3.4: Atomic Mass and Atomic Number (2024)

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    Learning Outcomes
    • Define atomic and mass numbers.
    • Determine the number of protons, neutrons, and electrons in an atom.
    • Identify the charge and relative mass of subatomic particles.
    • Label the location of subatomic particles in the atom.
    • Determine the mass of an atom based on its subatomic particles.
    • Write A/Z and symbol-mass format for an atom.

    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. Since neutrons do not affect the charge, the number of neutrons is not dependent on the number of protons and will vary even among atoms of the same element.

    Atomic Number

    The atomic number (represented by the letter Z) 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 (see figure below) 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. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on.

    3.4: Atomic Mass and Atomic Number (2)

    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. Helium, with two protons, will have two electrons. In the chemical classroom, the proton count will always be equivalent to an atom's atomic number. This value will not change unless the nucleus decays or is bombarded (nuclear physics).

    3.4: Atomic Mass and Atomic Number (3)

    Mass Number

    Experimental data 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 (represented by the letter A) is defined as the total number of protons and neutrons in an atom. Consider the table below, which shows data from the first six elements of the periodic table.

    Table \(\PageIndex{1}\): Atoms of the First Six Elements
    Name Symbol Atomic Number (Z) Protons Neutrons Electrons Mass Number (A) (rounded to two decimals)
    hydrogen \(\ce{H}\) 1 1 0 1 1.01
    helium \(\ce{He}\) 2 2 2 2 4.00
    lithium \(\ce{Li}\) 3 3 4 3 6.94
    beryllium \(\ce{Be}\) 4 4 5 4 9.01
    boron \(\ce{B}\) 5 5 6 5 10.18
    carbon \(\ce{C}\) 6 6 6 6 12.01

    Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since \(2 + 2 = 4\), we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.

    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.

    \[\text{Number of neutrons} = \text{ rounded mass number} - \text{atomic number} \nonumber \]

    Atoms of the element chromium \(\left( \ce{Cr} \right)\) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:

    \[52 - 24 = 28 \: \text{neutrons in a chromium atom} \nonumber \]

    The composition of any atom can be illustrated with a shorthand notation called A/Z format. Both the atomic number and mass are written to the left of the chemical symbol. The "A" value is written as a superscript while the "Z" value is written as a subscript. For an example of this notation, look to the chromium atom shown below:

    \[\ce{^{52}_{24}Cr} \nonumber \]

    Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr-52. In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values.

    Example \(\PageIndex{1}\)

    Calculate each of the three subatomic particles and give specific group or period names for each atom.

    1. mercury
    2. platinum
    3. bromine

    Solutions

    1. Hg (transition metal)- has 80 electrons, 80 protons, and 121 neutrons
    2. Pt (transition metal)- has 78 electrons, 78 protons, and 117 neutrons
    3. Br (halogen)- has 35 electrons, 35 protons, and 45 neutrons
    Example \(\PageIndex{2}\)

    Write both A/Z and symbol-mass formats for the atoms in Example \(\PageIndex{1}\).

    Solutions

    1. \(\ce{^{201}_{80}Hg}\) and Hg-201
    2. \(\ce{^{195}_{78}Pt}\) and Pt-195
    3. \(\ce{^{80}_{35}Br}\) and Br-80
    Example \(\PageIndex{3}\)

    Identify the elements based on the statements below.

    1. Which element has 25 protons?
    2. Which element has 0 neutrons?
    3. Which element has 83 electrons?

    Solutions

    a. manganese

    b. hydrogen

    c. bismuth

    Need More Practice?

    • Turn to section 3.E of this OER and answer questions #1-#2, #4, and #8.

    Contributors and Attributions

    • Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)

    3.4: Atomic Mass and Atomic Number (2024)

    FAQs

    What is the answer of atomic number and mass number? ›

    The number of protons present in an atom is known as the Atomic number. The mass number is calculated by taking the sum of the number of neutrons (n) and protons (p) of an atom. Isotopes have a similar atomic number.

    What has an atomic mass of 34? ›

    Selenium

    How do you answer the atomic number? ›

    The atomic number of an atom is equal to the number of protons in the nucleus of an atom or the number of electrons in an electrically neutral atom. For example, in a sodium atom, there are 11 electrons and 11 protons. Thus the atomic number of Na atom = number of electrons = number of protons = 11.

    How do I find the atomic mass with the atomic number? ›

    Relationship. Atomic number and mass number both consider the number of protons present in a given element. The atomic number is simply the number of protons in the given element (Z), and the mass number is the number of protons + neutrons in an atom of the element (A).

    Which is atomic mass and number? ›

    Be careful you don't confuse atomic number and mass number. While the mass number is the sum of the protons and neutrons in an atom, the atomic number is only the number of protons. The atomic number is the value found associated with an element on the periodic table because it is the key to the element's identity.

    What is the atomic formula? ›

    1 Definition. An atomic formula is either the logical constant ⊥ or an expression of the form (rt1 … tn), where r is an n-ary relation symbol in ∑, and t1, …, tn are algebraic terms over ∑. In particular, each nullary relation symbol makes an atomic formula.

    What is the atomic mass of an atom equal to ______________ ______________? ›

    The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus.

    What has 36 atomic mass? ›

    Krypton is a chemical element with symbol Kr and atomic number 36. Classified as a noble gas, Krypton is a gas at room temperature.

    What has a mass number of 34 atomic number 16? ›

    Properties of S-34 Isotope:
    Properties of S-34 Isotope:S-34
    Atomic Number (Z)16
    Mass Number (A)34
    Nucleon Number (A)34
    Proton Number (Z)16
    25 more rows

    How to write atomic number and mass number? ›

    Mass Number(A) = Number of Protons + Number of Neutrons

    An isotope of any element can be uniquely represented as AZX, where X is the atomic symbol of the element, A is the mass number and Z is the atomic number. The isotope of carbon that has 6 neutrons is therefore 126C.

    What is atomic mass with an example? ›

    Therefore, the atomic mass of an individual atom can be said to equal the total number of protons and neutrons. For example, fluorine atoms typically have 9 protons and 10 neutrons, and therefore have an atomic mass of 19 amu.

    How to solve average atomic mass? ›

    For this calculation we multiply the masses by their percentages and sum them up to find the weighted average. Keep in mind for percentages to be used in a calculation like this they need to be in decimal form. So 80% is 0.80 and 20% is 0.20. This means the calculation is: 5(0.80) + 6(0.20) = 5.2 amu.

    What is the atomic mass ____ number? ›

    The mass number (represented by the letter A) is defined as the total number of protons and neutrons in an atom.

    What is the atomic number 6 and mass number 12? ›

    Carbon-12: with 6 protons and 6 neutrons and an atomic mass of 12.

    Where can I find the mass number? ›

    On the periodic table, the mass number is usually located below the element symbol. The mass number listed is the average mass of all of the element's isotopes. Each isotope has a certain percentage abundance found in nature, and these are added and averaged to obtain the average mass number.

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