std430 struct alignment

Question

What are the rules to align a struct according to std430?

I have this struct on the CPU side:

struct material {
    vec3 kd;
    vec3 ks;
    vec2 alpha; // Anisotropic roughness
    vec3 emission;
    vec3 emissionStrength;
};

Where should I add padding and how much should I add so that this can be sent to this ssbo:

GPU code:

struct Material {
    
    vec3 kd;
    vec3 ks;
    vec2 alpha; 
    vec3 emission;
    vec3 emissionStrength;

};


layout (std430, binding = 1) buffer Materials {
    Material materials[];
};

Also, how do I ensure that all the array's individual elements are also aligned?