The modern atomic theory states that atoms of one element are the same, while atoms of different elements are different. What makes atoms of different elements different? The fundamental characteristic that all atoms of the same element share is the number of protons. All atoms of hydrogen have one and only one proton in the nucleus; all atoms of iron have 26 protons in the nucleus. This number of protons is so important to the identity of an atom that it is called the atomic number.The number of protons in an atom is the atomic number of the element. Thus, hydrogen has an atomic number of 1, while iron has an atomic number of 26. Each element has its own characteristic atomic number.
In neutral atoms, the atomic number is also the number of electrons. Since the atomic number is unique to each element, it strongly determines the chemical properties of that element. Because an electron has negligible mass relative to that of a proton or a neutron, the mass number is calculated by the sum of the number of protons ((p^+)) and neutrons ((n)). Each proton and neutron's mass is approximately one atomic mass unit (AMU) and the two added together results in the mass number of the atom:
Atomic hydrogen. Polystyrene,mw 190000. Hydrogen, atomic. Polystyrene, Cellulose powder. Hydrogen molecule. Elemental hydrogen. Powdered cellulose. Monoatomic hydrogen. Polystyrene, HIPS. Cellulose Nanoparticles. Latex beads,polystyrene. Polystyrene,mw 1200. The atomic weight of hydrogen has been based on mass-spectrometric measurements since 1938. In its report for 1961, CIAAW recommended A r (H) = 1.007 97(1) based on the average and the range of ²H concentrations measured in hydrogen extracted from fresh- and saltwaters; however, it was noted that substances other than water could have a wider. Hydrogen is a chemical element with atomic number 1 which means there are 1 protons and 1 electrons in the atomic structure. Hydrogen is a chemical element with atomic number 1 which means there are 1 protons in its nucleus. Total number of protons in the nucleus is called the atomic number of the atom and is given the symbol Z. The total electrical charge of the nucleus is therefore +Ze, where e (elementary charge) equals to 1,602 x 10-19 coulombs. Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass.
[A=p^+ + n]
Atoms of the same element can have different numbers of neutrons, however. Atoms of the same element (i.e., atoms with the same number of protons) with different numbers of neutrons are called isotopes (discussed in next section). Most naturally occurring elements exist as isotopes. For example, most hydrogen atoms have a single proton in their nucleus. However, a small number (about one in a million) of hydrogen atoms have a proton and a neutron in their nuclei. This particular isotope of hydrogen is called deuterium. A very rare form of hydrogen has one proton and two neutrons in the nucleus; this isotope of hydrogen is called tritium. The sum of the number of protons and neutrons in the nucleus is called the mass number of the isotope.
Example (PageIndex{1})
- The most common carbon atoms have six protons and six neutrons in their nuclei. What are the atomic number and the mass number of these carbon atoms?
- An isotope of uranium has an atomic number of 92 and a mass number of 235. What are the number of protons and neutrons in the nucleus of this atom?
Solution
- If a carbon atom has six protons in its nucleus, its atomic number is 6. If it also has six neutrons in the nucleus, then the mass number is 6 + 6, or 12.
- If the atomic number of uranium is 92, then that is the number of protons in the nucleus. Because the mass number is 235, then the number of neutrons in the nucleus is 235 − 92, or 143.
Exercise (PageIndex{1})
The number of protons in the nucleus of a tin atom is 50, while the number of neutrons in the nucleus is 68. What are the atomic number and the mass number of this isotope?
Atomic number = 50, mass number = 118
When referring to an atom, we simply use the element's name: the term sodium refers to the element as well as an atom of sodium. But it can be unwieldy to use the name of elements all the time. Instead, chemistry defines a symbol for each element. The atomic symbol is a one- or two-letter representation of the name of an element. By convention, the first letter of an element's symbol is always capitalized, while the second letter (if present) is lowercase. Thus, the symbol for hydrogen is H, the symbol for sodium is Na, and the symbol for nickel is Ni. Most symbols come from the English name of the element, although some symbols come from an element's Latin name. (The symbol for sodium, Na, comes from its Latin name, natrium.) Table (PageIndex{1}) lists some common elements and their symbols. You should memorize the symbols in Table (PageIndex{2}), as this is how we will be representing elements throughout chemistry.
Element Name | Symbol | Element Name | Symbol |
---|---|---|---|
Aluminum | Al | Mercury | Hg |
Argon | Ar | Molybdenum | Mo |
Arsenic | As | Neon | Ne |
Barium | Ba | Nickel | Ni |
Beryllium | Be | Nitrogen | N |
Bismuth | Bi | Oxygen | O |
Boron | B | Palladium | Pd |
Bromine | Br | Phosphorus | P |
Calcium | Ca | Platinum | Pt |
Carbon | C | Potassium | K |
Chlorine | Cl | Radium | Ra |
Chromium | Cr | Radon | Rn |
Cobalt | Co | Rubidium | Rb |
Copper | Cu | Scandium | Sc |
Fluorine | F | Selenium | Se |
Gallium | Ga | Silicon | Si |
Germanium | Ge | Silver | Ag |
Gold | Au | Sodium | Na |
Helium | He | Strontium | Sr |
Hydrogen | H | Sulfur | S |
Iodine | I | Tantalum | Ta |
Iridium | Ir | Tin | Sn |
Iron | Fe | Titanium | Ti |
Krypton | Kr | Tungsten | W |
Lead | Pb | Uranium | U |
Lithium | Li | Xenon | Xe |
Magnesium | Mg | Zinc | Zn |
Manganese | Mn | Zirconium | Zr |
A simple way of indicating the mass number of a particular atom is to list it as a superscript on the left side of an element’s symbol. Atomic numbers are often listed as a subscript on the left side of an element’s symbol (Figure (PageIndex{1})).
Figure (PageIndex{1}) shows an easy way to represent isotopes using the atomic symbols where (X) is the symbol of the element, (A) is the mass number, and (Z) is the atomic number. Thus, for the isotope of carbon that has 6 protons and 6 neutrons, the symbol is
[ce{_{6}^{12}C} nonumber]
where (C) is the symbol for the element, 6 represents the atomic number, and 12 represents the mass number.
Example (PageIndex{2}):
- What is the symbol for an isotope of uranium that has an atomic number of 92 and a mass number of 235?
- How many protons and neutrons are in (ce{_{26}^{56}Fe})
Solution
- The symbol for this isotope is (ce{_{92}^{235}U})
- This iron atom has 26 protons and 56 − 26 = 30 neutrons.
![Hydrogen Atomic Number Hydrogen Atomic Number](https://mammothmemory.net/images/user/base/Chemistry/2. Atomic Structure/2.2.15 Hydrogen block.jpg)
Exercise (PageIndex{2})
How many protons are in (ce{_{11}^{23} Na})
Hydrogen Atomic Number Of Neutrons
11 protons
Hydrogen Atomic Number Mass And Structure
It is also common to state the mass number after the name of an element to indicate a particular isotope. Carbon-12 represents an isotope of carbon with 6 protons and 6 neutrons, while uranium-238 is an isotope of uranium that has 146 neutrons.
![Symbol Symbol](/uploads/1/1/8/2/118218918/493180723.png)
Key Takeaways
- Atoms themselves are composed of protons, neutrons, and electrons.
- Each element has its own atomic number, which is equal to the number of protons in its nucleus.
- Isotopes of an element contain different numbers of neutrons.
- Elements are represented by an atomic symbol.