3. an atom has 17 protons and 18 neutrons. identify the element and write its mass number.\n4. chlorine has…

Explanation:

Step1: Identify the element

The number of protons determines the element. Since there are 17 protons, the element is chlorine (Cl).

Step2: Calculate the mass number

The mass number is the sum of protons and neutrons. Given 17 protons and 18 neutrons, the mass number $A=17 + 18=35$.

Step3: Electron - configuration rules

Electrons fill orbitals in the order of increasing energy: $1s<2s<2p<3s<3p$ etc. The $s$ - orbital can hold a maximum of 2 electrons, and the $p$ - orbital can hold a maximum of 6 electrons.

Step4: Determine the electron - configuration of chlorine

Chlorine has 17 electrons.

• First, fill the 1s orbital: 2 electrons are placed in 1s, so we have $1s^{2}$.
• Then, fill the 2s orbital: 2 electrons are placed in 2s, giving $2s^{2}$.
• Next, fill the 2p orbital: 6 electrons are placed in 2p, resulting in $2p^{6}$.
• Then, fill the 3s orbital: 2 electrons are placed in 3s, so we get $3s^{2}$.
• Finally, the remaining 5 electrons are placed in the 3p orbital, giving $3p^{5}$. The electron - configuration of chlorine is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}$.

Answer:

3. Element: Chlorine (Cl), Mass number: 35
4. Electron - configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}$