electrons filling up the low-energy shells closer to the nucleus before they move into the higher-energy shells further out. The shell closest to the nucleus, 1n, can hold two electrons, while the next shell, 2n, can hold eight, and the third shell, 3n, can hold up to eighteen.
Rules for electron structure in atoms.
- Atoms want a full outer shell.
- Atoms usually fill the innermost shells first. (not the naughty transition metals)
- Each shell can contain only a fixed number of electrons.
- The general formula is that the nth shell can in principle hold up to 2(n2) electrons.
The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons. In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule. Some atoms can be stable with an octet even though their valence shell is the 3n shell, which can hold up to 18 electrons. We will explore the reason for this when we discuss electron orbitals below.
Examples of some neutral atoms and their electron configurations are shown below. In this table, you can see that helium has a full valence shell, with two electrons in its first and only, 1n, shell. Similarly, neon has a complete outer 2n shell containing eight electrons. These electron configurations make helium and neon very stable. Although argon does not technically have a full outer shell, since the 3n shell can hold up to eighteen electrons, it is stable like neon and helium because it has eight electrons in the 3n shell and thus satisfies the octet rule. In contrast, chlorine has only seven electrons in its outermost shell, while sodium has just one. These patterns do not fill the outermost shell or satisfy the octet rule, making chlorine and sodium reactive, eager to gain or lose electrons to reach a more stable configuration.
The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons. In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule. Some atoms can be stable with an octet even though their valence shell is the 3n shell, which can hold up to 18 electrons. We will explore the reason for this when we discuss electron orbitals below.
Examples of some neutral atoms and their electron configurations are shown below. In this table, you can see that helium has a full valence shell, with two electrons in its first and only, 1n, shell. Similarly, neon has a complete outer 2n shell containing eight electrons. These electron configurations make helium and neon very stable. Although argon does not technically have a full outer shell, since the 3n shell can hold up to eighteen electrons, it is stable like neon and helium because it has eight electrons in the 3n shell and thus satisfies the octet rule. In contrast, chlorine has only seven electrons in its outermost shell, while sodium has just one. These patterns do not fill the outermost shell or satisfy the octet rule, making chlorine and sodium reactive, eager to gain or lose electrons to reach a more stable configuration.
ELECTRON CONFIGURATIONS
Electron configurations describe where electrons are located around the nucleus of an atom. For example, the electron configuration of lithium, 1s²2s¹, tells us that lithium has two electrons in the 1s subshell and one electron in the 2s subshell.